landslide hazard assessment and landslide …landslide hazard assessment and landslide precipitation...

EARTH SCIENCES CENTRE GÖTEBORG UNIVERSITY B428 2004

LANDSLIDE HAZARD ASSESSMENT AND LANDSLIDE PRECIPITATION RELATIONSHIP

IN VALPARAISO, CENTRAL CHILE

Ingelöv Erikson Jenny Högstedt

Department of Physical Geography GÖTEBORG 2004

GÖTEBORGS UNIVERSITET Institutionen för geovetenskaper Naturgeografi Geovetarcentrum

LANDSLIDE HAZARD ASSESSMENT AND LANDSLIDE PRECIPITATION RELATIONSHIP

IN VALPARAISO, CENTRAL CHILE

Ingelöv Erikson Jenny Högstedt

ISSN 1400-3821 B428

Projektarbete Göteborg 2004 Postadress Besöksadress Telefo Telfax Earth Sciences Centre Geovetarcentrum Geovetarcentrum 031-773 19 51 031-773 19 86 Göteborg University S-405 30 Göteborg Guldhedsgatan 5A S-405 30 Göteborg

SWEDEN

Abstract The city of Valparaìso (33.04ºS and 71.37ºW) is situated on 42 hills on the coast in central Chile. The city’s population has grown the past 50 years which has lead to extensive building on the hills. Most of the houses are built on steep slopes and many houses in marginalised areas do not follow safety regulations recommended by the Municipality. Valparaíso has a Mediterranean climate, with a mean annual precipitation (1944-2002) of 369.2 mm. The main part of the precipitation falls during intense rainstorms during the austral winter (May –August) which annually create problems with inundations and shallow landslides (debris flows and mud flows). These rainstorms are intensified during El Niño years. The inhabitants and municipality are well aware of the problems but lacks resources to deal with them in a sustainable way. The governmental agency SERNAGEOMIN (Servicio Nacional de Geologia y Mineria) in Chile has identified six risk areas in Valparaíso. The study area, Quebrada Francia, is one of them. The main factors causing landslides in Valparaíso are precipitation and anthropogenic activity in combination with the steepness of the slopes. Seismic activities, a high degree of weathering and unfavourable joint structures are also factors of importance. During records from 29 years a total amount of 202 mass movements, causing at least 62 deaths were recorded. It was established that failures occur after a few days of rainstorms after at least 100 mm of precipitation the past two months. Landslides can occur after intensities over 14.6 mm/24 hour. They occur frequently after 35 mm/24 hours and intensities over 50 mm/24 hours will most certainly cause slope failures. It has been proved that earthquakes create favourable circumstances for landslides but that rainfall acts as the main trigger. The most unsafe areas of Quebrada Francia are characterised by being steep, built on with houses of low quality, containing few safety measures and with large amount of loose filling material. They are situated on highly-completely weathered material, considered unsafe due to its loose characteristics and high infiltration capacity. These areas are found on the flanks between Avenida Francia and Avenida Alemania and along the ravine continuing up from Avenida Francia. Along the flank south of Camino El Vergel immediate risk areas are also found. KEYWORDS: Valparaíso, Chile, landslide hazard map, precipitation threshold value, landslide factors

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Resumen La ciudad de Valparaíso (33.04ºS and 71.37ºW) está situada sobre 42 laderas en la costa del Chile central. El crecimiento de la población de la ciudad en los últimos 50 años, ha llevado a extensivas construcciones sobre las colinas. La mayor parte de las casas están construídas en escarpadas pendientes, y muchas de las ubicadas áreas marginales no siguen las regulaciones de seguridad recomendadas por la Municipalidad. Valparaíso tiene un clima Mediteriano y el precipitación anual es 369.2 mm (1944-2002. La mayor parte de la precipitación cae durante intensas tormentas de lluvia del invierno austral (Mayo - Agosto), las que cada año crean problemas de inundaciones y derrumbes superficiales (flujos de barro y flujos detriticos). Estas lluvias se intensifican durante los años en que se presenta el fenómeno de El Niño. Los habitantes y la Municipalidad están conscientes de estos problemas pero carecen de recursos para enfrentarlos de manera sostenida. El SERNAGEOMIN, Servicio Nacional de Geología y Minería de Chile, ha identificado seis áreas de riesgo en Valparaíso. El área de estudio, Quebrada Francia, es una de ellas. Los principales factores causantes de derrumbes en Valparaíso son las precipitaciones y las actividades antropogénicas en combinación con la acentuada inclinación de las pendientes. Las actividades sísmicas, rocas con grandes alteraciones, fisuras y estructuras no favorables, también son factores de importancia. En 29 años documentados, se registran 202 derrumbes que causaron al menos 62 muertes. Se estableció que las derrumbes ocurren luego de algunos días de lluvias después de por lo menos 100 mm de precipitaciones en los últimos dos meses. Deslizamientos pueden ocurrir después de intensidades por sobre los 14.6mm/24 horas; ocurren frecuentemente luego de 35mm/24 horas, en tanto que intensidades de más de 50mm/24 horas casi con seguridad provocarán fallas de la pendiente. Se ha comprobado que los sismos crean circunstancias favorables para derrumbes, pero es la lluvia el principal detonante. Las áreas más inseguras de Quebrada Francia se caracterizan por ser muy inclinadas, con viviendas de baja calidad constructiva, presentando pocas medidas de seguridad y con grandes porciones de material de relleno suelto. Estas se ubican sobre material completamente o altamente removido, considerado inseguro debido a su baja compactación y a su alta capacidad de infiltración. Estas áreas se encuentran entre los costados de Avenida Francia y Avenida Alemania, y a todo lo largo del barranco que continúa hacia arriba por Av. Francia. Es también posible encontrar áreas de riesgo inmediatas muy cercanas a todo lo largo del costado sur del Camino El Vergel.

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Preface This study is a Masters thesis, equivalent of 20 Swedish credits in Physical Geography for Ingelöv Erikson and Jenny Högstedt at the Earth Science centre, University of Gothenburg, Sweden. The study was founded by The Swedish International Cooperation Agency (SIDA) (ASDI in Spanish) through a Minor FIield Study (MFS) scholarship given to us through the University of Karlstad. The study was supervised by Assistant Professor Mats Olvmo at the Physical Geography department at the University of Gothenburg. Two fantastic months of field studies were conducted in Valparaíso and Santiago, Chile between the 9th of October and 9th of December 2003. The field studies were supervised by Geologist Sr Arturo Hauser at SERNAGEOMIN in Santiago and by Civil Engineer Sr Gino Muzio at the Municipality of Valparaíso. This thesis is a product of teamwork between the two authors. Both of us have been deeply involved in all the processes from collecting and analysing data to conclusions. Jenny Högstedt has however had the main responsibility of the execution of the maps and Ingelöv Erikson has had the main responsibility of the correlation between rainfall, earthquake and landslide data. All photos are published with authorisation from the photographer.

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Acknowledgements This project would not have been possible without the help, support and encouragement from a number of persons. Firstly we would like to thank Monica Mancilla, whose help led us to Chile and to Professor Eduardo Valenzuela, whom we actually never met. But by help from him, we came in contact with Arturo Hauser at SERNAGEOMIN (Servicio Nacional de Geología y Minería) in Chile. Arturo Hauser was our supervisor in Chile, and we are extremely thankful to him for all help, information and time he gave us. Ha gave us invaluable help in finding a suitable study area and designing the project. To Gino Muzio, at the Municipality of Valparaíso, we would like to express our gratitude in Spanish: Muchas gracias para nos ha dado tiempo, información y consideración, no solamentecon objetivo del proyecto, pero en todas maneras. Thanks also to Mats Olvmo, for being our supervisor in Sweden and for answering all of our questions and giving good advice. Rodrigo Rauld also gets our deepest gratitude, for help during field work, with data collecting, for pedagogically answering our endless questions, and last, but absolutely not least, for giving us unforgettable moments in Chile. Thank you very much for putting in such an effort in showing us your country (especially the Andes!). It was much appreciated! Pablo Perez became a dear friend of ours, and has given us valuable internet links. ¡Muchas gracias a los dos! For all kind of help during field work, with photos and image formatting and for non-stop support and patience in big and small things, we give big hugs and thanks to Erik Florberger. Cecilia Johansson, thank you for being model on one of the photos, for being a great friend, for help with the Spanish language and for all the other things you helped us with during our stay in Chile. We also say thank you to: Fredrik Lindberg in GIS issues, Björn Holmer for constructive critique, Niclas Jacobsson and Lennart Nilsson at Ramböll Sweden AB, for help with liquid and plastic limits, as well as hydraulic conductivity, SIDA and the University of Karlstad for the MFS scholarship, the busdrivers and colectivo-drivers of Valparaíso and Viña del Mar for exiting busrides, to everyone not mentioned above who has been helping us in one way or another and family & friends for putting up with us and supporting us during the project.

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CONTENTS

1 INTRODUCTION.............................................................................................................................................. 7

1.1 OBJECTIVES............................................................................................................................................... 8 2 MASS MOVEMENT PROCESSES AND HAZARD MANAGEMENT ...................................................... 9

2.1 HILL SLOPE PROCESSES ......................................................................................................................... 9 2.1.1 Rapid mass movements - definitions ...................................................................................................... 9

2.2 GEOLOGICAL FACTORS .......................................................................................................................... 9 2.2.1 Slope steepness..................................................................................................................................... 10 2.2.2 Weathering ........................................................................................................................................... 10 2.2.3 Discontinuities as a weakening factor.................................................................................................. 10 2.2.4 Soil properties ...................................................................................................................................... 11 2.2.5 Infiltration ............................................................................................................................................ 11 2.2.6 Vegetation ............................................................................................................................................ 12 2.2.7 Earthquakes ......................................................................................................................................... 12

2.3 CLIMATIC FACTORS .............................................................................................................................. 13 2.4 HUMAN IMPACT ..................................................................................................................................... 13 2.5 LANDSLIDE RISK MANAGEMENT....................................................................................................... 13 2.6 HAZARD MAPPING ................................................................................................................................. 14

3 THE STUDY AREA ........................................................................................................................................ 15 3.1 CHILE – SOCIAL AND ECONOMIC BACKGROUND.......................................................................... 15 3.2 VALPARAÍSO ........................................................................................................................................... 16

3.2.1 Geography and geology of the city....................................................................................................... 16 3.2.2 Climate ................................................................................................................................................. 17 3.2.3 Slope geomorphology in the Valparaíso area ...................................................................................... 17 3.2.4 The study area ...................................................................................................................................... 18 3.2.5 Fatal landslide at Cerro de la Cruz ..................................................................................................... 18

4 METHODS ....................................................................................................................................................... 20 4.1 DEFINITIONS............................................................................................................................................ 20 4.2 RELATIONSHIP LANDSLIDES – PRECIPITATION ............................................................................. 20

4.2.1 Precipitation data collection................................................................................................................ 20 4.2.2 Landslide data collection ..................................................................................................................... 20 4.2.3 Earthquake data collection .................................................................................................................. 21

4.3 SOIL ANALYSIS ....................................................................................................................................... 21 4.3.1 Infiltration capacity.............................................................................................................................. 21 4.3.2 Soil sampling........................................................................................................................................ 22

4.4 LANDSLIDE HAZARD MAP ................................................................................................................... 23 4.4.1 Weathering classification..................................................................................................................... 23 4.4.2 Urbanisation classification .................................................................................................................. 25 4.4.3 Slope steepness..................................................................................................................................... 25 4.4.4 Weighting of the maps.......................................................................................................................... 26 4.4.5 Multi-factor hazard map ...................................................................................................................... 26

5 RESULTS ......................................................................................................................................................... 27 5.1 QUEBRADA FRANCIA............................................................................................................................ 27

5.1.1 Urban character................................................................................................................................... 29 5.1.2 Surface runoff management ................................................................................................................. 30

5.2 RELATIONSHIP LANDSLIDES - PRECIPITATION.............................................................................. 31 5.2.1 Fatality statistics .................................................................................................................................. 31 5.2.2 Precipitation and mass movement........................................................................................................ 32

5.2.2.1 Amount of precipitation................................................................................................................................... 32 5.2.2.2 Rainfall Intensity ............................................................................................................................................. 36

5.2.3 Earthquakes and mass movements ....................................................................................................... 37 5.3 PROPERTIES OF THE MATERIAL IN QUEBRADA FRANCIA .......................................................... 39

5.3.1 Discontinuities in Quebrada Francia................................................................................................... 39

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5.3.2 Weathering profiles.............................................................................................................................. 39 5.3.3 Characteristics of the analysed weathering classes ............................................................................. 41

5.3.3.1 Completely weathered ..................................................................................................................................... 41 5.3.3.2 Highly-completely weathered .......................................................................................................................... 43 5.3.3.3 Highly weathered ............................................................................................................................................ 44 5.3.3.4 Filling material ............................................................................................................................................... 45

5.4 MAPS.......................................................................................................................................................... 46 5.4.1 Weathering map ................................................................................................................................... 47 5.4.2 Urbanisation map................................................................................................................................. 48 5.4.3 Slope steepness..................................................................................................................................... 49 5.4.4 Weighting system.................................................................................................................................. 53

5.4.4.1 Weighting of the weathering classes ............................................................................................................... 54 5.4.4.2 Weighting of the urbanisation classes ............................................................................................................. 56 5.4.4.3 Weighting of slope steepness ........................................................................................................................... 57 5.4.4.3 Weighting of slope steepness ........................................................................................................................... 58

5.4.5 Combined factor maps ......................................................................................................................... 58 5.4.5.1 Comparison of the EH weighting map and the RMS weighting map............................................................... 59 5.4.5.2 Areas prone to mass movements...................................................................................................................... 59

6 DISCUSSION ................................................................................................................................................... 62 6.1 THE SENSITIVE AREA OF VALPARAÍSO............................................................................................ 62 6.2 RELATIONSHIP LANDSLIDES - PRECIPITATION.............................................................................. 62

6.2.1 Fatalities .............................................................................................................................................. 62 6.2.2 Amount of rainfall ................................................................................................................................ 62 6.2.3 Rainfall intensity .................................................................................................................................. 63 6.2.4 Urbanisation ........................................................................................................................................ 64 6.2.5 Earthquakes and mass movements ....................................................................................................... 65 6.2.6 Relation between El Niño and landslides............................................................................................. 66

6.3 LANDSLIDE HAZARD MAP ................................................................................................................... 66 6.3.1 The multi-factor hazard map................................................................................................................ 66 6.3.2 Characteristics of the immediate risk of mass movements areas ......................................................... 67 6.3.3 The weathering map............................................................................................................................. 68

6.3.3.1 Discontinuities as a weakening factor............................................................................................................. 68 6.3.4 The urbanisation map .......................................................................................................................... 68

6.3.4.1 Vegetation in Quebrada Francia..................................................................................................................... 70 6.4 CAUSES OF MASS MOVEMENTS ......................................................................................................... 70 6.5 GIS – PRECISENESS AND OBJECTIVITY?........................................................................................... 71

7 CONCLUSIONS .............................................................................................................................................. 72 8 RECOMMENDATIONS................................................................................................................................. 73

8.1 CONSTRUCTION AND MAINTENANCE .............................................................................................. 73 8.2 AFTER EARTHQUAKES.......................................................................................................................... 73 8.3 BEFORE AND DURING THE RAINFALL PERIOD............................................................................... 74 8.4 TECHNICAL ENGINEERING SOLUTIONS ........................................................................................... 74 8.5 HAZARD PREDICTION ........................................................................................................................... 75 8.6 INFORMATION TO PEOPLE LIVING IN RISK AREAS....................................................................... 75 8.7 START A LANDSLIDE RECORD............................................................................................................ 75

9 REFERENCES................................................................................................................................................. 77 9.1 ORAL AND WRITTEN COMMUNICATION.......................................................................................... 79 9.2 INTERNET ................................................................................................................................................. 79

APPENDIX.......................................................................................................................................................... 83

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1 INTRODUCTION The last decade of the 20th century was declared the International Decade for Natural Disaster Reduction (IDNDR) by the United Nations. The purpose of the IDNDR was to make each country identify and evaluate local natural hazards as well as implement prevention and preparedness plans (UNESCO). Landslide hazards annually cause a lot of threat of death and destruction. According to Dai et al. (2002) the world-wide landslide activity is increasing, despite improvements in hazard recognition, prediction, mitigation measures and warning systems. Reasons for this are defined by Schuster (1996) as increased urbanisation in landslide-prone areas, continued deforestation in landslide-prone areas and increased regional precipitation caused by climate change. A great deal of research effort has been put into the landslide issue regarding causing and triggering factors and hazard management, especially in expanding urban areas such as Hong Kong, Rio de Janeiro (Thomas 1994). Even though knowledge exists, there seems to be a gap between those who have the knowledge and the decision-makers. A big challenge for scientists and researchers is therefore to present the information in an easily understood and at the same time accurate way. In this task, Geographical Information Systems (GIS) may be useful. The main area of use of GIS for landslide assessment researchers should be as an analyse tool. Many studies also deal with the influence of precipitation of mass movements (Pallàs et al., 2003, Chowdhury & Flentje 2002 and Pasuto & Silvano 1998). Attempts to establish threshold values for rainfall amount and intensity have been made, as it can be an important help in landslide warning systems. These threshold values differ for each place, because of differences in underlying material and urban use of the area. It is therefore important to establish site specific threshold values. In this thesis it is dealt with a rapidly developing urban area in Valparaíso, Chile, where landslides are a frequently occurring problem. The municipality of Valparaíso is well aware of the problem, but lacks resources to deal with it in a sustainable way. The thesis does not provide extra resources, but helps identifying areas where safety measures may be most efficient and during which periods the area is most prone to landslide hazards. Problems with mass movements, such as rock falls and debris flows, are very common in the urbanized coastal areas throughout Chile (Hauser 2000). The problems occur mostly during the rainy seasons, and are widely associated with intense precipitation. Worst affected are underprivileged people, living in marginalised, “self-urbanised” areas (usually hill slopes), that has not been able to implement the recommended safety measures. SERNAGEOMIN (Servicio National de Geologia y Mineria) in Chile has started a project, to identify risk zones in urban areas in Chile. In Valparaíso, six different sectors have been recognised as unstable. These sectors were identified during the austral winter of 2002, when several mass movement incidents were reported as a consequence of rainstorms (Hauser 2003). This study was carried out in Quebrada Francia, Valparaíso, which has been pointed out as one of these sectors.

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1.1 OBJECTIVES The objectives of this thesis were developed in co-operation with the following supervisors in Chile; the Municipality of Valparaíso and SERNAGEOMIN, to suit their needs of research and knowledge concerning landslide hazard, causes and management. The first aim of the study is to identify hazard areas in Quebrada Francia, Valparaíso. This is needed by the Municipality of Valparaíso for them to take action in appropriate areas in this part of the city. The concrete task was to produce a landslide hazard map of Quebrada Francia. Included in this is also a proposal of reasonable and suitable safety measures, obtainable by the municipality. The second aim of the study is to see if the landslide frequency of Valparaíso and its close surroundings correlates with rainfall data. It is generally believed that landslides are rainfall-induced in the area, both by geologists and locals, but has never been scientifically proved (Hauser, oral communication). To be able to make a relevant landslide-rainfall correlation, several factors causing landslides need to be understood and considered. The third aim is to establish a precipitation threshold value concerning landslide risk, to be a help in forewarnings to habitants when landslide hazard is getting severe. The three main questions to be answered are:

• Which specific areas in Quebrada Francia are most prone to landsliding and why? • What are the most important causes of mass movement in Valparaíso? • Is there a precipitation threshold value when landslides are triggered?

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2 MASS MOVEMENT PROCESSES AND HAZARD MANAGEMENT This section gives a background for the results and discussion of the study.

2.1 HILL SLOPE PROCESSES Transformation of slopes is a natural process driven by gravity, and exists in all areas with a slanting character. Hill slope related processes can be slow such as creep or solifluction and rapid such as landslides, debris flows or rock falls. The focus of this section will be on rapid processes, because they are the ones that severely affect the lives of the people in Valparaíso and its neighbouring areas.

2.1.1 Rapid mass movements - definitions Rapid mass movements include those mass movements taking place during a limited amount of time. The opposite is slow mass movements, such as creep, that continues for a long period of time. Hansen (1984) defines movements faster than 1.5 meters/day as rapid movements and 0.3 meters/minute as very rapid. The rapid mass movements of interest in the study area will be characterized below. The word landslide is loosely a used term, which is defined as follows; the speed is fairly rapid, the movement involves falling, sliding or flowing and it only affects a limited part of a hillside (Hansen 1984). Rock fall refers to the process of falling of rock material, including large rock masses, single blocks or gravel to boulder sized particles (Selby 1993). They normally occur from steep cliffs or from rock slopes modified for transport lines or quarrying (Whalley 1984). Debris flows are closely related to precipitation. They develop where a material is put into movement by the addition of water and consists of all fractions. Hill slopes with colluvial material or saprolites are typical for debris flow (Selby 1993). Mudflows occur when mud is liquefied. It occurs in clays with high liquid limits in areas with high precipitation. Both mudflows and debris flows can occur on slopes with low angle. (Selby 1993)

2.2 GEOLOGICAL FACTORS Terzaghi (1950) classified the factors that influence the occurrence of mass movements as external causes, that result in an increase in the shearing stress of the material and internal causes, that result in a decrease in shearing resistance of the material (Hansen 1984). External changes in stability conditions may be geometrical changes or unloading due to undercutting, erosion or artificial excavation e.g., loading due to addition of material, artificial or natural causes of shocks and vibration (earthquakes, construction works, traffic) as well as changes in the water regime. Internal changes in stability conditions can be progressive failure (following lateral expansion or fissuring), weathering or seepage erosion.

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It is often difficult to identify one specific cause of a mass movement. It is also hard to determine what specific courses of actions are natural or induced/accelerated by human activity. A number of the causing factors that are of importance in the study area are presented in more detail in the following sections.

2.2.1 Slope steepness Of major importance for the slope stability is its inclination. Pallàs et al. (2004) establishes critical slope angles for debris flows failures as ranged from 20° to 49° in a hazard mapping study made in Nicaragua. The elevation of the slope in comparison with its length also influences its stability; hence a long steep slope is more instable than a short steep slope (Olvmo, oral communication).

2.2.2 Weathering Because of differences in temperatures and pressures between surface conditions and conditions during formation, rocks are altered by weathering to new material. Weathering is influenced by several factors. The most important of these factors are the climate and the physical and chemical composition of the parent rock. A residual soil is the product of extensive weathering of the bedrock. The amount of clay minerals in a residual soil affects the behaviour of the soil. Selby (1993) has made the following classification: • Less than 15% of clay; the material behaves as granular material • 15 - 40% of clay; an intermediate situation occurs • More than 40% of clay; behaves mostly as clay.

2.2.3 Discontinuities as a weakening factor According to Selby (1993) the main difference between sedimentary soils and saprolitic soils is the inheritance in saprolites of relict joints and other structural features of the bedrock. The relict joint is likely to be a plane of preferential shear or different pore water pressure. A stability analysis requires knowledge of the distribution, geometry and engineering properties of the discontinuities in the mass. The discontinuities in the mass affect its stability as it acts a weakening factor of the material. Soils formed through weathering may contain adverse joints and will require “rock” methods of investigations and analyses. (Hencher 1987) Layers between different weathering grades can act as planes for slides Hencher (1987) states that it is important to class the type of discontinuity found in the mass of interest. Different types of discontinuities acts in different ways, with a varying degree of danger. The most probable joints found in the study area are: Tectonic joints are the most common type of discontinuity. They are persisting fractures which results from orogenic stresses in the earths crust, and are common in all rock types. They often occur as related groups or sets. Faults are fractures along which displacement has occurred. They are associated with sheared or shattered rock and often have relatively high permeability, and may carry a lot of

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groundwater. Another danger with faults is that they may be active, and cause movements in the ground. Discontinuities have a controlling influence of groundwater flow through the mass. Joints which carry a lot of water may be particularly weathered and coated with deposits of oxides and clay.

2.2.4 Soil properties In order to understand the geomorphologic processes in an area it is important to have knowledge about the rheological behaviour of the material. The Atterbergs limits (figure 1) can be used as an estimation of three states of cohesive soils: solid, plastic and liquid. They are being delimited by the shrinking limit (SL), the plastic limit (PL), which is the boundary between brittle solid and plastic soil and the liquid limit (LL), the moisture content at the transition from a plastic to liquid state. The SL and PL are expressed as moisture content percentages of dry weight (Goudie 1981).

Figure 1. The Atterberg limits (Goudie 1981)

2.2.5 Infiltration Infiltration is the process by which water enters the surface horizon of the soil. The infiltration capacity of the soils in a slope has an affect on its stability. A soil with a potentially small infiltration capacity has a high runoff potential and hence high surface erodability. High infiltration on the other hand leads to a high pore water pressures unless the soil has a high permeability, and the water is transported away from the critical area. A rise in pore-water pressure decreases soil strength and can eventually act as a trigger of a landslide (Selby 1993). Hydraulic conductivity is a measure of the ease in which soil pores permit water movement through the soil. In saturated soils it is equivalent of permeability. The permeability of a soil varies with effective pressure and void ratio and will be influenced by soil stratification. In many soils permeability is greater in horizontal direction through one soil horizon that is vertical through several horizons (Selby 1993).

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2.2.6 Vegetation Greenway (1987) classifies the controlling factors of vegetation on slope stability in two – hydrological and mechanical mechanisms. Each mechanism is either beneficial or adverse to the stability of the slope, and may or not may be relevant for the specific situation. Hydrological factors

1. The leaves and crown of the tree intercept the rainfall, which decreases the amount of rain water available for infiltration or runoff. Leaves on the ground contribute to this as well. The leaves are also responsible for evaporation of water from the tree, which also increases water available to the ground. BENEFICIAL

2. Roots and stems increase the roughness of the ground; that is breaking up the soil and making the infiltration capacity increase. The roughness of the soil can decrease the surface runoff velocity, and hence increase the infiltration capacity. ADVERSE

3. Roots extract moisture from the surrounding soil – lowering the soil moisture and the pore water pressure. BENEFICIAL

4. Depletion of soil moisture through the roots may cause desiccation of the soil – causing cracking in the soil, which increases the infiltration. ADVERSE

Mechanical factors

1. Roots increases soil strength, by reinforcing the soil. BENEFICIAL 2. Tree root s may anchor in to a lower, firmer strata (or jointed bedrock), which supports

the soil mantle not to slide. BENEFICIAL 3. Tree roots may seek its way into joints in rock slopes, wedging blocks apart and

making them fall. On the other hand, the trees may act as a protection barrier for these rock falls. ADVERSE/BENEFICIAL

4. The load/weight of the trees surcharges the slope, increasing normal force and downhill force. ADVERSE/BENEFICIAL

5. Vegetation that catches the wind transmits dynamic forces through the tree into the slope. If the tree falls, the increase erosion and infiltration will occur. ADVERSE!

6. The roots may also bind surface soil particle to them, reducing the erosion. BENEFICIAL

2.2.7 Earthquakes Earthquake vibrations can create mass wasting processes in two ways, firstly by weakening the hill slope, and secondly as a trigger. Earthquake waves can as any other waveform be damped or amplified (Selby 1993). Weak soils, especially soils with high water content can act as an amplifier. Topography can also cause amplification; ridges or convex parts of hills are the parts of a hill slope where surface waves get their strongest amplitude. Earthquake magnitudes that have caused earthquakes differ widely. The smallest earthquake that caused local landslides in USA had a M=4 on the Richter scale. Selby also declares that the minimum magnitude for soil liquefaction and lateral flows is M=5.

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2.3 CLIMATIC FACTORS The major climatic factors which influence runoff and erosion are precipitation, temperature, and wind. Precipitation is by far the most important (Selby 1983). It acts as a weathering agent and precipitation affects the pore-water pressure as well as surface runoff. Intense precipitation can trigger landslides. The importance of precipitation to landslide triggering is stated by several authors (Pallàs et al 2004, Chowdhury & Flentje 2002, Pasuto & Silvano 1998, Thomas 1994).

2.4 HUMAN IMPACT Thomas (1994) emphasises the increase of landslide activity in areas where construction has altered the natural slope profiles. Construction is here considered both to be man made structures and changes in vegetation cover made by man. The stability (or instability) of slopes are increasingly controlled by man-made processes (Wasowski 1998), emphasises the importance of gaining knowledge about anthropogenic influence on slope processes.

2.5 LANDSLIDE RISK MANAGEMENT To cope with the risk of landslides in an area, different approaches are possible. Dai et al. (2002) categorizes landslide risk management into four categories: planning control, engineering solution, acceptance, and monitoring and warning systems. Selby (1993) makes a slightly different grouping, excluding acceptance and separating engineering solutions into two categories: reducing causing forces and increasing restricting forces. Planning control is a cost effective way to reduce landslide risk by decreasing the vulnerability. In practice this could be removing, discouraging or restrict urban development in hazard areas. Reducing landslide risk by planning control presupposes that information on restrictions and regulations reaches the habitants of a hazardous area. It is also of importance that it is made possible for the target group to follow and realize the restrictions and regulations. No result will be achieved if these conditions are not fulfilled. Engineering solutions can reduce causing forces or increase restricting forces. This is a direct and often expensive way of reducing landslide risk. Reducing causing forces can include a functioning drainage system to decrease water pressure, remove weight, such as soil or rock from the head of the slope or decrease length of slope by terracing. Examples of increasing restricting forces are retaining walls and gabions1 to support the slope, shotcrete2 and mesh curtains to fasten loose slope surface, and rock anchors to prevent joints from expanding (Selby 1993). Monitoring and landslide warning is another option for unstable slopes, where engineering solutions is not cost-effective or unfeasible in relation to the specific risk. It is also an alternative where resources are scarce; engineering solutions are desirable but not

1 Wire baskets filled with rock material. 2 Concrete ”sprayed” on a slope.

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economically achievable. This contributes in the first place to save human lives and not property. Monitoring programs can include, according to Dai et al (2002): magnitude, rate and location of slope deformation, measurements of pore pressures and piezometric levels and seismic acceleration. Real-time landslide warning systems works to warn locals via radio- and TV-stations when factors known to cause landslides prevail. Such a system takes its base in a) empirical or theoretical relationships between landslide initiation and rainfall duration and intensity; b) hazard maps identifying vulnerable areas and c) weather forecasts and rain gauges (Dai et al., 2002). Acceptance of a given risk may be necessary. In an economic context this can be explained as to there has to be a balance between the costs of preventing measures and the economic loss of landslides, for example during a year. There is also a human aspect of the acceptance issue – how much is human life worth? How much can it cost to minimise the risk for human deaths in landslide events?

2.6 HAZARD MAPPING Hazard maps are used in many different research fields. They are made to identify specific hazard areas, generally by scientists or consultants as a help to decision-makers. Common hazard maps are for example seismic hazard maps (Kayabali & Akin 2003), flood hazard maps (Islam & Sado 2000), and groundwater contamination hazard maps (Ducci 1999) and as in this thesis landslide hazard maps (Pallas et al. 2003). Currently, hazard maps are generally made using GIS as an analyzing tool. A commonly used approach is to identify the influencing factors, map them individually and then combine the maps into a hazard map. There is no universally accepted system of mapping techniques in different environments (Van Westen et al. 1999). Goudie (1981) describes some techniques in geomorphological mapping, but emphasizes that mapping is more or less subjective and dependant upon the skill and experience of the constructor of the map. Another way of producing a hazard map is by entering the backdoor. This is done by mapping already known affected areas from historic records or by remote sensing, and then analyzing the characteristics of these areas. From these characteristics, new hazard areas can be identified. To use a combination of the two techniques would generate a detailed hazard map where all the possible information about the area has been considered. There are no standard methods of producing hazard maps, and many different approaches are found in scientific published material. The purpose of the map, the character of the studied area and the available information needs to be specified when approaching how to construct the hazard map.

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3 THE STUDY AREA

3.1 CHILE – SOCIAL AND ECONOMIC BACKGROUND Chile is a long and narrow country situated along the west coast of South America. From north to south the country reaches 4200 km, but is generally not wider than 200 km, with an area of 756 945 km2 (Swedish Institute of National Affairs 2002). The geography of Chile is dominated by the Andes with peaks reaching more than 6000 meters in the east, followed by the fertile Central Valley. The Coastal Range, with peaks of more than 2000 meters, separates the Central Valley from the coastal planes next to the Pacific Ocean. The population was during 2000, 15.3 millions (Swedish Institute of National Affairs 2002) and the majority lives in the central parts of the country, most of them in the cities. The official language is Spanish, but the Indian languages Mapuche and Queuchua are also spoken. The main part of the population is mestizos, but there are also Europeans and Indians.

Figure 2. Map over Chile. Chile was a Spanish colony and became independent in 1818 when democracy was implied, although only a privileged amount of people were allowed to vote. The history in Chile has been rather turbulent during the last 35 years. In 1973, Augusto Pinochet took the power of the country in a military coup from socialist Salvador Allende, starting a 15 year long dictatorship. These actions caused turbulence in the Chilean economy, with ups and downs both before and during the dictatorship. In 2004, Chile has had democracy for 15 years and is experiencing the most successful economy in Latin America; the economy is growing, export is increasing, inflation is under control and there are constantly high investments (Swedish Institute of National Affairs 2002). Despite of the growing economy there is still a great part of the population living in poverty. The old system with class barriers remains since the

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colonial times. The combination of political turbulence, poverty and great exposure to geographic instability has led to a situation where the preventing measures regarding natural disasters have been neglected.

3.2 VALPARAÍSO

3.2.1 Geography and geology of the city Valparaíso is situated at latitude 33.04 and longitude 71.37. Valparaíso is situated in a subduction zone, where the Nazca plate is disappearing in under the South American plate. The area suffers from a large amount of seismic activity. Between 1970 and 2002, 53 tremors with a magnitude over five occurred within the Valparaíso region in Chile. The city of Valparaíso has a very distinctive geography, seen in figure 3. The urban area is situated on a narrow coastal terrace and stretches up to the surrounding hills. Nine levels of marine terraces have been distinguished, the highest in the urban area of Valparaíso reaching 450 meters above sea level. (Grimme & Alvarez Sch. 1964). These terraces are eroded and dissected by watercourses and ravines. The inclination of the hills ranges between 15°-35°, and at places as steep as 60°-65° (Hauser 2003), is a serious obstacle for the expanding of the city.

Figure 3. Valparaíso seen from the southern part of the city. In the distance the coastal mountain range can be seen. (Photo: I. Erikson) The centre of the city is built on artificial filling material upon a sandy material. A narrow band of weathered and recent colluvium is located between the flat coastal terrace and the surrounding hills. The colluvial material also reaches 100-250 meters up the gorges. The hills,

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or terraces, consist of more or less weathered pre-cambrian, gneiss (Grimme and Alvarez Sch., 1964). In situ-weathered material is locally known as maicillo. The maicillo layer is at some sites as deep as 30 meters (Grimme & Alvarez Sch. 1964). Maicillo is known to be very vulnerable to water erosion (Rauld & Fernández 2002).

3.2.2 Climate The climate of Chile is extremely diverse due to its large latitudinal and altitudinal coverage. It ranges from the arid Atacama Desert in the north of the country to the very humid southern parts. The two parallel mountain chains effectively generate differences between maritime and continental climates in the country. The coastal zone around the city of Valparaíso has a Mediterranean climate, with strong maritime influence. In the Köppen system the climate is termed Csbn, which refers to a warm temperate climate with warm, dry summers and rainy, cloudy winters. The average temperature of January is 18°C and of July 11.5°C (Gispert et al. 1999). Mean annual precipitation (1944-2002) is 369.2 mm, concentrated to the months of May to August. The main part of the precipitation in Valparaíso falls during the austral winter (May, June, July and August). A lot of the precipitation reaches central Chile through large rainstorms. Most of the time they are related to active cold frontal system outbreaks. (Salazar 1993) The rainstorms generally last for several days and the precipitation is very intense (article studies El Mercurio 1949-2003). The El Niño phenomenon has had a clear influence on the precipitation pattern in the area. It has been shown that ENSO related rainfall is highly seasonally dependent in central Chile, with most significant relationship during the austral winter (June –August). During strong ENSO events the rainfall in central Chile increases. (Montecinos 1998 and Montecinos et al. 2000.)

3.2.3 Slope geomorphology in the Valparaíso area SERNAGEOMIN has produced reports of the larger landslide events in the areas over the past 5 years. They describe possible causes and triggers of the events. In all of these reports precipitation is considered being the cause of failure. A study by Grimme and Alvarez Sch (1964) considering the soils and geology of Valparaíso. This study states that the slopes become unstable between 40° to 45°, depending on the degree of alteration. Information about studies made at Universidad de Playa Ancha in Valparaíso was found in a news article in El Mercurio 15th of April, 2003. This article declares that geographer Carolina Martinéz has stated a precipitation threshold value for landslide initiation of 15 mm/24 hours in the most sensitive areas in Valparaíso. Articles about the stability problems read in El Mercurio revealed a general public awareness about the landslide problems.

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3.2.4 The study area The study area was chosen in one of the ravines surrounding Valparaíso, called Quebrada Francia. This specific area is more thoroughly described in section 5.1.

3.2.5 Fatal landslide at Cerro de la Cruz To demonstrate the conditions of the area, an event of June 2002 will be presented. May of 2002 had a total rainfall of 196.5 mm (compared to an average of 62.6 mm during the months of May 1944-2002). This led to a series of mass movements in Valparaíso and Viña del Mar, some of them with fatal damages (Rauld & Fernández 2002) and one event blocking the only road between Viña del Mar and Valparaíso (Fernández 2002). In the study area of this thesis, several mass movements and floods have taken place. One of them, at Cerro de la Cruz, took place at 5 AM, June 3rd. It caused the death of two people, one house completely destroyed and another damaged. The event, classified by the authors of this thesis as a debris flow, is thoroughly described by geologists at SERNAGEOMIN (Rauld & Fernández 2002).

Figure 4. The slope failure at Cerro de la Cruz viewed from the opposite side of the gully. (Photo: R. Rauld)

Figure 5. The slope failure at Cerro de la Cruz viewed from above. (Photo: R. Rauld)

The slide consisted of artificial material, which had been used to construct a car road, situated 12 meters above the affected houses. A broken sewer was also found in the slope failure. Figure 4 gives an idea of the affected area. The surface of the slide scar is 36 m2, and the inclination of the affected slope around 45°. This filling material was not secured by any safety measure. Furthermore heavy trucks using the road caused vibrations in the material, weakening the internal strength of the material. The trigger of the slide was the heavy rains of that night, increasing the pore pressure in the material to a critical level. The broken sewer

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may by leaking have increased the pore water pressure additionally. The scarp of the slide, as well as the location of the broken sewer is shown in figure 5. This mass movement can be seen as very typical for Valparaíso and Viña del Mar, with unfavourable natural conditions, worsened by human interference and the trigger being heavy rain.

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4 METHODS The methods are divided into four parts. First comes a short definition of key terms. Secondly are presented the data collecting and analyzing of the landslide data and precipitation relationship. As a third part comes the method used for soil analysing. In the last section the methodology of the mapping is presented.

4.1 DEFINITIONS There are a number of different terms concerning the landslide problem used in the literature. These are not always consistently used, and therefore need to be defined in this thesis. The definitions can be seen in table 1.

Table 1. Definitions

Term Definition Source Hazard Probability of occurrence within a specified period of

time and a given area. Selby (1993); p 377

Vulnerability Degree of loss to a population/properties/economic activities etc resulting from the occurrence of a natural phenomenon of a given magnitude.

Selby (1993); p 377

(Specific) Risk Hazard • vulnerability. The expected degree of loss due to a particular natural phenomenon.

Selby (1993); p 377

Cause A process of long term influence (such as a several day long rainfall) contributing to a landslide event.

Wasowski (1998); p 209

Trigger A process of very short duration (such as an intense 24h rainfall or earthquake) starting a landslide event.

Wasowski (1998); p 209

4.2 RELATIONSHIP LANDSLIDES – PRECIPITATION

4.2.1 Precipitation data collection Rain fall data between the years of 1944 and 2002 was collected from two different sources, Servicio Hidrográfico y Oceanográfico de la Armada de Chile (SHOA) and Dirección Metereológica de Chile. None of the sources had complete records; therefore data was received from two different sources. The records were searched for monthly information of total precipitation, 24 hours maximum, date of 24 hours maximum and number of days with more than 10 mm of precipitation. The nearest meteorological station is Punta Angeles, situated in the southern part of Valparaíso. This was the first-hand choice in the search for information. During the years of 1987-1992, as well as 1995-1996, Punta Angeles was missing in the records. For these periods the information came from the meteorological station Quintero, which is situated by the coast 30 km north of Valparaíso. In the data from Dirección Metereológica de Chile, there was no information available on 24 hours maximum, date of 24 hours maximum and number of days with more than 10 mm of precipitation.

4.2.2 Landslide data collection Since no official landslide records are held in Valparaíso, the most exhaustive source of information is newspapers. SERNAGEOMIN is keeping a register with all articles relating to heavy rains, flooding and landslides in the Valparaíso Region from the years of 1944, 1948, 1949, 1962, 1966, 1970, 1972, 1974, 1977-1980, 1982, 1984-1985, 1988 and 1990-1996, the

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months of May-August. The articles were collected from El Mercurio de Valparaíso, the main daily newspaper in Chile, during December 2002 to March 2003 (mail communication, Rauld, 2004-01-12). SERNAGEOMIN library staff carried out the search. For the years 1997-2003 all articles published in El Mercurio were available on the Internet. Information about landslide events during this period of time was therefore searched at their web page (www.mercuriovalpo.cl). Because of storage limitations, it is probable that not all articles written in a newspaper are published and kept on their web pages, since new publications are made all the time. It is therefore highly possible that the amount of landslide data searched for on the internet is not as representative as data collected straight from the paper. There are only landslide records from 29 years. The reason for this is difficulties to get access to the Library archive of El Mercurio papers. The correlation between the articles and the precipitation records would be more accurate if we would have had access to more articles, and hence have more data to correlate. The articles were in Spanish, which is not the native language of the authors. Misinterpretations may have occurred due to language. Since the articles were taken from newspapers, there is no scientific model for how the journalists have described landslides. All articles from these two sources, concerning Valparaíso and Viña del Mar with close surroundings, were scanned for information about date of event, place, type of landslide along with probable cause and consequences. The most commonly occurring kind of slope failure is landslide, classified by El Mercurio as “derrumbe”. Events classified as flows were in the newspaper mainly called “dezlisamiento de tierra”. Events classified by the newspaper as “dezlisamiento de rocas” or “desprendimientos de rocas” are here called rockfalls. Muros are here called retaining walls.

4.2.3 Earthquake data collection Earthquake data was received from SERNAGEOMIN. All earthquakes 1966-2001 within two degrees latitude and longitude from Valparaíso, above a magnitude of 4 on the Richter scale was included, with information about date, latitude, longitude, depth and magnitude. The magnitude of the earthquakes has not been modified from the original data, regarding depth and distance from epicentre to the study area (Valparaíso and Viña del Mar). This means that some earthquakes, with a large magnitude but a distant epicentre, should not have been included.

4.3 SOIL ANALYSIS

4.3.1 Infiltration capacity Infiltration and soil samples were taken were taken at 12 sites, 2 measurements were taken at each site (figure 6). To measure the infiltration capacity of the weathering classes a Dual Ring Infiltrometer was used. It consists of two circular cans. The inner can was scaled in centimetres. The two cans were firmly put about two centimetres into the soil. The measurements were made by filling the inner can to the top with water. The water was then refilled every minute, and the amount

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http://www.mercuriovalpo.cl/

of water that had infiltrated in the soil was noted. Each test was carried out for twenty minutes. The measurements were performed on completely, highly-completely and highly weathered as well as filling material. Remaining classes were not considered relevant for this test since they were more rock than soil. For each class of weathering the infiltration capacity was measured in three different locations. There were five measurements taken from completely and highly weathered, six from highly-completely and seven from filling material. An error was made during the infiltration tests in field; only the inner can was filled, instead of both cans. The water of the outer can would have worked to fill the pore spaces beyond the width of the inner can, and hence make the water of the inner can infiltrate straight downwards

Figure 6. Map indicating where infiltration measurements are made.

4.3.2 Soil sampling Soil samples were collected at each of the places where infiltration capacity measurements were carried out. Some of the samples from the same location were mixed, where it was regarded as very similar material. 13 soil samples were analysed. Sieve analysis was used to determine fractions larger than 0.075 mm. To analyze fractions smaller than 0.075 mm, the pipette method was used. The Swedish Standard procedure (Ståhl

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1972) was followed in both cases, but the procedures are also described in international literature (Goudie 1981). The hydraulic conductivity was calculated from the particle size distribution, with help from Lennart Nilsson and Niclas Jacobsson at ScandiaConsult AB (presently Ramböll Sverige AB). The liquid limits of soil sample 4 and 5 were analysed with a cone penetrometer. The cone had an angle of 60° and a weight of 60 g, according to Swedish Standard (Karlsson, 1974). The plastic limits of soil sample 4 and 5 were derived from a rolling test. The test was conducted according to the Swedish Standard (Karlsson 1974), but are also described in international literature (Goudie 1981). Liquid and plastic limit analyses were only conducted on sample 4 and 5 since they had a combined clay and silt content of more than 50 % and therefore could be regarded as cohesive soils (Selby 1993). The soil samples were collected during the infiltration measurements. Some of the samples were taken from locations where the rock was hard enough to not crumble whilst touched. These samples were therefore collected by the aid of a geologic hammer. The samples may have been affected by vibrations due to the long journey across the Atlantic. There is always a risk of error during analyzing: laboratory mistakes and calculation errors.

4.4 LANDSLIDE HAZARD MAP The landslide hazard map is constructed from three maps: degree of weathering, type of urbanisation/vegetation and slope steepness. These maps were combined to produce a multi-factor landslide hazard map. All maps are produced by the authors of this thesis. The field mapping was made with help from two maps. For the lower, urbanised part of the study area a printed copy of the digital primary map (from Departamento de Obras) of Valparaíso was used. The scale of the printed map was approximately 1:4000. For the easter, not yet built on part of the area, topographic maps from Instituto Geográfico Militar de Chile (for details on this map, see reference Instituto Geográfico Militar de Chile) were used. The original scale was 1:25000, but enlarged copies were made to a scale of 1:12200. The weathering grade and the grade of urbanisation were mapped by both walking in the area and by car.

4.4.1 Weathering classification The different stages of weathering used in the map are based on the Dearman classification table (Selby 1993), but has been modified by the authors during field observations. Since the weathering classification needed to be more specified, extra classes were added between the original classes. This has been done to provide a more precise measure of weathering in order to create accurate hazard maps. The classifications are described in table 2. Classes written in italics are invented by the authors during the field study and therefore suites the area. Classes marked by grey were not

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found or not mapped. The observations were made in cut profiles and other visible locations. A layer of more than 0.4 meters was required for a material to be fitted in a classification. If there was less than 0.4 meters, the underlying material was regarded (see descriptions in table 2). In addition to the state of the weathering, discontinuities were also noted for areas less weathered than moderately-highly. They were classified and given safety values according to the Rock Mass Classification (RMS) (Selby 1993). The parameters that were considered in field can be seen in table 3. Two parameters recommended in Selbys RMS classification were not carried out during the study. It was not possible to consider the outflow of groundwater from joints, since the fieldwork took place during the dry period of the year and it only rained once during the two month stay in Valparaíso.

Table 2. Weathering classification (Dearman classification (Selby 1993). Modified by the authors)

Grade of weathering Description and character Fresh bedrock Completely fresh exposures of rock with little or no discoloration and no

superficial alteration. All the discontinuities are structural Slightly weathered > 90% rock. Slightly discoloured. The existing intact rock is not

noticeably weaker than fresh rock. It is not possible to remove parts of rock by hand.

Slightly-moderately weathered 90- 70% rock, or a thickness of less than 0,4 meters of moderately weathered material overlaying rock in a lower grade of weathering.

Moderately weathered 50-90% rock. Less than half of the rock mass disintegrated. Corestones still largely rectangular and interlocked. Parts of rock can rather easily be removed from original rock.

Moderatel -highly weathered A thickness of less than 0,4 meter highly weathered rock overlaying moderately to slightly weathered rock.

Highly weathered <50% rock. Pallid silty sand with few rounded corestones. Can be excavated with a geological hammer. Parts of rock can very easily be removed, and these pieces fall apart with very little effort.

Highly-completely weathered Falling apart with little effort. Highly crumbled material with a yellowish colour. Structures are seen in the profiles. Also if a layer with a thickness of less than 0,4 meter of completely weathered rock overlaying rock in a lower grade of weathering..

Completely weathered Red brown sandy clay. Changed to soil, some corestones/ghosts may be present. When taken apart, feeling clayey.

Residual soil A pedological soil containing horizons and no sign of original rock fabric.

Filling material Material transported by people to serve as construction material. Could have its origin locally or be taken from a distant site.

Subjectivity in these types of classifications is unavoidable. Someone else could have made another judgement, and another division of the weathering scale. Furthermore there is always an amount of subjectivity when it comes to visual analysis. The evaluation of the grade of weathering is dependant upon the skill, experience and personal opinion of the viewer. There

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were also some difficulties to see the underlying material as Quebrada Francia is a heavily urbanised area, and the soil often was covered by either housing or filling material. There is always a risk of error during analyzing: laboratory mistakes and calculation errors. An obvious step away from the reality (the real properties of the collected samples), is the preparation of the samples, when a decompositioner was used. The chemical and mechanical treatment probably fractioned the material more than any natural process would have done. This is believed to have affected highly and highly-completely weathered material most, but as all samples were treated similarly, the error should be similar in all samples. Table 3. Geomorphic rock-mass strength classification and ratings by Selby (1980).

Orientation Rating Spacing Rating Continuity, Infill Rating Width Rating Weathering Rating

Very favourable. Steep dips into slope, cross joints interlock

20 >3 m 30 None continous No infill

7 <0,1 mm

7 Unweathered 10

Favourable. Moderate dips into slope

18 3 – 1 m 28 Few continous No infill

6 0,1 – 1 mm

6 Slightly weathered

9

Fair. Horizontal dips or nearly vertical

14 1 – 0,3 m

21 Continous No infill

5 1 – 5 mm

5 Moderately weathered

7

Unfavourable. Moderate dips out of slope

9 300-50 mm

15 Continous Thin infill

4 5 – 20 mm

4 Highly weathered

5

Very unfavourable. Steep dips out of slope

5 <50 mm 8 Continous Thick infill

1 >20 mm

2 Completely weathered

3

4.4.2 Urbanisation classification The urbanisation classification was invented during fieldwork in order to grade the quality of the housing in field. The information about what could be classed as good or bad housing was received from Gino Muzio (Municipality of Valparaíso) during field excursions. The classes were invented to make an urbanisation map, which is a visible way to classify the dangers in building styles, as they are closely related to the slope processes that occur in the area. Filling material has not been classified in either the weathering or the urbanisation maps, due to the difficulty of detecting them as they generally are built upon. All the urbanised areas have used filling material, but areas classified as low or middle quality of housing are where the filling material is most easily affected by the climate.

In table 4, the types of urbanisation and vegetation are described, as well as the most common safety measures for each class.

4.4.3 Slope steepness Of major importance for the slope stability is the steepness of the slope. Pallàs et al. (2004) establishes critical slope angles for debris flows failures as ranged from 20° to 49° in a hazard

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mapping study made in Nicaragua. This study was used as a base for the slope steepness classification. Three classes were chosen; <20º, 20º-50º and >50º. It was decided to use three classes because it is valuable to know which slopes or parts of slopes are steeper than 50º. The slope steepness mapping was made manually based on the contour lines on the maps.

Table 4. Types of urbanisation/vegetation.

Type of urbanisation/vegetation

Description Character of safety measures

Forest Mainly eucalyptus - Scrub Bushes, low vegetation. - Low quality of housing Unpaved soil streets, stairs

homemade (some of concrete). Houses or huts are self-constructed, mainly by wood. The cut-and-fill method is very common.

No or few safety measures. The existing retaining walls consist of tires or sand bags. Supportive pillars are made of wood.

Middle quality of housing Most streets paved, most stairs made of concrete. Most of the houses built according to building standards set by the city planning office. The cut-and-fill method is common.

More examples than in preceding class of safety measures in the cuttings. Retaining walls are made of stone, tires or sand bags. Sporadic cases of shotcrete. Supportive pillars are made of concrete, and regarded as stable.

Good quality of housing All streets are paved, few places where rain can infiltrate into soil. Practically every house is built according to standards of the city planning office.

Retaining walls are made by stone, and are common. Shotcrete is common. Supportive pillars are made by concrete.

4.4.4 Weighting of the maps The weighting system is described in section 5.4.2.

4.4.5 Multi-factor hazard map The digital topographic map was converted from CAD to MapInfo. The field maps, of weathering and urbanisation grade, were scanned and provided with accurate coordinates to fit the original map. The paper maps, used for the non urbanised (eastern) part of the study area, were also scanned and added to the digital map. These maps were used as a background when digitalizing the classified areas, to minimize the map drawing errors. The weathering map, urbanisation map and inclination map were digitalized in MapInfo, and given values according to the weighting system (explained in section 5.4.2). The maps were converted from vector files to raster files. With the overlay function of ArcMap, the three maps were added together to form a multi-factor hazard map. When mapping different features in field, a certain amount of subjectivity is always present. The result is dependant upon the skill, experience and personal opinion of the person.

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5 RESULTS The results are divided into four parts. First comes a thorough description of the study area. In section 5.2 the results of the analysing of the landslide, precipitation and earthquake data are presented. In section 5.3 the properties of the analysed material is presented, and finally in 5.4 the maps are presented.

5.1 QUEBRADA FRANCIA The study area was concentrated to one of the valleys, Quebrada Francia (also known as Quebrada Jaime), in the urban area of Valparaíso. The study area is seen in figure 7, and the location in the city is seen in figure 8. The ravine is situated between the hills of Cerro de la Cruz, Cerro Monjas and Cerro Mariposa. This area was chosen because it is a densely populated area where damage-causing mass movements occur almost every year. It is an area with increasing population and urbanization. It was pointed out as one of six sectors with slope instability in Valparaíso in a study made by Hauser (2003). A study of the factors supposed to cause landslides in the ravine was therefore desired.

Figure 7. View of the upper three valleys of Quebrada Francia and Jaime in November 2003. (Photo: I. Erikson) The mapped area in the urbanised part is approximately 1.0 km2, and in the upper part of the studied area circa 2.4 km2. The altitude stretches up to 512 metres above sea level. It is formed by three smaller ravines becoming one towards the city centre. The upper part of the area is mainly covered with eucalyptus trees and scrub like vegetation. The main rock type in the ravine is gneiss, in different states of weathering. Fractures and joints with close spacing, visible at the surface of the rock are commonly seen in the area. The most common types of mass movement in Quebrada Francia are mudflows, debris flows and rock falls (Hauser, mail communication).

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Figure 8. Map of Valparaíso, with the investigated area (Quebrada Francia) marked. Figure 9. Schematic picture of a slope where a cut and fill construction technique has been used. The slope is affected by a mass movement. (Rauld & Fernandez 2002).

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5.1.1 Urban character The population of Quebrada Jaime is estimated to 5000 above Avenida Alemania. The population below Avenida Almana is unknown. Around 80% of the households have drinking water, and 50% are connected to the sewage system. Electricity is available to every house. (Muzio, oral communication). According to Rauld & Fernández (2002), the “cut and fill” technique, seen figure 9, is very common throughout hill slope areas on the entire Central Coast of Chile. The process is used to prepare place for house, road or stairs constructions. This excavation of the slopes increases the steepness and cuts of lateral support which makes the slope unstable. Placing the cut off material or artificial filling just beside the excavation is very common to make the building site larger. This produces an increased pressure (increased shear stress) on the down slope area. A change in hydrology, such as increased/decreased infiltration may also be a result of these processes. The lower part of the ravine has a dense urban character; the middle part has a rural/village urban character whereas the upper part is vegetated. Next to the paved main streets, the houses are generally of good quality. The lower quality houses, mainly situated along the unpaved roads and on the flanks of the ravine, correspond to the terms sheds or huts. To overcome the problem with steep slope angles, several approaches can be seen. Some houses are partly built on poles to compensate for the differing ground level. To stabilise the cuts, a number of methods can be seen, although many of the cuttings are not stabilized at all. Some commonly used stabilising methods seen in Valparaíso is shotcrete, tires or sacks filled with sand stapled on each other to make a kind of retention wall, tarpaulins and more properly built retaining walls. See figures 10 and 11.

Figure 10. Shotcrete used to protect a slope on Avenida Alemania to the right. To the left a zink plate is covering the slope for protection. (Photo: I. Erikson)

Figure 11. Retaining wall on Avenida Alemania. (Photo: I. Erikson)

No fresh rock or residual soil was detected. Filling material was present in varying quantities and thickness close to nearly every construction. The urbanised areas have generally used filling material to compensate level differences and to create flat areas to build on.

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5.1.2 Surface runoff management Tunnels and ditches transport surface runoff from the hills. The tunnels continue beneath the major central streets running towards the sea. Eroded material that is being transported with the surface runoff accumulates in the tunnels. These tunnels have to be cleaned once or twice every year, depending on the amount of rain. To minimize the speed and consequently the eroding power of the running water from the hills, several so called sabo dams (figures 12 and 13) has been built to collect sediment at different levels. They are normally being emptied with sediment around four times per year (Muzio, oral communication).

Figure 12. A sabo dam in Quebrada Francia seen from above. (Photo: I. Erikson)

Figure 13. The same sabo dam as in photo 7, seen from below. (Photo: I. Erikson)

The emptying of the dams and tunnels is essential for the surface runoff system to function well. The dams must be inspected regularly and also after more than five days of continuous raining or a rain with an intensity of 70mm/24 hours or more. During especially intense rain events, the surface runoff exceeds the capacity of the system. In other words, the municipality does not have the resources to empty the dams with necessary frequency. This occurs 3-4 times every year, according to Gino Muzio (Municipality of Valpraíso), and responsible of this issue at Municipality of Valparaíso. The consequences are flooding of the ditches and watercourses in the ravines and on severe occasions also the streets in the city centre. (Muzio, oral communication) The main problems the municipality has to deal with, concerning the dams and sedimentation, are, according to Muzio:

• Garbage thrown in the dams, and on the flanks of the ravines. Especially plastic bags, which do not decompose, blocking the holes in the sabo dam walls very effectively.

• The cutting of trees in the upper part of the ravines. This increases surface runoff and eventually the amount of sediment accumulated in the sabo dams.

• Modification of the slopes in the ravines, for the purpose of house or path constructions. Since these cuttings often end up unstable, they produce large amounts of sediment due to slope failure.

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5.2 RELATIONSHIP LANDSLIDES - PRECIPITATION In this chapter the correlation between precipitation and mass movements, as well as seismic activity and mass movements is presented. The type of recorded mass movement events and statistics about fatalities occurring from mass movements are also presented. Mass movements are generally presented as days with mass movements/month. This was the most accurate way to use the data since information about the amount of events was considered somewhat inconsequent. It is important to consider that all mass movement data presented here origins from articles read in the newspaper El Mercurio de Valparaìso and that some slope failure events may not have been covered by them. Exceptions are made whilst examining the role of the intensity of the rain, and in appendix, where the numbers of events are presented, instead of the number of days with mass movement. The constructed landslide record is presented in appendix 1.

5.2.1 Fatality statistics In figure 14 mass movement related fatalities can be seen. Out of 29 years with mass movement data, deaths took place during 15 of the years. Deaths from mass movements have been a problem in the area since at least the forties. The events which have caused fatalities are widely spread over the years. It is though worth to notice that mass movement data is available throughout the nineties, where there is a relatively low amount of mass movements caused deaths. It is also worth noticing that people have been killed by landslides every year during the new millennium.

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Figure 14. Mass movement related fatalities during the 29 years with existing mass movement statistics.

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The type of mass movement, amount of incidents and related disasters with fatal consequences can be seen in table 5. From 29 years, records of a total amount of 202 slope failures were found. During these events 62 deaths were reported from a total of 22 events. Viewed statistically, an average of 0.3 people dies per mass movement. The most dangerous mass movements are flows where 0.48 deaths per mass movement, closely followed by bursting retaining walls with an average of 0.45 deaths per mass movement. Table 5. Type of mass movements, and amount with fatal consequences. Type of movement

Amount of incidents

Amount of deadly movements

Amount of fatalities

Landslide 107 12 23

Retaining walls which have burst

42 4 19

Flow 33 4 16

Rockfall 14 2 4

Total 202 (196 + 6 unspecified) 22 62

5.2.2 Precipitation and mass movement Figure 15 presents the yearly precipitation along with number of reported days with mass movement. No mass movement data was available for years where number of reported landslide is zero. From 1997 and forward, days with reports are counted for all months during the year, whereas from 1996 and back only May-August are included. During 1997 and 1998 all mass movements took place during other months than May-August. On the other hand, from 1999-2003, no mass movements are reported outside from May-September. This means that during the years with data from 1944-1996, the number of days with reported landslides could have been higher if newspapers from all months of the year would have been available. The annual precipitation average in Valparaíso is 370 mm. Generally, the number of days with reported mass movements corresponds well to annual precipitation, though there are a number of exceptions. In 1992, when precipitation was 623 mm 17 days with mass movements occurred. This is a lot even if it is compared to the large amount of precipitation. Compared to 1997, when precipitation was 754 mm for the whole year, only four days with reported mass movements. 1944 show the same kind of exception; for a precipitation of 601 mm, only one day reports mass movements. It is evident that other parameters than just the amount of precipitation affects the amount of mass movement days.

5.2.2.1 Amount of precipitation The correlation between mass movement days and the monthly precipitation can be seen in figure 16. The determination coefficient is 0.46 thus showing a correlation between the two parameters of almost 50%. There are a large number of mass movement days even for months with a low precipitation. It can be noted that months with more than one day of mass

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movement does not occur until it has rained 50 mm/month, and can frequently be seen while it is raining around 100 mm/month. Four points with differing values are explained further. The two exceptionally high values, (above the regression line), occur in June 1991 (185 mm, 7 mass movement days) and July 1977 (177 mm, 6 mass movement days). In July 1977 cracks from tremors are given as a specific reason for one of the days. On the 24th of June 1977 three tremors of a magnitude

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Figure 15. The yearly precipitation and the number of events of mass movements as well as days with reported mass movements 1944-2002. Note that for years with zero mass movements, no data was available. The left axis displays mm of precipitation and the right axis displays number of events or days. between 4.6 and 4.9 occurred in the Valparaíso region. Cracks from these tremors may have caused mass movements several weeks after the event. Two of the mass movement days are the first and the second of the month, and can therefore also be related to precipitation in late June 1977. June 1991 also has mass movement days during the two first days of the month, but apart from that a satisfying explanation to the high amount of mass movement days could not be read in El Mercurio. The two lower values are from June 2000 (341mm, 2 mass movement days) and July 2001 (330 mm, 2 mass movement events). In July 2001 the two days took place the 27th and the 28th; hence the rains came fairly late in the month. It is possible that this rain attributed to mass movements during august 2001. Further explanation about these two events will be given in section 6.2.2.

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R2 = 0,46

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Figure 16. Monthly precipitation plotted against amount of mass movement days/month. (May- August, 1944-2002), events clearly caused by man has been excluded. Figure 17 shows the amount of reported mass movement events for one month in relation with the precipitation which have fallen the month of the mass movement and the previous month. If the events occur in July, the combined amount of precipitation which has fallen during booth June and July are shown. The determination coefficient for figure 17 is 0.26. The correlation is thus lower between the two month and one month precipitation and the amount of mass movement days. It is though interesting to notice that the minimum precipitation value for mass movement events to occur is at 11.7 mm during the month before and the month of the event, occurring in august 2000. No specified cause for the failures could be read about. The next event where mass movements occur is after 34 mm/mm of precipitation and after 40 mm of rain one mass movement day occurs frequently. Months that have more than one mass movement day occur when the precipitation is more than 68 mm. These months occur frequently when the cumulated precipitation is more than 109 mm. The values in figure 17 are widely scattered. Whilst looking more closely at the data it can be seen that for the months with high precipitation and a low amount of mass movement days, the precipitation value is dependent on the month before, which generally has had a high amount of rainfall value. For the months with a low precipitation value and a high amount of mass movement days the situation is reversed. This situation is clarified with the summer of 1984, marked by two circles in the diagram. Only one mass movement day is marked in figure 7 for August of 1984 although the consolidated precipitation is 454 mm. In July 1984 it rained 390 mm whilst it only rained 64 mm in August, one mass movement day is therefore a reasonable result. July on the other hand had a very high precipitation and therefore caused a lot of mass movement days, whereas it only rained 32 mm in June, which did not increase the total precipitation for the two months very much. The same pattern can be seen for the third point circulated in the diagram, which is the value for May and June 1991 (257 mm, 7 events). June 1991 has as mentioned earlier a high amount of mass movement days. These examples show that the amount of mass movements is the largest during the month with the most precipitation.

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R2 = 0,26

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Figure 17. Mass movement days during the latest month plotted against precipitation for the two previoumonths. Using data for May –August, excluding events clearly caused by man.

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Figure 18. Monthly precipitation and amount of mass movement days. May – August (1944-19889 excluevents clearly caused by man.

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Figure 19. Monthly precipitation and amount of mass movement days. May- August, (1991-2002) excludevents clearly caused by man.

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August 1984

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In figure 18 and 19 the same data are shown as in figure 16. They are shown in two different time series, 1944-1988; which is represented by 30 monthly values and 1991-2002 which are represented by 25 monthly values. It is worth noticing the difference in determination coefficient between the two time series. It is 0.63 for figure 18 whereas the exactly same parameter for figure 19 is much lower and has a determination coefficient of 0.28.

5.2.2.2 Rainfall Intensity Figure 20 shows the amount of mass movement events that occurred two days after the monthly 24 hour precipitation intensity maximum. The determination coefficient is 0.012 and thus pointing towards a low correlation between rainfall intensity and mass movement events. It should be noted that sufficient data to compare this only existed for 50 events, out of these dates 23 events suffered from mass movements the following two days. It is though possible to find some important values in the figure. The minimum rainfall intensity for mass movements to occur, according to the data available, is 14.4 mm/24 hour event. With one exception, the 24 hour intensity needs to be at least 33 mm/24 hour for more than one mass movement to occur. There is a bulk in the values between 40 mm and 60 mm/24 hour where more than four mass movement incidents has occurred. The low regression value is influenced by the exceptionally high number of events for a rather low intensity marked in the diagram. That is for June 1972. In June 1972 there is information about a tree which has fallen and caused landslides, this event may have increased the amount of reported slope failures after the high intensity precipitation. It is also worth noticing that the event with the highest intensity (114 mm/24h, May 92) only caused three mass movement events. In May it rained 142 mm, and in April 26.9 so the low value could be explained by this very intense rain falling on dry ground, being nearly the first rain falling for the year. Hence the infiltration capacity of the ground can be regarded as low.

R2 = 0.012

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Figure 20. 24 hour maximum intensity and amount of reported mass movements during the two days following the event.

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Table 6. The ten events with highest rain intensity in 24 hours without any recorded landslide events the following 2 days.

Month and year Maximum 24/h

May -74 47.0

August -95 43.4

May -91 41.4

July -72 35.2

May -85 35.0

August -72 33.5 August -84 32.3

July -88 31.8

August -77, -85 28.0

May -72 26.1

Table 6 presents the ten highest rainfall intensities without any recorded mass movements. The main part of the values is around 35 mm/24 hours. But there are three events with intensities higher than 41 mm/24 hours, which have not created any recorded mass movements. The highest intensity without any mass movements in May 1974 may be explained by having been the first rains of the year. The precipitation in April 1974 was 0.1 mm. May and August are the most represented months were high rainfall intensity have not lead to any failures. Table 6 can be used as an indicator that the ground needs pre-wetting in order to reach an unstable state. The main parts of the intensities which have not caused mass movements are from 35 mm/ 24 hours and below. This is considered the critical intensity value for Valparaíso area, but as the results shows, both this and higher intensities can occur without resulting in any mass movements. Having viewed both figure 20 and table 6 it can be estimated that intensities higher than 50 mm/hour causes mass movements.

5.2.3 Earthquakes and mass movements Whilst considering earthquake data within two latitudes and longitudes of Valparaíso, 58% of the months between May and August 1966-2001 were affected by an earthquake of magnitude 4 or larger. All magnitudes considered in this paper are measured according to the Richter scale. 1985 was a seismically active year; a total amount of 191 events with a magnitude above 4.5 and with an epicentre between latitude 32º S and 34,5º S and longitude 70º E to 72º E were recorded. Although the annual precipitation was rather low (269.6 mm) there were five recorded days with mass movements. They occurred in June and July, and four of the five incidents were retaining walls which burst. The reasons for the incidents mentioned in the articles in El Mercurio were fissures from earthquakes in the retaining walls, and rain. The fact that four out of five incidents are retaining walls which have burst shows how sensitive man made structures are to seismic ground movements. Five incidents where tremors or fissure from tremors where given as a reason for the failure were found in El Mercurio. Whilst looking at earthquake data six months before the failure it was found that the failures only occurred when there had been earthquakes with a magnitude

37

over five within the past two months. Three of the failures where preceded by tremors over 6 within the past three months. Many tremors with magnitudes over five are recorded without any records of slope failures. In order to statistically show a relationship between earthquake activity and mass movements in Valparaíso the following diagrams were produced. Figure 19 and 20 shows data from 1966-2001 from May-August, since seismological data only exist from these years. 24 of these years had corresponding mass movement data. These 24 years are used in the diagram. Figure 21 shows that the mass movement day frequency increases with approximately 0.3 days when there has been a tremor compared to occasions when there has not been any tremors with a magnitude over 4 the past two months.

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1.53

1.361.23 1.23

Figure 21. Mass movement vulnerability due to seismic activity, within one latitude and one longitude of Valparaíso, showing the amount of mass movement days per month.

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Figure 22. Mass movement vulnerability due to seismic activity, showing data for two latitudes and longitudes. Figure 22 considers earthquake data which have epicentres within two latitudes of Valparaíso instead of one. The result is shows a slight increase, 0.03, in the affect that earthquakes have on the amount of mass movement days. The difference between the two diagrams is small.

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Since the area is to viewed from a risk perspective it is however worth using two latitudes of data if a similar study is to be done. Figure 21 and 22 shows the interesting fact that it is the rainstorms which are the triggers of mass movements and not tremors or earthquakes. If earthquakes were triggers the staple showing days/month (for days not affected by tremors) would be very small. Considering the fact that the main part of the slope failures are mud or debris flows it is understandable that a large amount of intense precipitation is needed for mass movements to occur.

5.3 PROPERTIES OF THE MATERIAL IN QUEBRADA FRANCIA

5.3.1 Discontinuities in Quebrada Francia The geology of Valparaíso was investigated by Alvarez Sch. (1964). The structures were explained as follows; the metamorphic rocks have at various parts a high degree of schistosity. The direction of dip varies between the north and the north east, the inclination is in general strong towards the east. The metamorphic rocks also contain complicated microstructures which deforms the schisostisy. The Valparaìso area is highly affected by a number of faults. The largest is the Marga Marga fault which runs directly through the neighbouring city of Viña del Mar, in an east – westerly direction. The closest fault to Quebrada Francia which run between Cerro Baron and Cerro de Los Placeres is parallel to the Marga Marga fault but of minor importance. During the fieldwork the weathering classifications between moderately-highly to slightly weathered material where analysed using the RMS classification. In the remaining weathering classes it was too difficult to find structures so a RMS classification could not be done. The nature of the discontinuities was not classified. It is likely that the main part of the discontinuities would be classified as joints. It was also noted in field that there are a great amount of micro fissures in the rock mass due to weathering. This was mainly notified in highly weathered material and highly-completely weathered material. The presence of biotic weathering was also noted. The roots from scrub vegetation penetrate and open up minor joints (figure 23). A failure which may have its origin due to biotic weathering occurred in Quebrada Francia in 2002 when a rock fall killed two persons. The consequence can be seen in figure 24.

5.3.2 Weathering profiles An important aspect of the slope stability is the depth of the weathered material. The depth has impact on how much material that will be brought along in case of a possible failure. It was only possible to study weathering profiles in road cuts. No digging could be made. The weathering studies have resulted in a closer study of a few weathering profiles which can be seen as “standard models” for the area in general. Figure 25 and 26 shows weathering profiles classified as completely weathered seen in road cuts. The bright red area (darker grey if seen in greyscale) is the completely weathered section. The border between completely weathered and highly weathered materials is clear in these cuts.

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It is possible to see that the layer of completely weathered material rarely is thick. The layers seen in field varied between 1 dm an 15 meters, but the general depth was noted to be between 0.4 and 0.6 meters, as shown in the photos. The large-scale profile in figure 25 gives a good indication of how the natural forest vegetation grows on the hill.

Figure 23. Biotic weathering opening up joints in the bedrock. (Photo I. Erikson)

Figure 24. Consequences of a rock fall on Avenida Francia in 2002. Two people lost their lives. (Photo I. Erikson)

Figure 25. Large scale weathering profile in roadcut in the northern part of the studied area. The difference between completely weathered material and highly weathered material can clearly be seen. Cecilia on the photo is 159 cm tall. (Photo: I. Erikson)

Figure 26. Weathering profile in the west part of the area. (Photo: I. Erikson)

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Figure 27 shows an area cut out along the main road which makes the upper (western) border of the study area. This profile is an excellent example of the resistance to weathering of the quartz lines in the rock. The quartz can be seen as the light parts in the profile. At the very top of the profile there is a thin layer of residual soil over the completely weathered part.

Figure 27. Weathering profile in completely weathered material with relict structures. (Photo: I. Erikson)

5.3.3 Characteristics of the analysed weathering classes The results from the grain-size analysis and the in-situ infiltration measurements will be presented in the section. Soil samples and infiltration measurements were made on filling material, completely, highly-completely and highly weathered material. A triangle diagram with the grain-size results is presented in figure 28. A map with the infiltration measurement sites is shown in figure 6. A table with the complete results of grain size distribution, infiltration rate, liquid limit, plastic limit and hydraulic conductivity for each sample is found in appendix 2.

5.3.3.1 Completely weathered The completely weathered samples (sample 4 and 5) show a similar grain size distribution and contained the highest amount of clay and silt particles, 50% respectively 54%. This indicates that completely weathered material acts as a cohesive soil (Selby 1993). The infiltration capacity, shown in figure 29, is measured in five sites. The infiltration rates are similar for all sites and show a low infiltration capacity, all below 1 cm/min. The infiltration capacity is constant throughout the 20 minutes the measurement lasted. This indicates that the response of the material to rain is not changing during at least 20 minutes. The low infiltration capacity indicates that the surface runoff from the completely weathered material is high. High surface runoff creates a favourable environment for surface erosion.

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Figure 28. Triangle diagram showing grain size distribution of filling material, completely, highly-completely and highly weathered material. The hydraulic conductivity of completely weathered rock is very low, with an average of the two samples of 0.01 mm/h. The liquid limits of samples 4 and 5 are 40% and 30%, whereas the plastic limits are 21% and 17%. The plasticity index (PI) and the activity of the samples are presented in table 7. From this result, sample 4 is normal clay, while sample 5 is right between normal and inactive clay. Both of the samples could be classified as medium plasticity (Karlsson 1975).

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Figure 29. Infiltration capacity for completely weathered material.

Table 7. Plasticity index and activity for the completely weathered samples.

Sample Plasticity Index Activity

4 19 0,95 5 13 0,74

Average 16 0,85

5.3.3.2 Highly-completely weathered The soil samples (1A, 1B, 10 and 10B) within this class show a good resemblance. They could all be described as gravely sand material, which means that they act as frictional soils. The silt and clay content is below 10% in all samples except sample 10B, where the content is 16.5%, this measurement was taken in kaolinite.

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Figure 30. Infiltration capacities for highly-completely weathered material

The infiltration capacity for highly-completely weathered rock is shown in figure 30. 1B has three very high results between the 6th and 12th minute of the measurement. This is due to water spilling out from under the tube; hence these results are wrong and due to technical problems during the measurement. These values were excluded in calculations of steady state rate. Measurement 11B shows a comparably high infiltration rate. This measurement was taken in a fissure with kaolinite. The infiltration rates for 1A and 1B are also very high; here the highly-completely weathered material was dug out beneath filling material and showed a very loose character which easily fell apart. The infiltration capacity varies between the different measurements sites, but is generally high compared to filling material and completely weathered material. There are generally higher infiltration capacities during the first two or three minutes. The hydraulic conductivity of the four samples varies between 105 mm/h and 224 mm/h, except sample 10B, which have a hydraulic conductivity of only 12 mm/h. This is due to the higher clay and silt content in the kaolinite material.

5.3.3.3 Highly weathered The particle size distribution differs slightly within the highly weathered samples (samples 2, 3 and 8). They have a very low clay content (<0.7 %) in common, but the sand/gravel ratio is not consistent (50% resp. 46%, 75% resp. 18% and 31% resp. 65%). Since highly weathered material included larger units of rock, with a fragile texture, these units might have broken into smaller fragments during the handling of the samples. If these units originally had a

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different strength, they probably would have de-composed differently. This difficulty to compare granular weathered saprolites is also discussed by Thomas (1994). The infiltration capacity, shown in figure 31, in the highly weathered material shows quite a high variation between the five measurements sites. Samples 2A and 2B show the highest infiltration, whereas the remaining three sites are all under two cm/min.

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Figure 31. Infiltration capacities for highly weathered material.

The hydraulic conductivity of the three highly weathered samples are 296, 697 and 790 mm/h. The areas classified as highly and highly-completely weathered material can be interpreted as granular weathering. The local term maicillo refers to these grades of weathering. As seen in figure 5, the particle size distribution indicates that highly-completely weathered material contains a larger percent of the clay/silt fraction, than does highly weathered. This is consistent with the classification system, where highly weathered is meant to be less weathered than highly-completely weathered. The fact that the clay fractions in both classes are below 15 %, means that they behave as a granular material (Thomas, 1994).

5.3.3.4 Filling material The particle size distributions of the filling material samples (samples 1F, 7, 9 and 12A), generally contain a low share of clay (2-4%). The filling material is not supposed to show a consistent particle size fraction, since this material may origin from any of the weathering classes or from an extern source. Figure 28 indicates that some of the filling material (sample 9 and 12A) origins from highly-completely weathered material, since the grain size

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distribution is very similar to that of highly-completely weathered. These filling material samples are probably rests of old slide material, reused in construction works. Samples 1F and 7 have a larger fraction of sand, and also contained rounded stones, which might mean that they are originally fluvial material. The material could have been transported by locals from the bottom of the ravine, where a small water stream flows, for construction purposes. The infiltration capacities, figure 32, are quite similar for the different sites, except for 12A which shows a very high infiltration capacity. This could be explained by the fact that 12A is the only site where infiltration is measured in a material which is classified as “slope failure material”. 1C has an exceptional high value after 12 minutes; this is due to a technical measurement problem and should not be considered as a value of importance. When calculating steady state rate, this value has been excluded.

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Figure 32. Infiltration capacities for filling material

The hydraulic conductivity of the four filling material samples is 22, 4, 14 and 65 mm/h. The highest value belongs to sample 12A, which also had the lowest clay and silt content.

5.4 MAPS The maps are more detailed further down the ravine, due to smaller scale original maps in this area. The limits between more and less detailed maps are clearly visible on the three base maps; polygones are larger in the upper part of the ravine and smaller in the lower part. The accuracy of the field observations are the same for both parts, even though the geographic accuracy may be better in the lower part.

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The more detailed part coincides with the urbanised part of the area, while the upper part of the ravine mainly is non urbanised forest- and scrub-land. Therefore it is not as crucial to have a more detailed picture of the upper part.

5.4.1 Weathering map The weathering map can be seen in figure 34. No fresh rock was detected. Filling material was present in varying quantities and thickness close to nearly every construction.

Table 8 shows that the studied area mainly consists of rock that is more weathered than the class regarded as highly weathered. Less weathered material than highly together represents 3.3 %, while completely, highly-completely and highly weathered material represents 61.5 %, 23.9 % and 5.4 %, note that the class regarded as most dangerous concerning landslide hazard, is found in nearly a fourth of the area. The weathering map only expresses in which state of weathering the material is. It does not consider the joints or other types of discontinuities which also affects the stability of the material. If more weathered material is considered more unstable a risk analyse of this map would be as follows. The western part (towards the city centre) of the studied area has a low degree of weathering, making it less vulnerable to hazard of landslides. On the other hand, the low degree of weathering makes it prone to rock falls, depending on the nature of the structures in the area and the steepness of the slope. There is a fair amount of highly weathered material at the western side of the study area. This material is prone to landslides and debris flows but can take larger amounts of rain due to high permeability and its quite high infiltration capacity. The main part of the roads marked on the map runs through areas of highly and highly-completely weathered material. The retaining walls along these roads are extra sensitive since they are in material with a high infiltration capacity. There is a great risk that they act as water stoppers, creating high pore water pressure in the material behind the retaining wall. There is a fair amount of highly-completely weathered material which coincides with urban parts of the study area, these parts are considered most prone to failures. Small areas of less weathered material are marked on the western part of the study area, where the original map was more detailed. Less weathered areas still exist in the eastern part of the map, but since it was less detailed they were considered to small to be mapped accurately. Almost all the material in the eastern, less urbanised part, of the study area is highly-completely or completely weathered. This weathering grade of the material creates a lot of debris and mudflows during rainstorms which generate a lot of sediment in the alluvial areas. Table 8. Distribution of the weathering grades.

Grade of weathering Area km2 Percentage of study area %

Completely 2.03 61.5 Highly-completely 0.79 23.9 Highly 0.18 5.4 Moderately-highly 0.061 1.8 Moderately 0.019 0.6 Slightly-moderately 0.015 0.4 Slightly 0.018 0.5 Fluvial 0.18 km2 5.5

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A couple of meters on each side of a water stream were not classified as residual rock, but fluvial material. This has therefore been classified outside the weathering classification and is not given any risk factor value. The reason of presenting these areas in the weathering map is that even though the fluvial material does not fit in either of the weathering grades, it still represents 5.5 % of the study area. The fluvial material is further discussed in section 6.3.1.

5.4.2 Urbanisation map The urbanisation map is seen in figure 35. The upper part of the ravine, seen in figure 33, consists exclusively of vegetation; scrub in the southern two thirds, to the left in figure 7, and eucalyptus forest in the northern third. Together the two vegetation classes make 78.1 % of the study area. The limits between scrub and forest are subject to constant change with cutting, replanting and natural change. Therefore the limits on the map should not be considered as an absolute truth. The hazard of mass movement is not considered to differ very much between these two types of vegetation, why the limits between scrub and forest are not of major importance. In the more central part of the study area, some scrub areas can be found, but no forest at all. These central scrub areas are found on slopes too steep to be built on.

Figure 33. The quebrada seen from the upper non-urbanised part. The limit between scrub and forest is visible. (Photo: I. Erikson)

Areas with low quality of housing are considered the most vulnerable. They are generally present at the flanks and the bottom of the ravines in steep areas. A large part of the low quality of housing is situated in the western parts (toward the city centre) in the study area. The quality of housing is well mixed. Parts with middle quality of housing are sometimes found below areas with low class of housing. These houses are then situated in a vulnerable location, since all mass movements travel down slope. For the most parts areas with middle quality of housing are situated above areas with less quality of housing, which creates safety both for the upper and lower parts of the slope. The most dangerous parts on this map are found in the western part of the study area, since this is where the people live. The upper part of the hills where the housing has a high quality is where the secure areas are found. 21.7 % of the area is considered to be urbanised, with nearly half of that area classified as lowest class of housing. Only 3.5 % of the study area is classified as city housing.

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It also has to be said that most of the areas classified as the lowest quality of housing has a rather small population. Areas that have been populated earlier, or modified for road constructions or similar, have also been classified as lowest class of housing. This is because the contributing risk for mass movements was considered similar for these areas. Generally in the studied area, constructions of lower quality (both houses and roads) were found in more sparsely urbanised areas, commonly very steep slopes. More densely built-on areas generally contained well-built roads and houses. Therefore these two factors were combined into one, the urbanisation grade. The urbanised areas have generally used filling material to compensate level differences and to create flat areas to build on. Filling material has not been classified in either the weathering or the urbanisation maps, due to the difficulty of detecting areas consisting of filling material as they generally are built upon. All the urbanised areas have used filling material, but areas classified as low or middle quality of housing are where the filling material is most easily affected by the elements.

Table 9. Distribution of the urbanisation/vegetation classification.

Type of urbanisation/vegetation Area km2 Percentage of study area %

City housing 0.11 3.2

Middle class of housing 0.26 7.9

Lowest class of housing 0.35 10.6

Scrub 1.76 53.3

Forest 0.82 24.8

5.4.3 Slope steepness The inclination map is seen in figure 36. The steepness of the slopes can be seen in figure 7, is naturally low on the heights and in the bottoms of the ravines. The flanks are mainly has a steepness between 20º-50º, with areas of even steeper inclination. Table 10 shows that only 2.9 % of the area is steeper than 50º, but 52.3 % of the area has an inclination stronger than 20º. Figure 34 (on opposite page). The weathering map. Figure 35 (to the left on next spread). The urbanisation map. Figure 36 (to the right on next spread). The inclination map.

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There is a considerable amount of areas with an inclination over 50º in the study area. A lot is concentrated in the narrow western part. These areas can be considered very vulnerable to mass movements since the strong inclination makes the areas very easily affected by gravity. They are generally not occupied since it is almost impossible to construct houses in such steep areas. Areas with steepness less than 20º are found in the eastern part of the study area. In the western part they are often found in combination with the infrastructure visible on the maps. Whilst compared to the urbanisation map it can be seen that the main part of the good quality of housing is situated in the areas which are less steep than 20º. More than half of the area is steeper than 20º, knowing this it is possible to understand the great difficulties of constructing safe living areas for the inhabitants of Valparaíso. The main parts of the urbanised areas are situated in areas steeper than 20º, especially areas with low or middle class of housing. Table 10. Distribution of slope steepness.

Inclination Area km2 Percentage of study area %

<20º 1.57 47.6

20º-50º 1.63 49.4

>50º 0.095 2.9

5.4.4 Weighting system Three maps containing different information about the risk situation in Quebrada Francia are used. These maps are: weathering classification, urbanisation classification and inclination. The different maps used for the weighting system do not have an equal risk factor. A grading between the three weighting classes has therefore been made by the authors. To recognize this grading each weighting class has been given a unique rating span, in table 11 presented as used span, where 1 is the highest possible risk factor and 74 is the lowest risk factor. 74 have been chosen as the lowest risk factor as it coincides with the strongest possible rating in Selby’s RMS classification. It was therefore natural to have 74 as the strongest possible value for the remaining classes. In the actual table, the span 1-10 is not used, as no area with such little stability was considered found. Neither is the span 70-74 used, as no area with that amount of safety was considered found. Based on this reasoning, 63 is the most unsafe value in the risk factor grading, whereas 200 is the safest value that can be given when the three maps are combined. Table 11. Grading between the weighting classes and the used span.

Weighting classes Risk factor Weighting value Used span

Urbanisation Highest 1 - 74 10-70; 60 units

Slope steepness Modest 1 - 74 20-65; 45 units

Weathering grade Lowest 19 - 74 RMS used span 3-65; 62 units EH used span 33-65; 32 units

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5.4.4.1 Weighting of the weathering classes For the weathering classes two sets of grading systems were used. One of them is the original RMS classification (section 4.4.1, table 3). The other one is based on the RMS classification but adjusted by the authors in the Erikson Högstedt (EH) grading system to suit the specific conditions in Quebrada Francia, where the degree of alteration of the rock shows a large variation over a small area. The EH grading system only differs from the RMS classification for the three most weathered classes. The grading system is adjusted to the fact that the Valparaìso area has characteristics of rocks, weathered material and clayey soil. The difference between the two systems is that the EH system gives a much lower (more unsafe) value to highly and highly–completely weathered areas, and a much higher value (safer) to the completely weathered areas. Since no signs of relict joint structures could be seen in the completely weathered material during the field studies, this value had to be adjusted to the fact that the material classified by the authors as completely weathered had the properties of a clayey soil. In the material classified by Selby as completely, signs of joint structures are still visible.

Table 12. Weighting values of the weathering classes

Weathering Classes EH weighting value RMS weighting value

Slightly 9 + joint rating 9 + joint rating

Slightly-moderately 8 + joint rating 8 + joint rating

Moderately 7 + joint rating 7 + joint rating

Moderately-highly 6 + joint rating 6 + joint rating

Highly 40 5 + av. joint rating (41) = 46

Highly-completely 37 4 + av. joint rating (41) = 45

Completely 42 3 (there are no joints) Highly–completely weathered Highly-completely is considered to be the most dangerous material, because it is easy to divide into minor parts. It is also easily disturbed by construction work or similar. Due to the high infiltration rate and lower hydraulic conductivity there is a great risk for high pore water pressure. Highly weathered material Highly weathered is classified as the second most dangerous material. The infiltration rate varies between measured sites but is generally high. Combined with the high hydraulic conductivity it can be assumed that high pore water pressure only occurs at extreme precipitation. It was noted in field that the material is relatively firm and therefore the most durable of the three most weathered classes. Completely weathered Completely weathered is classified as the third most dangerous material because it is a clayey soil. It generally occurs in a rather thin layer which easily causes small mudflows. They can trigger larger movements and causes sedimentation. Under completely there is always a layer of highly-completely, which can be dangerous during construction. The infiltration rate and hydraulic conductivity is low, which indicates that danger due to high pore water pressure

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rarely occurs. A low rise of the water content makes the material act as a plastic mass, because of its large clay content. For the less weathered classes (highly-moderately, moderately, moderately and slightly weathered material) the most influencing factor on the landslide risk is the presence and characteristics of joints. The risk due to joints could only be seen in these four classes. The values for each location in the map (figure 37) can be seen in table 13. The joint values for each site have been added to the risk due to weathering in order to make the risk prediction for each class as accurate as possible. For highly and highly-completely weathered material no joint values could be seen in field. This is not equivalent with it not existing any weaker parts in the material due to discontinuities. Since it is an altered mass there will be weaker parts because of inherited structures from the origin rock. To avoid this problem a mean value of the sites where joint investigations were fully conducted was calculated. In the RMS classification this mean value has then been added to the rating for the highly and highly-completely weathered material. The completely weathered material was considered not to contain any relict structures since it can be classified as a laterite (oral communication, Mats Olvmo). Therefore there have been no joint risk values added to this class. It should be considered that the discontinuities at times were difficult to see due to vegetation and the very steep conditions of the area.

Figure 37. Map of the sites where a complete joint investigation were made. See also table 13.

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Table 13. Risk values for locations where joint investigations were conducted. Site numbers refers to figure 37.

Site number

Rating, orientation

Rating, spacing

Rating, continuity,

infill

Rating, width

Joint risk

rating

Rating Weathering

Summarized risk rating

1 14 21 1 4 40 8? 48

2 5 21 4 5 35 7 42

3 8 21 2 2 33 7 40

4 5 15 4 2 26 7 33

5 14 21 4 4 43 9 52

6 5 24 6 4 39 8? 47

7 14 21 6 4 45 8 53

8 9 21 4 4 38 9? 45

9 18 15 6 5 44 6? 50

10 18 15 4 4 41 8? 49

12 14 28 6 5 53 7 60

14 14 28 6 6 54 7 61

18 18 28 6 5 57 8 65

20 ?(14) 21 6 4 (31) 8 (39)

22 9 15 5 4 33 7 40

41 5 21 4 4 34 8 42

44 14 21 5 5 45 8 53

46 14 15 6 6 41 7 48

47 ?(14) 21 6 6 (33) 7 (40)

55 ?(14) ?(21) 1 2 (3) 8 (11)

Average joint rating 41,2 49,1

5.4.4.2 Weighting of the urbanisation classes During field work it was noted that the urbanised areas seemed to be most affected by hill slope related problems. Therefore the urbanisation has been given the largest span, and is allowed to affect the final multi-factor map more than the other maps. The weighting within the urbanisation class is made based on the span of 10-70. The safest class of the urbanisation classifications should be city housing, since the infiltration capacity is extremely small in this class. The more or less appropriate safety measure taken in this area is also contributes to safety. This however does not mean that landsliding don’t occur in this area, only that it is less probable. This class was given a weighting value of 70. The most unsafe class (figure 38) would be the lowest class of housing; since the slopes here have been modified in a way which increases infiltration (filling material) and increases shear stress by not take the appropriate safety measures. Small debris flows and slides would be very expected in these areas. A weighting value of 10 was given this class.

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Table 14. Weighting vales of the urbanisation classification

Urbanisation class Weighting value

City housing 70

Scrub 50

Forest 45

Middle class of housing 25

Lowest class of housing 10

The urbanised areas have generally used filling material to compensate for level differences and to create flat areas to build on. The filling material has not been classified in any of the maps, due to the impossibility of detecting areas consisting of filling material as they generally are built upon. All the urbanised areas have used filling material, but in areas classified as low or middle quality of housing, the filling material is most easily affected by the elements. The urbanisation map is given the largest rating units in this hazard map. It should though be considered that this classification is subjective and that each class can contain occasional buildings that fit in another class.

Figure 38. Lowest class of housing, with few or no safety measures. (Photo: I. Erikson)

Figure 39. Middle class of housing, with few safety measures. (Photo: I. Erikson)

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5.4.4.3 Weighting of slope steepness The slope steepness classes were decided as the second most important factor to landslide hazard in Quebrada Francia. Slopes that were less steep than 20º were considered safe, and therefore given a high value (65). Since the weighting value span of the urbanization classes were 10-70, the weighting of the steepest inclination class had to be higher than 10. Consequently, slopes between 20º and 50º were given a weighting value of 30, while slopes steeper than 50º were given a value of 20. The areas between 20º-50º are given a relatively low grading number; this is because of the large span which includes areas which are very steep and the immediate risk that steep slopes naturally present. Parameters not considered in the weighting of the slope steepness are the height and length of the slopes.

Table 15. Weighting values of the slope steepness classes

Slope steepness Weighting values

0º - 20º 65

20º -50º 30

Over 50º 20

5.4.5 Combined factor maps Two multiple hazard maps were made – one displaying the result using the EH weighting system (figure 40) and the other one displaying the result of the RMS weighting system (figure 40). The multiple hazard maps display four categories of areas; they can be seen in table 16. Table 16. Categories in the multiple factor maps

Category Colour on multiple

hazard map Weighting Authors recommendations

1. Immediate risk of mass movements Red <82

Urgent need of improved safety measures, such as proper retaining walls or improved

house construction technique.

2. Risk of mass movements Yellow 83-97 Need of improved safety measures, see above.

3. A small risk of mass movements Green 98-160

Need of improved safety measures, but should not be prioritized before the above

two categories. 4. Not at immediate risk of

mass movements Blue >161 No immediate need of improved safety measures.

1. Immediate risk of mass movements This category is characterised by a grade of weathering with a value less than 42 in the EH weighting system. It includes highly, highly-completely and completely weathered, and less weathered rock with an unfavourable joint structure. The inclination is more than 20º, and the lowest class of housing is required for an area to be placed in this category. It is within this category the authorities are recommended to prioritize improved safety measures.

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2. Risk of mass movements The same weathering and inclination categories as in the above class are needed. The difference is that middle class of housing instead of lowest class of housing characterize the second category. This category is also experiencing a risk of mass movements and in the need of improved safety measures. 3. A small risk of mass movements An urbanisation character of lowest or middle class of housing, combined with an inclination of more than 20º, with a weathering value between 42 and 50 is considered to have a small risk of mass movements. 4. Not at immediate risk of mass movements A weathering value of more than 50 combined with an inclination of less than 20, as well as city housing, forest or scrub vegetation is required.

5.4.5.1 Comparison of the EH weighting map and the RMS weighting map Using the RMS system on the weathering map gives a range in the combined map of 33-186, where 186 is the safest value. The EH system on the weathering map gives a range of 65-186 in the combined map. This difference, where the RMS system in fact gives a wider range, is mainly due the different judgement of completely weathered rock. The RMS system gives completely weathered rock a safety value of three (which is very unsafe), while the EH classification gives it a value of 42. Concerning highly and highly-completely weathered rock, the RMS system gives them values of 46 resp. 45, while the EH system considered it less safe and gave values of 40 resp 37. The greatest difference is seen in the upper part of the ravine, where the EH multiple hazard map classify it as the third category of risk (green on the map), while the RMS multiple hazard map show areas of first, second and third category of risk (red, yellow and green). There is also a difference between the two maps in general where the EH map identifies consequent immediate risk of mass movements, the RMS map tend to classify big parts of the same area as risk of mass movements.

5.4.5.2 Areas prone to mass movements The two maps have in common that they identify more or less the same areas as more prone to mass movements, as well as areas with no immediate risk of mass movements. The areas identified as having a risk of mass movements are well synchronised with the areas classified as low quality of housing on the urbanisation map. They can be found along Avenida Alemana, especially on the flanks towards Avenida Francia, as well as from the continuing of the ravine from the top of Avenida Francia along the southern side of the northern ravine. Where the ravine is divided into three in the upper part some immediate risk areas can also be found; one in the lower southern ravine along the bottom, and one on the northern side of the central ravine. These two areas also correspond to highly-completely weathered rock on the weathering map. Areas with no immediate risk of mass movement corresponds to an extent to the areas classified as city housing on the urbanisation map, found along the lower part of Avenida Francia, and along the lower part of Calle Vergel

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60

Figure 40. The EH landslide hazard map.

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Figure 41. The RMS landslide hazard map.

6 DISCUSSION

6.1 THE SENSITIVE AREA OF VALPARAÍSO The high amount of slope failures in the Valparaíso area can not be related to only one reason. Then why do so many failures occur? To answer this one must look at the whole area and not only at statistics. Chowdhurry et al. (2002) suggests that in urban areas, it is necessary to consider: (a) the long term and delayed effects of previous deforestation and urbanisation (b) continuing and new urbanization/ deforestation processes, and (c) external disturbances such as individual excavations, significant fills and changes in

surface and subsurface drainage conditions. According to the information gained whilst studying the articles in El Mercurio and whilst talking to experts at SERNAGEOMIN (Hauser, oral communication), mass movements only occur during heavy rainstorms.

6.2 RELATIONSHIP LANDSLIDES - PRECIPITATION

6.2.1 Fatalities It is a hard task to make people’s deaths into numbers. But it is vital to show that slope failures are a virtual life-threatening problem for the people in Valparaíso. This is proved by the fact that more than 60 people have been reported dead in accidents related to mass movements, during the past 60 years. If looked upon more closely the numbers presents us with the fact that man made structures are the largest killers per event. Weak retaining walls which burst are a major problem in Valparaíso. It causes deaths as well as large bills for the municipality.

6.2.2 Amount of rainfall According to experts (Hauser, oral communication) as well as common knowledge (articles read in El Mercurio), the main trigger to slope failures in Valparaíso is rain. A study of rainfall triggered landslides in Italy by Pasuto & Silvano (1998) states that precipitation plays a fundamental role in the onset, as well as spatial and temporal evolution of mass movements. One of the main goals in several research projects has been the identification of critical threshold values of precipitation, to facilitate the avoidance of hazard. Since geological and climatologic parameters differ from one place to another, each specific site needs to be investigated by its own. This is confirmed by the result of this study. It is evident that all slope failure events can not be related to the amount of precipitation. This could be valuable for decision makers whilst organising safety measures, emptying of dams and planning for future development of the city.

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In a study made in a cut slope in unsaturated residual soils in Hong Kong by Ng & Q (1998), the duration of rainfall needed for the cut slope to reach a critical state was found to be between 3 and 7 days. The duration of the rainfall was considered as a very important factor to slope stability because of its effect on pore water pressure. As mentioned earlier, high pore water pressure has a negative effect of the stability of the slope. No such exact study of the amount of precipitation days and their effect on slope stability could be done for the Valparaíso area, since everyday precipitation data was not available. This study shows that the main part of the rainfall-induced failures occurs the same month as the heavy precipitation (figure 17). From information gained in El Mercurio, the main part of slope failures occurs after a few days of heavy rainstorms. It can be estimated that the 3-7 days duration found in Hong Kong can be regarded as an approximate value for Valparaíso as well. It was noted whilst reading the article material, that several rainstorms that occurred in May created headlines about the lack of slope failures and other incidents such as inundation. Although this is rather vague and not scientific information, it can be seen as an indicator that a certain amount of rainfall is needed for mass movements to occur.

6.2.3 Rainfall intensity It is clear that there are other parameters acting upon slope stability than just the amount of rainfall. Another aspect of rainfall is its intensity, which is shown in figure 18. In the Hong Kong study by Ng & Q (1998) it was found that the factor of safety of the slope decreases as rainfall intensity increases for a given set of soil permeability, slope geometry and initial ground water conditions. According to Hauser (2000), the great majority of flows in regolithic granite in central Chile are responses to rainfall with high intensity. Initially they are landslides (deslizamientos multirotacionales) but after spontaneous liquefaction flows develop. They are related to an increase of the pore water pressure, which decreases the resistance of the slope and increases the load in the granular soil. The smallest rainfall intensity needed for slope failure to occur was 14.5 mm/24 hours. This result correlates with the results in a study made at Universidad de Playa Ancha, found in El Mercurio 15/4 2003, in which geographer Carolina Martinéz has found a precipitation threshold value for 15 mm/24 hours in the most sensitive areas of Valparaíso. Unlike the study by Ng & Q (1998), there are, according to figure 18, generally not more mass movements the more intense the rainfall is. This result can be due to the source of information. The intensity correlation is made based on events and not mass movement days. El Mercurio can not be seen as an accurate source for the amount of mass movements, which actually have taken place. It should be considered as an almost impossible task for the newspaper to get information about all the events that have occurred. Hauser (2002) has established that rainfall intensity in the preandian zone of Region Metropolitana (east of Santiago) of more than 60 mm/24hours is producing debris flows. During discussions with Hauser and Rauld it was established that debris flows and mudflows are the most common types of mass movement in the Valparaíso area. According to data presented in this thesis rainfall intensities over 50 mm/24 hours will create mass movements. The large amount of events occurring between the rainfall intensities of 35 mm/24 hours and 70 mm/24 hours shows that the slopes easily reach instability at rainfall intensity from 30-35 mm/24hours.

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The most probable reason for the significantly lower threshold values found for Valparaíso is that it is an urban area. The negative effect of urbanisation on slope stability is well documented and it can therefore be considered probable that the lower threshold value found in Valparaíso is accurate. An exact threshold value of the intensity and amount of rain needed for slope failures to occur is difficult to find. Therefore: A dangerous scenario for the inhabitants in Valparaíso is a rainstorm continuing for several days with a precipitation intensity of around 35 mm/24 hours or more, preceded by at least 100 mm of rain during the past two months. This would most certainly cause several mass movements for more than one day. It is obvious that the closer the preceding rainfall is to the high intensity rainfall, the more dangerous is the occasion. According to knowledge gained from articles in El Mercurio the area is most vulnerable to mass movements after several following days with intense precipitation. It should be noted that mass movements will still occur at rainfall values and intensities lower than this.

6.2.4 Urbanisation In 1970, 75% of the population in the 5th region (Valparaíso) lived in urban areas. In 2002 the urban population had risen to 86.2 %. The past 20 years the population of the region has increased with 36% (Instituto Nacional de Estadisticas, 2004). These numbers gives an indication of the strain due to increased population the environment has gone through. The human role in creating landforms and modifying the operation of geomorphological processes such as weathering, erosion and deposition is a theme of great importance. (Goudie 2000) Goudie (2000) further states that the range of the human impact on both geomorphological forms and processes is considerable. Indirect anthropogenic processes often produce mass movements. They are therefore often less easy to recognise, not least because they tend to involve, not the operation of new processes, but the acceleration of natural processes. The situation in Valparaíso is similarly complicated by the fact that it is a highly urbanised area. Little of the construction taking place in the hilly areas is supervised by construction engineers (Muzio, oral communication). Goudie (2000) focuses on the problems with the cut-and-fill method which is also commonly used in Valparaíso for constructing roads and housing. Hong Kong, Kuala Lumpur and Rio de Janeiro are cities in the humid parts of the low latitudes which are subject of increasing pressure of the land, with frequent slope failures as a consequence (Goudie 2000, Thomas 1994). Valparaìso differs from these cities because of its dry climate, but the fact that virtually all the rain comes within a period of 3-4 months explains the city’s vulnerability. Two possible explanations for the differences between the two time scales (figures 17 and 18) are discussed. One is the Internet reliability explained in section 4.2.2. A second reason for the differences in determination coefficient between the two time series may be the effect that man has had on the slopes. Between 1992 and 2001 the population of Valparaíso increased with 7.7 % (Instituto Nacional de Estadisticas, 2004). The growth of the population will

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definitely have affected the area, more houses will have been built and more steep areas will have been built upon. The past 14 years Chile has experienced economic growth. During the nineties the annual growth was 7 % and during the first years of the new millennium it varied between 3-5 %. (Swedish Institute of National Affairs).The economic growth has lead to development, where more people can afford cars travelling up the roads (Muzio, oral communication), causing vibrations that may trigger mass movements. It is also possible that the development has led to heavier houses. The large annual growth during the nineties may have contributed to more safety measures and better care of the existing ones. This may explain the lower number of mass movements and mortality rate during the same period.

6.2.5 Earthquakes and mass movements It is a well known fact that vibrations from tremors and earthquakes acts as triggers for mass movements (Selby 1993). According to the data presented in this paper, there is clearly a greater vulnerability when seismic activity has been present the past two months in Valparaíso. There is a tiny increase in the data for two latitudes. It is recommended to use seismic data for two latitudes if a similar study is done. This increase is interesting, how far does the increase reach? Results presented on the web from an ongoing research by Rodriguez (Imperial College of London, internet reference) which modifies previous results by Keefer shows the area affected by landslides in relation to seismic activity. In relation to our study it can be said that larger earthquakes produces landslides within a larger area. A tremor of a magnitude of 4 within two latitudes of Valparaíso is sufficient to increase the amount of landslides in Valparaíso. This is slightly more than what is presented in figure 42 and can be related to the urban character of the area.

Figure 42. Area affected by landslides as a function of magnitude MS. The solid line is the upper bound as determined by Keefer (1984), and dashed line is the proposed modified bound. The depths of the epicentre of the earthquakes have not been considered in this correlation. It is though worth mentioning that a shallow tremor with the same magnitude as a profound one generally causes much more damage Whilst looking at the magnitude where the reason of the failure according to El Mercurio is tremors, a magnitude of 5 seems to cause dangerous fissures in retaining walls or cracks in the

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ground. But the data used for this conclusion contains only five events of which three occurred in 1985. Since many tremors over five have occurred without any following failures it is probable that the depth of the epicentre of the tremor is relevant. It can however be said that a shallow earthquake with a magnitude over five, occurring within two latitudes of Valparaíso is likely to cause cracks or fissures in the ground or in construction works which can facilitate slope failures. It is proved in figure 22 that rainfall and not earthquakes acts as a trigger for landslides. This is an interesting result compared to failures occurring in south western Sweden, where rain builds up high porewater pressures, and traffic causes vibrations which acts as triggers for the movements. In Valparaíso it works the other way around, tremors and other vibrations weakens the ground and rainfall acts as a trigger.

6.2.6 Relation between El Niño and landslides Monteceinos et al. (2003) concludes that El Niño associates with above average rainfall in central Chile (30° S -35° S) during austral winter (June, July and August), while La Niña produces below average precipitation. Regional and large-scale circulation features during extreme rainfall conditions share significant similarities with those during extreme ENSO phases. The El Niño episodes contribute to northward migration of the storm track in the SE Pacific and causes wetter than average conditions in central Chile (30° S -35° S). The influences of ENSO in austral summer are though relatively weak and cause no major differences in the climate around Valparaíso. (Information about El Niños effects on Chile can also be found on the homepage of Instituto Nacional de Estadisticas en Chile www.ine.cl/17ambient/V-5pdf). The possibility to plan for a year with expected high rainfall values exists in Valparaíso. Since years with extreme ENSO phases are known to cause extreme rainfall, extra precautions related to slope failures can be taken during these years. In practicality this does not need to be very difficult. Strong El Niño events reaches Ecuador and Peru every two to seven years, and is often accompanied by beneficial rainfall in the arid coastal regions of these two countries (NOAA). Since the same event reaches Valparaíso during the austral winter, there is a time span between the events in which preparations can be made. It can be assumed that heavy El Niño events and associated hazards in Peru will be reported in the news in Chile. Decision makers will therefore gain knowledge about the event through media. If they (this is the tricky part) are aware of the relationship between El Niño events in Peru and destruction due to slope failures in Chile, extra money can be given to preparations before expected high precipitation. Planning like this, using known facts and common sense can save lives and money.

6.3 LANDSLIDE HAZARD MAP

6.3.1 The multi-factor hazard map Both the EH multi-factor map and the RMS multi-factor map are functional in identifying risk areas. The EH map has the advantage of being more distinct and less scattered than the RMS map. They more or less indicate that the same areas are more prone to landslides, but put the areas at different risk intensity.

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http://www.ine.cl/17ambient/V-5pdf

The landslide hazard maps presented in this thesis are based on a system invented by the authors. The system was chosen because it was considered the best way to deal with the existing data, the knowledge of the authors and the possibilities in field. There are however several different ways to create hazard maps. Several authors emphasises the need for developing feasible methodologies of landslide hazard assessment and mitigation. Pallas et al. (2003) and Dai et al. (2002) emphasise the importance of new methodologies to better understand and present landslide hazards. It is widely spread to use mass movement scars as an aid of creating hazard maps (Pallas et al. 2003, Dai et al. 2002 and Pasuto et al. 1998). They can be found either by analysing aerial photographs or by observations in field. It is however a different situation in a highly urbanised area. Slope failures are often “cleaned” quickly from the area in order to secure it or to use the ground (or the landslide material) for other purposes. Aerial photographs at regular and frequent time interval would have been needed. Because of this it was considered difficult to use aerial photographs as a method in Quebrada Francia. It was instead decided that the hazard map of Quebrada Francia would be based on data about the actual physical properties in the study area. Dai et al. (2002) brings up another important aspect of slope failures which is important in a hazard evaluation of an area. It is to establish accurate predictions of the run-out behaviour of a landslide, such as how fast and how far a landslide travels once mobilised. The slope failures in the study area are usually small events and do not reach very far. They nonetheless cause a lot of damage. The gullies are thin and only contain enough space for a stream, at rare occasions there are people living close to the streams. The main problem in the gullies is the large amount of sediment and garbage which accumulate in the streams and the sabo dams, which creates inundations during events with a lot of precipitation. It would be interesting to evaluate the accuracy of the produced hazard map by mapping occurred slope failures on top of it. By doing this for a while a new and better hazard map can be produced, it would then show where the actual slope failures occur.

6.3.2 Characteristics of the immediate risk of mass movements areas It is very important to characterize the areas that have been identified to be at immediate risk of mass movements. The characterization is a future help to identify risk areas in the close surroundings of the study area that has not been geologically examined. The areas considered at immediate risk of mass movements in this study can be identified by the following parameters: the quality of the building styles are low and the inclination is above 20º. Since this is applicable on large areas throughout the Valparaìso area, it is also important to look at the geological characteristics. This study considers highly and highly-completely weathered material to be most prone to landslides, due to its character. In the multi-factor landslide hazard maps, it is within these weathering classes the risk areas are found. Houses just below areas identified as immediate-risk areas should also be considered at risk, since sliding material can hit the houses. This is exemplified in figure 44, where a tree situated in an immediate risk area has fallen, and creates a threat to the houses beneath. When

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asking the municipality if this tree would be removed by them the answer was negative. Threats like this are meant to be removed by the people living in the immediate risk area.

6.3.3 The weathering map The fluvial material and areas have specific problems due to its location in the bottom of the ravines. During the dry season, very little or no water flows in the water streams. On the other hand, flooding and undercutting causes damage to the people living there during the rainy season. Not surprisingly, these fluvial areas are also where the lowest quality of the buildings is found. The households in the bottom of the ravines, especially in the less urbanised, eastern part, therefore have to deal with 1) the risk of landslides, mudflows and other mass movement related events and 2) flooding. These risks are connected, since the flooding causes undercutting and in the prolongation possibly mass movements.

6.3.3.1 Discontinuities as a weakening factor Joints carrying groundwater are particularly prone to weathering. This may cause instabilities such as rock falls on surfaces with a lower degree of weathering. Weathered joints generally have an increased infiltration capacity; if the outflow of groundwater is slow high pore pressure may occur in the rock mass. This may contribute to an eventual failure during heavy rains. If the groundwater pattern and the factors contributing to changes in groundwater pressure are understood, modelling techniques may be used to predict the effect of changes such as increased infiltration or excavation. Hencher (1987) recommends the use of piezometers in order to measure and understand the groundwater patterns in the rock. Further studies about the joint structure and the groundwater patterns in the rock mass in Valparaíso could give more precise indications to where to find dangerous parts of the rock mass

6.3.4 The urbanisation map The common cut-and-fill method of preparing for house constructions need safety measures to produce a stable result. If no safety measures are taken, the filling material or the cut wall is prone to collapse during the rain period. This is of general knowledge among the inhabitants, also in the more inadequately built areas. “Home-made” safety measures are therefore common in the lowest class of housing, such as protection walls made from tires or sand bags, or even tarpaulins are used to prevent a slope to erode or failure. One of the most obvious risk factors for slope failure discovered during field observations is the construction of home-made retaining walls without drainage holes. The infiltrated water flows through the soil until it reaches a wall and stops. The water builds up a pressure and may cause the wall to collapse. Walls like this occur in all urbanised areas, although they are especially common in the lowest and middle class of housing. In city housing, most of the walls have drainage holes, following the regulations of the municipality. The municipality gives directions on the dimensions (thickness, height) of the retaining walls needed. Gino Muzio on the Departemento de Obras, however often experienced that these directions were neglected or that people were not aware of them, also in well-built areas. The types of walls made from tires, sand bags or other waste material generally do not have a satisfying drainage system. Water is seeping out in the narrow openings between the tires or

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bags, but on the other hand the resisting strength of the wall is not sufficient for even small amounts of pressure. A tarpaulin covering a slope, or a recent slide scar, is only a short term protection, in need of improvement. An example of this is seen in figure 43.

Figure 44. Fallen tree creating a risk for the houses beneath. (Photo: J. Högstedt)

Figure 43. Tarpaulins protecting a slope of further failure. (Photo: J. Högstedt)

It must be noted that the municipality does not support private persons with means for safety measures. The use of primitive safety measures such as tarpaulins is therefore understandable – you use the available resources to protect your house and road. Concerning public roads and public houses such as schools (if they are not private), the municipality however try to establish appropriate safety measures. The maintenance of these walls is not always high priority, especially not when it is needed the most, after long-duration intense rainfalls. At these moments the resources are directed towards the people directly affected by the consequences; homeless families, injured persons or destroyed roads. (Muzio, oral communication) Based on this discussion, it is obvious that the lowest class of housing is the most unsafe concerning stability, followed by the middle class of housing. The class that we called city housing is generally safe regarding stability, due to paved streets, well-built houses and appropriate safety measures.

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6.3.4.1 Vegetation in Quebrada Francia Mass movements did not seem to be a problem in these areas according to Muzio, and no visual evidence of slope movement activity was detected during fieldwork. However, this does not exclude that mass movements occur in the forested or vegetated areas, only that it does not affect humans. It is of interest to evaluate and estimate the influence of these types of vegetation on slope stability in the area, especially since the forested area is planned to be a new residential area, and roads are already being built for this purpose. A Chilean study evaluating the effects of forests made at Universidad Austral de Chile (see internet references), states that landslides in forests were responsible for the largest part of eroded sediment in the forests. Landsliding in the forests also showed to be closely connected to forest roads and the usage of heavy forest machines (together 70 % of the investigated landslides). With this study in mind, there is consequently reason to be careful at this moment, when roads have been constructed in the forested area, but not yet secured with retaining walls. The cut and fill method, with which the road were constructed, also induces a certain risk.

6.4 CAUSES OF MASS MOVEMENTS Here follows five causes discovered during the fieldwork, which all contributes largely to the instability of the slopes in Valparaíso:

1. The inclination of the slopes where the housing is constructed is sometimes very steep, and for the most part quite steep. This can be seen in the inclination map presented in figure 36.

2. The area is situated near a subduction zone; hence seismic activity shaking the ground

is rather common.

3. The area is situated on unstable material. It is well weathered and contain joints and other discontinuities with unfavourable characteristics

4. Nearly all precipitation falls in heavy rainstorms during the austral winter, creating

favourable conditions for slope failure.

5. Valparaíso is a growing city; therefore anthropogenic activity is severely affecting the area. Urban activities can be divided into several factors causing failure:

• Constructions, mainly weakening through cut and fill, but also by increasing

the load on the slopes by houses and roads. The main part of the construction taking place in Valparaíso is “uncontrolled home constructions” (Muzio, oral communication).

• Urban daily life, which causes vibrations of the ground due to cars and trucks travelling the roads, and trains travelling the railroad between Valparaíso and Viña del Mar.

An example of this is Avenida España, the main road between Valparaíso and Viña del Mar. This is a road with three lines in each direction, situated in between the sea and the beginning of the hills, with a steep cliff consisting of mainly unprotected weathered gneiss. Rockfalls

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and minor landslides frequently block the road. The events are usually quite small, but still cause great disruptions in the traffic between the two cities. (Observation and El Mercurio 2003)

6.5 GIS – PRECISENESS AND OBJECTIVITY? Carrara et al. (1999) states in their article about “GIS Technology in the Prediction and Monitoring of Landslide Hazards” that technologies such as GIS have raised great expectations as a potential means of coping with natural disasters. A landslide hazard map obtained by systematic data manipulation within GIS is assumed to be more objective than a comparable hand made product derived from the same input data and founded on the same conceptual model. This does not necessarily need to be the case. The maps presented in this thesis are all based on an original hand drawn map by the authors, and GIS has been used as an aid to produce the combined hazard map. The accuracy of the computerised map is completely dependent on the accuracy of the originally hand drawn map. GIS is an aid to see things, and should not be used as the primary goal of an investigation. Carrara et al. (1999), states that it is very important to have raw data of good quality and quantity since computerisation can not improve bad data. Hervas et al. (2003) explain the possibility of remote sensing as a powerful tool for collecting information on landslide occurrence and activity over wide areas. Field techniques, despite being very precise, are usually not sufficient to achieve this goal, since they mostly provide point-based measurements. Carrara et al. (1999) focuses on the fact that the distribution of GIS technology is still held back by factors such as the difficulty in acquiring appropriate raw data, the intrinsic complexity of predictive models, the lack of efficient graphical user interfaces, the high cost of digitisation and the persistence of bottlenecks in hardware capitals. One problem according to Carrara et al. (1999) is that hazards models are frequently developed by GIS experts who have little experience of natural catastrophes. The authors discovered a difficulty in finding GIS software which could realize our plan for a multi-factor hazard map, where three layers were combined. Two different soft wares had to be used – MapInfo for digitalization and ArcMap for adding together the three maps with the overlay function. The digitalisation and further manipulation of the data proved to be a very time-consuming task, since a lot of time was spent on gaining deeper knowledge about the programs as well as minor adjustments. It is clear now that the whole process would have been faster done by hand. Perhaps it would also have created a better feeling for the area, working with the maps manually, making analyses an easier and more natural task. During this study a two dimensional hazard map has been produced. It is a fact that slope processes always occurs in three dimensions. A development of the use of 3D GIS techniques in landslide hazards would produce more accurate maps. They would though be more complex and require more knowledge about slope processes in order to be understood.

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7 CONCLUSIONS Which specific areas in Quebrada Francia are most prone to landsliding and why? The unsafe areas of Quebrada Francia are characterised by being steep, built on with houses of low quality and few safety measures and situated on highly-completely weathered material. These areas are found on the flanks between Avenida Francia and Avenida Alemania and along the ravine continuing up from Avenida Francia. Along the flank south of Camino El Vergel, immediate risk areas are also found. Highly-completely weathered material is considered most prone to mass movements due to high infiltration capacity. The lowest quality of housing is considered most dangerous due to the cut-and-fill method during constructing as well as lack of appropriate safety measures. Of the three examined factors (weathering, urbanisation and slope inclination, the character of the urbanisation was regarded as the most influencing factor on landslide hazard. What are the most important causes of mass movement in Valparaíso? The most important trigger of mass movements in Valparaíso is precipitation. Nearly all the precipitation falls through rain storms during the austral winter. This is also when the mass movements occur. Anthropogenic activity can be regarded as a major cause of mass movements in Valparaíso. Seismic activity weakens constructions and weathered rock in the study area, but the trigger is mainly precipitation. Is there a precipitation threshold value when landslides are triggered? Slope failures in Valparaíso and Viña del Mar are possible at precipitation intensities of 14.5 mm/24 hours or more. Precipitation intensities of 50 mm/24 hours or more will create mass movements. The slopes of Valparaíso and Viña del Mar easily reach instability at rainfall intensities around 30-35 mm/24 hours. Generally, slope failures in the area occur after a 3-7 days, of intense rainstorms, after a total precipitation of the last two months of 100 mm. It is the precipitation which has fallen within the short period close to the failure which has the largest impact on the stability of the slopes.

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8 RECOMMENDATIONS

8.1 CONSTRUCTION AND MAINTENANCE

1. Retaining walls that support a cut should always be maintained so they withhold a good level of drainage. If this is not done there is a danger that the wall will burst due to high pore water pressure during or after a heavy rainfall. The type of maintenance is of different importance due to what material the rock or soil behind it is made of. The following advices are due to what material is found behind the contamination wall and organised with the most dangerous material at the top:

• Filling material: Can be of varied composition, but from the soil

analyses made they generally have a similar composition to highly-completely weathered material. Filling material is loose and can therefore cause both sediment-filled flows which can inundate urban areas and transport sediments into unwanted places. It can also cause high pore water pressure. Therefore it is very important to have a functioning drainage system as well as a working runoff system for surface runoff.

• Highly-completely weathered material: Since the material is prone to landslides due to high pore water pressure, it is very important that the drainage of the retaining wall is in a very good condition.

• Highly weathered material: Important to have a functioning drainage system.

• Completely weathered material: Walls supporting material in this material are not in any great danger of bursting due to high pore water pressure. Drainage should always be available, but the concentration of the maintenance work should be to create a good runoff environment and to avoid flooding of sediment filled water.

2. The drainage factor is always very important to consider. These recommendations are

also important to consider whilst constructing AND maintaining houses with one wall blocking the natural seepage of a cut or whilst using shotcrete or any other stabilizer.

3. Construction work in hilly areas should not take place during the rainfall period. All

construction works should be finished before the rainfall period starts.

4. Do not use former slope failure material as filling material. The risk of reactivation of this material is great.

8.2 AFTER EARTHQUAKES A shallow earthquake with a magnitude over five, occurring within two latitudes of Valparaíso is likely to cause cracks or fissures in the ground or in construction works which can facilitate slope failures. It would be wise to have a check up procedure of retaining walls after three or four such earthquakes have occurred. If fissures are detected during this check ups they should be mended.

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8.3 BEFORE AND DURING THE RAINFALL PERIOD Each year the rainfall period should be prepared for. The preparation should contain the following:

• All road or construction works in the hilly areas should be finished for the season. • Restrict the amount of heavy vehicles travelling on the roads in the hilly areas,

especially during and after a rainfall event. • All contamination walls should have been checked. It is important that their drainage

system is working well, and that the retaining walls don’t contain any fissures or other weak points due to earthquakes or general wore down.

• All sabo dams and drainage tunnels that exist within the city should be emptied.

The possibility to plan for a year with expected high rainfall values exists in Valparaíso. Since years with extreme ENSO phases are known to cause extreme rainfall, extra precautions related to slope failures can be taken during these years. In practicality this does not need to be very difficult. Strong El Niño events reaches Ecuador and Peru every two to seven years, and is often accompanied by beneficial rainfall in the arid coastal regions of these two countries (NOAA internet reference). Since the same event reaches Valparaíso during the austral winter, there is a time span between the events in which preparations can be made. It can be assumed that heavy El Niño events and associated hazards in Peru will be reported in the news in Chile. Decision makers will therefore gain knowledge about the event through media. If they (this is the tricky part) are aware of the relationship between El Niño events in Peru and destruction due to slope failures in Chile, extra money can be given to preparations before expected high precipitation. Planning like this, using known facts and common sense can save lives and money.

8.4 TECHNICAL ENGINEERING SOLUTIONS There are already a number of technical engineering devices present in Quebrada Francia. There are however more possibilities: Gabiones Gabiones are steel carriages filled with loose sediment from the area. The Gabiones can be used as stabilisers for roads and buildings. The carriage is constructed by double twisted steel wire net which is warm galvanised and surrounded with a plastic coating to protect from corrosion. This is assembled to a net carriage with walls between every meter and then filled with sediment. Since they are filled with loose sediment the drainage is self managing, if the sediments they are filled with have fractions between 80-150 mm. The steel wire net carriage is very durable and will manage through large vibrations, forces from soil pressure and soil movements. Geotextiles Geotextiles can be carpets made of natural and synthetic fibres and are used to cover the ground. They reduce erosion and infiltration as well as creating a good environment for vegetation.

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8.5 HAZARD PREDICTION Selby (1993) offers an idea of a technical device to foresee shallow landslides due to heavy rains. Knowledge of pre-existing soil moisture content from an automatic tensiometer, and of rainfall intensity from a recording rain gauge, can provide a warning system for critical urban areas. This system can provide a warning for when soil moisture content is getting too high and there is an immediate risk for a sliding event. This can be done in Quebrada Francia, but more information about the behaviour of the different soils is needed.

8.6 INFORMATION TO PEOPLE LIVING IN RISK AREAS The people that live in the risk areas do have a chance to make their own neighbourhood safer. The problem with sabo dams filled with garbage and plastic bags, stopping the drainage of water and causing inundations, can only be solved by garbage not being thrown into the ravine. By using the neighbours organisations and schools, information about how this behaviour affects the safety of the town can be spread. The information should be easily understood, and not be used to scare the people living in the area. To work with schoolchildren is often very effective, as they can teach their parents and affect the behaviour of their family. To have projects where the amount of garbage thrown in the right places should be increased can be a good way to start. The project should contain a goal to be reached, for example if the amount of garbage thrown in a specific area can be noticeably reduced. When this goal is achieved, the school or organisation managing the project will be rewarded by the municipally. The reward gained can for example be something that the school or organisation needs (new playground facilities or something similar). The money used for the reward, will be the equal amount to what the municipally consider that they have saved out of less work in emptying the dams and tunnels from garbage. Information about construction, and the dangers of using the wrong methods whilst constructing should also be easily available. It is important that the information also reaches people constructing houses or huts in dangerous areas. The information should be constructive and contain methods of how to construct a safe house and paths. All the information given should focus on ways of acting and what kind of result the acting produces. To deny the existence of hill slope related problems is not the right way to confront the problem. With the right information reaching the right people, habitants in risk areas can act themselves to prevent problems. If less slope failures occur, money and lives will be saved.

8.7 START A LANDSLIDE RECORD The availability and quality of historic landslide database cannot be overemphasized since they constitute the bases for all components of landslide risk assessment (Dai et al. 2002).

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To start a landslide record supervised by the municipality would be a reasonably easy way to get a view about how many landslides that occur. This record would be useful in many ways, for example as background information or foundation for political and economic decisions. It will also facilitate further studies about landslide and their effects. It would be excellent if each city with landslide disasters could run there own landslide record in Chile. A landslide record should contain the time, location, possible trigger and caused damage of the event. The landslide record could be administered by the Centro de Emergencia (Emergency Center), as they are the ones who receive all emergency calls. It is then important that they are given enough time and resources to be able to note all landslide occasions which have occurred. Some statistics of landslide events already exists but was not possible for us to use since a lot of data were missing due to scarce resources during catastrophic events.

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9 REFERENCES Alvarez Sch. L (1964), Geología del Area Valparaíso-Viña del Mar, Instituto de Investigagaciones Geológicas Chile. Boletin 16, 28-48 Carrara A, Guzetti F, Cardinali M & Reichenbach P (1999), Use of GIS technology in the prediction and monitoring of landslide hazard, Natural Hazards 20, 117-135 Chowdhury R & Flentje P (2002), Uncertainties in rainfall induced landslide hazard, Quarterly Journal of Engineering Geology and Hydrogeology 35, 61-70 Cruden D & Fell R (1997), Landslide Risk Assessment. Balkema, Rotterdam, pp 371 Dai F.C., Lee C.F. & Ngai Y.Y. (2002), Landslide Risk Assessment and Management: an overview, Engineering Geology 64, 65-87 Ducci D (1999), GIS techniques for mapping groundwater contamination risk, Natural Hazards 20, 279-294 Fernandez C (2002), Deslizamiento del 24 de Agosto de 2002 en el camino Troncal, Comuna del Viña del Mar, Servicio Nacional de Geología y Minería, Subdireccion Nacional de Geología, Departemento de Geología Aplicada, Santiago, Chile, pp 6 Gispert et al (1999) Enciclopedia de Chile, Volumen 1, MMII Oceano Grupo Editorial, S.A., Chile, 39-66 Gispert et al (1999) Enciclopedia de Chile, Volumen 2, MMII Oceano Grupo Editorial, S.A., Chile, 501-507 Goudie A (2000), The Human Impact on the Natural Environment, Blackwell Publishers, 261-327 Goudie A with the assistance of Anderson M, Burt T, Lewin J, Richards K, Whalley B & Worsley P (1981), Geomorphological Techniques, School of Geography, University of Oxford, pp 395 Greenway D R (1987), Vegetation and slope stability, in Slope Stability, Geotechnical Engineering and Geomorphology, edited by Anderson M.G & Richards K.S. John Wiley & Sons Ltd, Plymouth, Great Britain, 187-231 Grimme K & Alvarez Sch L (1964), El Suelo de fundacion de Valparaiso y Viña del Mar, Provincia de Valparaíso, Instittuto de Investigaciones Geológicas de Chile, 1-27 Hansen M (1984), Strategies for classification of landslides, in Slope Stability, edited by Brunsden D & Prior D B, John Wiley & Sons Ltd, Salisbury, Great Britain, 1-26 Hauser A (2002), Diagnostico Geológico Geotécnico de Seis Sectores de Laderas y Taludes Inestables, En Casco Urbano de la Ciudad de Valparaíso, Quinta Región: Alternativas de Tratamiento. Servicio Nacional de Geología y Minería, Santiago, Chile, pp 43

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Hauser A (2000), Remociones en masa en Chile (versión actualizada), Servicio Nacional de Geología y Minería, Santiago, Chile, pp 89 Hencher S.R (1987), The implications of joints and structures for slope stability, in Slope Stability, Geotechnical Engineering and Geomorphology, edited by Anderson M.G & Richards K.S. John Wiley & Sons, Plymouth, Great Britain, 145-187 Hervas J, Barredo J I & Rosin P L (2003), Monitoring landslides from optical remotely sensed imagery: the case history of Tessina landslide, Italy,Geomorphology 54 (1-2), 63-75 Instituto Geografico Militar de Chile (1981) maps: 1) Valparaíso-Viña del Mar, 330000-713000, Seccion “E” No 47 N.E., 2) Valparaíso-Viña del Mar, 330000-713730, Seccion “E” No 47 N.O., both in scale 1:25000, topographic lines every 25 metres. Islam M & Sado K (2000), Flood hazard assessment in Bangladesh using NOAA AVHRR data with geographical information system, Hydrological Processes 14, 605-620 Karlsson R (1975), Konsistensgränser, förslag till Geotekniska laboratorieanvisningar, del 6, Statens råd för byggnadsforskning Kayabali K & Akin M (2003), Seismic hazard map of Turkey using the deterministic approach, Engineering Geology 69, 127-137 Montecinos A (1998), Seasonal rainforest forecast in central Chile (in spanish), MS Thesis Department of Geophysics, University of Chile, Santiago, pp 116 Montecinos A & Aceituno P (2003), Seasonality of the ENSO related Variability in Central Chile and Associated Circulation Anomalies, Journal of Climate 16, 281-296 Ng C.W.W. & Q. Shi (1998), A Numerical Investigation of Unsaturated Soil Slopes Subjected to Transient Seepage, Computers and Geotechniches 22, 1-28 Pallás R, Vilaplana J M, Guinau M, Falgás E, Alemany X & Muñoz A (2004), A pragmatic approach to debris flow hazard mapping in areas affectd by Hurricane Mitch: example from NW Nicaragua, Engineering Geology 72, 57-72 Pasuto A & Silvano S (1998), Rainfall as a trigger of shallow mass movements. A case study in the Dolomites, Italy, Environmental Geology 35, 84-189 Rauld R & Fernández C (2002), Deslizamientos ocurridos durante los temporales de Mayo y junio de 2002 en Valparaíso y Viña del Mar, V Región, Servicio Nacional de Geología y Minería, Subdireccion Nacional de Geología, Departemento de Geología Aplicada, Santiago, Chile, pp 8 Salazar R G (1993?), Configuraciones atmosféricas regionales durante grandes tormentaspluviales en Chile Central, Deparamento de Geofísica, Universidad de Chile, Santiago, Chile, pp 8 Schuster R L (1996), Socioecomic significance of landslides, in Landslides: Investigation and Mitigation, Special Report 247, edited by Turner A.K. & Schuster, R.L., Transportation

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research Board, National Research Council. National Academy press, Washington DC, pp 12-35 Selby M J (1993), Hillslope Materials and Processes, Second edition, Oxford University Press, Great Britain, pp 451 Ståhl (1972), Kornstorleksgränser, Statens råd för byggnadsforskning Terzaghi H (1950), Mechanism of landslides, Geological Society of America, Engineering Geology, 83-123 Thomas M (1994), Geomorphology in the tropics, John Wiley & Sons, Great Britain, pp 443 Utrikespolitiska institutet (Swedish Institute of International Affairs) (2002), Chile länder i fickformat, nr 716, pp 33 Wasowski J (1998), Understanding rainfall-landslide relationships in man-modified environments: a case history from Caramanico Terme, Italy, Environmental Geology 35 (2-3), 197-209 Van Westen CJ, Seijmonsbergen SC & Mantovani F (1999), Comparing landslide hazard maps, Natural Hazards 20, 137-158 Whalley W B (1984), Rockfalls, in Slope Stability, edited by Brunsden D & Prior D B, John Wiley & Sons Ltd, Salisbury, Great Britain, 217-257

9.1 ORAL AND WRITTEN COMMUNICATION Hauser, Arturo, geologist, Head of Applied Geology (Geología Aplicada), Servicio Nacional de Geología y Minería, Santiago, Chile, 2003 Muzio, Gino, civil constructor, Head of Operative Technical Department (Departamento Tecnico, Area Operativa), Municipality of Valparaíso, Chile, 2003 Rauld P, Rodrigo, geologist, Servicio Nacional de Geología y Minería, Chile, 2003 and 2004 Olvmo, Mats, Ass Prof, Physical Geography, Earth Science centre, University Of Gothenburg, Sweden, 2003 and 2004

9.2 INTERNET El Mercurio de Valparaíso, April 15th, 2004 www.mercuriovalpo.cl IDNDR, May 10th 2004 International Decade of Desaster Reduction http://www.unesco.org/science/earthsciences/disaster/disasterIDNDR.htm

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http://www.mercuriovalpo.cl/

http://www.unesco.org/science/earthsciences/disaster/disasterIDNDR.htm 2004-05-10

Imperial College of London, March 7th 2004 Soil Mechanics Group, C.E Rodriguez http://cvfeller.cv.ic.ac.uk/carlos.html Instituto Nacional de Estadisticas en Chile, August 7th 2004 http://www.ine.cl/17ambiente/1-2pdf-Bef http://www.ine.cl/17ambient/V-5pdf http://www.ine.cl/i-frecuentes.htm NOAA Climate Prediction Center, National Oceanic and Atmospheric Administration, May 10th 2004 Impacts of El Niño http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/impacts/warm_impacts.html UNESCO, February 2 2004 The World Heritage List http://whc.unesco.org/nwhc/pages/doc/mainf3.htm

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http://cvfeller.cv.ic.ac.uk/carlos.html

http://www.ine.cl/17ambiente/1-2pdf-Bef

http://www.ine.cl/17ambient/V-5pdf

http://www.ine.cl/i-frecuentes.htm

http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/impacts/warm_impacts.html

http://whc.unesco.org/nwhc/pages/doc/mainf3.htm

Appendix 1. Landslide record Date Place Cause/trigger Type of movement Amount of movemments

11944-08-08 Avenida España muro 1948-05-07 Valpo rain derrumbe 51948-05-14 Valpo/other rain rockfall/derrumbe 51949-05-30 Valpo rain muro 11949-06-21 Valpo rain muro 11962-06-23 Valpo rain derrumbe 151962-06-25 Viña rain derrumbe 41962-06-27 Valpo rain derrumbe 11966-06-14 Valpo rain muro 11966-06-16 Valpo rain escurrimiento (mudflow) 11966-06-20 Valpo rain muro 11966-06-21 Valpo rain debris/mudflows 31966-07-11 Other rain derrumbe 21966-07-19 Valpo/Viña rain derrumbe 51966-07-21 Valpo rain derrumbe/muro 31970-07-16 Valpo rain derrumbe/flujos de detritos 81970-07-29 Viña rain/muro avalanche de rocas 11972-05-10 Valpo rain flujos detritos 11972-06-11 0ther rain/traffic deslizamientos de tierra 11972-06-13 Valpo rain derrumbe/muro 31972-06-14 Viña rain/tree fell derrumbe 101972-06-16 Other rain derrumbe 11974-06-09 Valpo rain desprendimiento 11974-06-20 Valpo rain derrumbe 31974-06-28 Valpo/Viña rain derrumbe 101974-06-29 Valpo rain derrumbe 31974-06-30 Other rain deslizamiento de rocas 11974-07-24 Quebrada Francia muro 11977-06-25 Valpo rain muro/deslizamiento 21977-06-30 Avenida España rain desprendimiento de rocas 11977-07-01 Quebrada Francia rain muro 11977-07-02 Valpo rain derrumbe 31977-07-11 Valpo rain muro 11977-07-13 Valpo rain muro 11977-07-21 Valpo rain derrumbe 31977-07-22 Valpo/Viña rain/cracks derrumbe/muro 10

1978-07-13 Valpo rain derumbes/deslizamientos de tierra 20

1978-07-16 Valpo rain muro 11978-07-17 Valpo rain derrumbe 11980-05-10 Valpo rain derrumbe 11980-06-25 Other rain derrumbe 11980-07-07 Valpo rain derrumbe 31980-07-07 Viña rain derrumbe 11980-07-18 Viña rain derrumbe 11980-07-23 Valpo derrumbe 11980-07-26 Valpo rain derrumbe/deslizamiento 91982-05-10 Valpo rain muro/derrumbe 2

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1982-05-12 Valpo rain derrumbe/avalanche de rocas 101982-06-10 Valpo rain muro/derrumbe 21982-06-11 Valpo rain derrumbe 31982-06-25 Valpo muro 11982-06-26 Valpo slope moist muros/derrumbe 31982-06-27 Valpo rain derrumbe 21982-06-28 Valpo rain derrumbe 31982-07-15 Valpo/Viña rain derrumbe 51982-07-16 Valpo/Viña rain muro/desprendimiento de rocas 21982-08-13 Valpo porewater pressure muro 11984-05-22 Viña rain derrumbe/muro 11984-05-26 Valpo rain derrumbe 21984-05-28 Other rain hundimiento 1

1984-06-05 other stress due to high load

derrumbe during house contruction 1

1984-06-12 Quebrada Francia fractures in soil derrumbe 11984-07-02 Valpo/Viña rain derrumbe >101984-07-03 Valpo rain derrumbe 21984-07-04 other rain deslizamiento de tierra 11984-07-05 Other rain deslizamiento de rocas 11984-07-05 Valpo rain derrumbes 7

1984-07-06 Viña/Valpo rain deslizamiento/mudflow 41984-07-08 Valpo rain deslizamiento/derrumbe 31984-07-15 Viña flow 11984-08-24 Valpo porewater pressure muro 11985-06-14 Viña terremoto/rain muro 11985-07-29 Viña terremoto/rain muro 41985-07-30 Valpo terremoto/rain muro 11985-07-30 Viña rain muro 11985-07-31 Valpo rain derrumbe 11987-05-29 Valpo rain muro 11987-06-01 Valpo derrumbe con basura 11987-07-11 Valpo rain derrumbe 21987-07-11 other rain derrumbe 11987-07-13 Valpo/Viña rain derrumbe 41987-07-16 Valpo derrumbe 11987-07-24 Valpo rain muro 11987-08-11 Valpo rain derrrumbe 21987-08-12 Valpo/Viña rain derrumbe 71987-08-14 Quebrada Francia rain deslizamiento de tierra 31988-08-01 Valpo rain derrumbe 11991-05-28 Valpo rain deslizamiento de tierra 71991-06-02 Valpo rain derrumbe 21991-06-02 Avenida España rain derrumbe 11991-06-02 Other rain derrumbe 71991-06-03 Viña derrumbe 11991-06-03 Valpo derrumbe 41991-06-17 Valpo rain derrumbe 11991-06-18 Valpo rain deslizamiento de tierra 11991-06-19 Viña rain muro 11991-06-19 Valpo rain derrumbe 11991-06-20 Valpo rain derrumbe 101991-06-27 Viña rain hundimiento 1

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1992-05-06 Valpo rain derrumbe 11992-05-06 Viña and Valpo rain deslizamiento de tierra 31992-05-08 Valpo rain derrumbe 11992-05-10 Viña rain derrumbe 51992-05-10 Viña rain deslizamiento de tierra 21992-05-12 Valpo broken waterpipe derrumbe 11992-06-05 Valpo deslizamiento de tierra 11992-06-07 Valpo rain derrumbe 11992-06-08 Valpo rain muro 51992-06-24 Other rain deslizamiento de tierra 11992-06-25 Valpo rain derrumbe 21992-06-25 Viña and Valpo rain deslizamiento de tierra 11992-08-03 Viña broken waterpipe desprendimiento de rocas 11992-08-21 Valpo derrumbe 1

1992-08-29 Valpo broken waterpipe/rain derrumbe 1

1992-08-29 Valpo rain hundimiento 11992-08-30 Viña rain derrumbe 11992-09-01 Valpo desprendimiento de rocas 11992-09-15 Valpo derrumbe 11992-09-17 Other rain desprendimiento de rocas 11992-10-04 Valpo bad construction muro 11993-05-20 Viña rain hundimiento 11993-07-02 Valpo rain derrumbe 21994-05-23 Quebrada Francia rain desprendimiento de rocas 11994-05-23 Valpo rain derrumbe 11994-05-23 Other rain arrastre 11994-05-23 Other rain deslizamiento de tierra 11994-05-24 Viña rain derrumbe 51994-05-24 Valpo rain derrumbe 21994-05-25 Viña and Valpo rain deslizamiento de tierra 101994-05-27 Other broken waterpipe arrastre 11994-07-19 Viña and Valpo rain derrumbe 51994-07-23 Valpo rain derrumbe 11994-07-24 Valpo rain derrumbe 11994-08-08 Viña and Valpo rain derrumbe 31995-05-14 Viña broken waterpipe derrumbe 11995-05-15 other wave erosion muro 11995-06-06 Viña lack of maintenance hundimiento 11995-06-16 Valpo/Viña rain derrumbe ca51995-07-04 Valpo rain deslizamientos 31995-07-05 Valpo/Viña rain deslizamientos 11995-08-20 Viña rain derrumbe 11996-05-16 Avenida España rain deslizamiento de tierra 11996-06-13 Valpo rain muro 11996-07-07 Valpo rain derrumbe ca101996-07-08 Valpo/Viña rain muro/derrumbe ca51996-07-24 Valpo/Viña rain+road vibrations desprendimiento de rocas 11997-09-06 Valpo rain derrumbe 11997-09-07 Valpo rain derrumbe 401997-09-08 Valpo rain derrumbe 21997-11-18 Quebrada Francia construction work derrumbe 11998-02-05 Viña broken waterpipe deslizamiento de tierra 11999-03-13 Viña rain deslizamiento de tierra 11999-06-28 Valpo rain deslizamiento de tierra

83

1999-09-06 Viña rain derrumbe 101999-09-07 Viña and Valpo rain derrumbe/deslizamiento 2000-06-12 Viña rain muro 12000-06-24 Valpo rain derrumbe 12000-07-14 Valpo rain derrumbe 32000-08-30 Valpo derrumbe 32000-09-10 Viña rain hundimiento 12001-07-29 Valpo rain derrumbe 30

2001-07-30 Valpo rain derrumbe/deslizamiento 120,002001-08-26 Viña and Valpo rain derrumbe/deslizamiento 102001-09-26 Valpo rain derrumbe 12002-05-25 Valpo rain derrumbe 32002-05-25 Viña rain deslizamiento de tierra 32002-06-03 Quebrada Francia rain, tremor deslizamiento de tierra 32002-06-04 Valpo derrumbe 32002-06-04 Valpo deslizamiento de tierra 1

2002-06-04 Quebrada Francia 2002-06-11 Other rain deslizamiento de tierra 12002-06-16 Viña and Valpo deslizamiento de tierra 682002-06-16 Viña and Valpo derrumbe 282002-07-23 Quebrada Francia rain desprendimiento de rocas 12002-07-23 Valpo rain derrumbe 32002-07-23 Viña rain derrumbe 32002-08-07 Quebrada Francia rain deslizamiento de tierra 12002-08-07 Valpo rain derrumbe 32002-08-07 Viña rain derrumbe 32002-08-14 Avenida España derrumbe 12002-08-19 Avenida España derrumbe 22003-05-20 Valpo rain derrumbe 42003-05-20 Valpo rain muro 82003-07-07 Other rain deslizamiento de tierra 12003-11-16 Other derrumbe 1

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Appemdix 2. Soil analysis results

0,

Weathering classification

Soil Sample ID Particle size distribution

Infiltration rate (steady state value)

Liquid limit wL

Plastic limitwP

1F

Clay: 3% Silt: 13,5% Sand: 78,5% Gravel: 5%

793 mm/h

7


A: 353 mm/h B: 267 mm/h

9

Clay: 2,5% Silt: 12,5% Sand: 52% Gravel: 33%

A: 372 mm/h B: 120 mm/h

12A

Clay: 2% Silt: 8% Sand: 50% Gravel: 40%

1872 mm/h

12B 72 mm/h

Filling material

Average 550* mm/h

4


A: 72 mm/h B: 60 mm/h 40 % 21

5


A: 54 mm/h B: 24 mm/h 30 % 17

6 234 mm/h

Completely weathered

Average 89* mm/h 35% 19

1A


1212 mm/h

1B

Clay: 0,5% Silt: 6% Sand: 53,5% Gravel: 40%

1793 mm/h

Highly-completely weathered

10A


546 mm/h

85

10B


378 mm/h

11 A: 912 mm/h B: 2358 mm/h

Average 1200* mm/h

2

Clay: 0% Silt: 3,5% Sand: 31% Gravel: 60,5%

A: 2490 mm/h B: 1644 mm/h

3

Clay: 0,5% Silt: 6,5% Sand: 75,5% Gravel: 18%

774 mm/h

8


A: 456 mm/h B: 147 mm/h

Highly weathered

Average 1102* mm/h

Comments

The limits between the particle sizes are based on the classification of British Standards Institute (Selby sid 12) and are as follows (unit is mm): Gravel: >2,0 Sand: 0,06-2,0 Silt: 0,002-0,06 Clay: <0,002

* Infiltration rate measurements made on sites where soil samples were not

taken are included in the average rate

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