Engineering Geology 116 (2010) 129–138

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Engineering Geology

j ourna l homepage: www.e lsev ie r.com/ locate /enggeo

Urban geological mapping: Geotechnical data analysis for rationaldevelopment planning

Moufida El May a,⁎, Mahmoud Dlala a, Ismail Chenini b

a Laboratory of Paleoenvironment, Geomaterial and Seismic Risk, Department of Geology, Faculty of Sciences, University of Tunis El Manar, 2092 Tunis El Manar, Tunisiab Laboratory of Minerals Resources and Environment, Department of Geology, Faculty of Sciences of Tunis, 2092 Tunis El Manar, Tunisia

⁎ Corresponding author. Tel.: +216 97674190.E-mail address: elmaymoufida@yahoo.fr (M. El May

0013-7952/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.enggeo.2010.08.002

a b s t r a c t

a r t i c l e i n f o

Article history:Received 16 June 2009Received in revised form 20 July 2010Accepted 5 August 2010Available online 13 August 2010

Keywords:Urban geologyRational-land-use planningGeotechnical zoningGeographic Information SystemTunisia

Urban geology provides information on urban geologic environments as a scientific basis for planners andengineers for rational land use planning and urban development. Such mapping can be classified in terms ofpurpose, content and scale. In this study, procedure for preparation of engineering geological mapping inTunis City (Tunisia) is given, as a case study. The main restricting factors for urban development: such aslithology; topography; slope; seismotectonic; water table depth; flooding susceptibility and seismic-inducedeffects were considered in the preparation of maps. All these information layers were manipulated using theGeographic Information System (GIS), and they were then combined to produce uniform engineeringgeological maps. Results are illustrated as a suitability map for construction in the study area. Consequently,the study area was categorized into four different zones as: (1) a high region with potential risk of superficialperturbation; (2) a low and flat zone with flooding risk; (3) a low zone with flooding and probability ofsliding risk; (4) a region with mud levels and settlement risk.

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© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Throughout the world, urban geology occupies a key place inplanning and mapping the geological environment of cities. In somecontext, it is considered a close synonym for environmental geology(Baker, 1975). It is also defined as the study of land resources andgeologic hazards related to the development and expansion of urbanareas (Fuchu et al., 1994).

In addition to data collection, geological analysis and the establish-ment of the map, another purpose of urban geology is to providegeological knowledge to planners and politicians for the establishmentof rational development planning. Several urban geological studies ofsome cities provide good examples for this approach (Baker, 1975;Akpokodje, 1979; Edbrooke et al., 2003; Haworth, 2003; Nott, 2003;Willey, 2003; Özsan et al., 2007; El May et al., 2009).

Within the framework of urban geology, we consider geotechnicalmapping as an important tool that provides geotechnical parametersto establish a suitable map for construction, which helps establish asafe urban extension.

Tunis City, the capital of Tunisia, is located inNorthAfrica (Fig. 1). It isan excellent example of a city with a budding program in geotechnicalmapping. Since 1994 to 2007 the city has grown from a population of2250000 to 2380000. The exploding population evolves the city's limitextending.

Thepurpose of this paper is to contribute to a rational urbanplanningand development using urban geological environment mapping. Awidespread geological and geotechnical study becomes of high interest.A multicriteria analysis is used to establish urban geological andgeotechnical mapping with the following objectives: (1) investigationof the geological environment of Tunis City, (2) studying of the urbangeology and (3) proposing further recommendation by combininggeological maps with geotechnical aspects to establish a suitability mapfor construction.

2. Methodology

A methodological approach for the establishment and compilationof engineering geological map in the rapidly extended and complexcities based on the example of Tunis City is presented in this paper.

2.1. Basic question on geological mapping

Tunis City, with complex geomorphologic features, is distinguishedby the complexity and variety of geological parameters. It causesdifficulties in geological and geotechnical mapping. However, an urbangeological mapping can be made using the amount of data available.

Some basic questions on urban geological mapping need to bediscussed and improved before the application of the presentedapproach: (1) choosing an appropriate scale for such mapping,(2) selection of an urban geologymappingmethod and (3) compilationof maps or the utility of the final output for planners and decisionmakers in the studied city.

Fig. 1. Lithology map of the study area.

130 M. El May et al. / Engineering Geology 116 (2010) 129–138

2.2. Type of required information

The urban geological mapping mission gathers and explores theavailable data related to the study area. This includes the nature ofsuperficial materials, topography, drainage network, water table level,seismotectonic conditions, geotechnical properties of materials, andgeotechnical tests. Correlations derived from borehole stratigraphicinformation are also required. The borehole data were observed,analyzed and evaluated in order to obtain the water table depth, aswell as geotechnical and liquefaction potential layers.

2.3. Choice of mapping scale

The choice of scale for the processing of a geotechnicalmap dependson several criteria. Firstly, the purpose for which the map is intended

and the amount of detail that has to be shown (Dearman, 1991). Thesecond factor that should be taken into consideration to determine themapping scale is the size of the country that must be mapped (Price,1981). The third criteria of importance in the choice ofmapping scale, isthe complexity of the terrain to be mapped (Price, 1981). Fourthly,complementary maps, such as geology, topographical sheets, aerialphotographs and soil maps (Price, 1981). An international scale rangewas proposed by the UNESCO guidebook (CEGM-IAEGC, No. 15, 1976)and is given as follows: Large-scale maps (1:10000 and larger),medium-scale maps (less than 1:10000 and greater than 1:100000),and small-scale (1:100000 and less).

Tunis City is characterized by a complicate geological history anda nonmonotonous morphology (Monchicourt, 1904), that is why inthis study mapping scale 1:25000 as the very large scale used in allcomplimentary maps mainly in topographic maps. In case of smaller

131M. El May et al. / Engineering Geology 116 (2010) 129–138

maps (e.g. 1:50000), the same regional criteria (e.g., fault, aquiferextension, and watercourses) and appropriate evaluation of geolog-ical conditions of area studied could not be possible. However, thescale considered for the geotechnical mapping of Tunis City (1: 25000) expresses the detail and enables the successful evaluation ofthe suitability for construction. It is considered, according to theinternational scale range proposed by the UNESCO, as a mediumscale.

2.4. Compilation and geotechnical mapping procedure

All required information was initiated by the construction of aworkable project in ArcView 3.2. First, topographic maps of Tunis Citywere converted to digital format and georeferenced. Then, all mapswere processed to form a Geographic Information System (GIS)database of spatial data linked to attribute information. Theengineering geological map is based on the use of many steps thatconsist of geological and geotechnical data collection.

– Geological study that is based on the assembly of the geologicalmaps of Tunis City, already done, on a scale of 1/50000 and theassessment of the sismotectonic map prepared by Dlala and Kacem(2007). This includes the structural pattern of the study area anddetailed lithology of outcrops.

– Studying the topography by digitizing elevation map using theavailable topographic map of Tunis City established by theCartography and Topography Office of Tunisia (OTC) in 1984 atthe scale 1:25000

– Studying the water table level depth using the kriging method onthe basis of the available piezometer data and the geotechnicalborehole data. The kriging technique was primarily developed tosolvemining and geological problems but has since found, throughthe latest few years wide applications in different fields such asgroundwater. This procedure can be implemented in all caseswhere some spatial correlation between sampling points isobserved and it produces no significant errors in properlyrepresenting the study area.

– Slope process automatically derived from the topography usingthe grid utility of ArcView 3.2. The intensity of the slope isorganized toward a geological genetic rock nature.

– Lineament map that indicates fault location, active faults, recentdeformation indices, and instrumental and historic epicenterdistribution. It is extracted from the seismotectonic map estab-lished by Dlala and Kacem (2007). This map was made based ondirect field observations.

– Bearing capacity assessment map will be superposed to theprevious ones for a suitable geotechnical mapping.

The geotechnical zoning of Tunis City is finally given by compilingall cited criteria and by superposing them to the geotechnical map.

The basic geological map, which summarizes geological andgeotechnical criteria, have a great effect on the urbanismdevelopmentand represent a basis for the evaluation of the engineering conditionsof the city.

3. Study area: environmental geology problem

3.1. Study area

Tunis City belongs to a complex geological structure. It contains theterminal plains of the most important river Tell called Medjerda andMilian. Between the tow plains, highmorphological structures occupythe central part. The J. Ammar structure, which is 328 m high, is thehighest. Other hills, with Cretaceous peak, are constituted by a simpledomes separated by synclinal basins. Hillsides are formed by tertiaryformations. Lower parts of the studied area are filled with Quaternarydeposits. Hills and basins are often plump of a limestone travertine or

tuff shells. In addition, previous studies of Pimenta (1995) haveshowed the existence of thick river channel deposits from a big valleybetween Belvedere hills and Tunis Lake, called Medjerda valley. Itgenerates alternation between clayey, coarse and fine sandy deposits.

3.2. Geotechnical and environmental geology problems

Tunis City is characterized by the dominance of recent sedimentaryformation, formed by heterogeneous deposits and little compact andsandy soil that constituted the shallow aquifers. Sediment properties(lithology, age of deposit, grain size) and hydrogeological conditions(groundwater level) make the site favorable for seismic-waveamplification. Consequently, this makes the soil prone to liquefactionupon seismic shaking (Stephen et al., 2004). For these reasons, in TunisCity, soil may significantly liquefy in the case of a possible earthquake.Thus, the damage may become higher.

Slope stability is one of the severest problems that affect urbanplanning in the study area, mainly in Sidi bou Saïd hill. As mentionedin some previous work (Mellouli, 1984; Hamrouni et al., 1989; El May,2004), there have been numerous landslides.

In Tunisia, a compressive tectonic phase has started since the upperMiocene and continues until the present with a NW SE shorteningdirection (Philip, 1987; Dlala, 1995). Compressive tectonics causedseveral important earthquakes. Epicenter distribution map for theperiod between 856 and 2000 shows that Tunis area is characterizedby a low magnitude of earthquakes (1.4 to 3.8) (El May et al., 2009).Earthquakes of magnitude MN4 are localized especially in Tunis,Ariana, Chaouat, Sidi Thabet and Utique region. The strongest earth-quakes were of 4.3 magnitude on the Richter scale, and they wererecorded at Jebel Ressas. Earthquakes with 5.1 magnitude on theRichter scale were at the Jedaida region during the Chouat and SidiThabet 1970 earthquake. These are the nearest epicenters to the studyarea and the more felt by the population (Fig. 3).

In the study area the soil surrounding the sebkha is made up ofclay, fill, sands, and loess, as well as loessial clay. If wetted, the soil hasthe potential to collapse. The soil is heterogeneous and hasengineering properties that markedly vary with location, whichcauses damage to urban building.

In Tunis City, which is characterized by an important streamnetwork, flooding is an environmental problem, which can restrictand influence urban development. The flooding problem is concen-trated to narrow drainage zone and depends on the intensity ofrainfall (Fuchu et al., 1994). In this study, careful mapping of streamnetwork is done to identify the area threatened by floods.

4. Results

As declared earlier, geotechnical mapping method adopted for thisstudy involves the establishment of some maps. Each map wasrepresented as a separate layer in the GIS database. GIS is capable toincorporate the related spatial data into traditional geotechnicaldatabases in order to present a more comprehensive view of thetarget region (Kaâniche et al., 2000). For geotechnical mappingstudies, GIS offers a spatial representation of complex geologicalsystems (Kolat et al., 2006; Xie et al., 2006; Yilmaz, 2008, 2009).

In the first stage of the work, a number of thematic maps reflectingparameters that directly influence the suitability of construction wereprepared from the gathered data. Thematic maps established usingthe GIS technique include: (1) lithology, (2) topography; (3) slope,(4) seismotectonic, (5) liquefaction (6) flooding susceptibility, (7) andwater table depth. A multi criteria analysis is used to integrate allqualitative thematic layers prepared from the data collected and toprepare a map that shows suitable areas for safety construction andurban extension. In the second stage of the study, zoning maps aresuperposed to the map showing the bearing capacity calculated from

Table 1Integration of thematic layers.

Thematic integrated layer Resultant layer

Lithology+slope layer T1T1+seismotectonic layer T2T2+liquefaction layer T3T3+flooding susceptibility layer T4T4+water table layer T5T5+bearing capacity layer T6 (final zoning map)

132 M. El May et al. / Engineering Geology 116 (2010) 129–138

borehole data based on the geotechnical test to provide the final mapwith more occurrence and precision.

Thematic layers are combined tow by tow. The lithologic layer issuperposed to the slope layer, and the resulting map T1 is superposedto the siesmotectonic layer and gives the T2. This layer is superposed tothe liquefaction layer to give T3. T3 is superposed to the floodingsusceptibility layer to give T4. This layer is superposed to the watertable layer to give T5. This final layer is superposed to geotechnicallayers to give the final zoning map (Table 1).

4.1. Lithology and soil layer

The lithologic aspect of Tunis City can be classified according to itsgeneral morphology. The flatlands are occupied by formations of theQuaternary and Holocene age (Fig. 1):

– Recent alluvial deposits are formed by crust, clay, sand, and mud.The Quaternary marine deposits and sebkha deposits are made ofmud, silts, and evaporites.

– Holocene formations are composed of the shelly sands of theSoukra area.

Fig. 2. Seismotectonic m

Hillsides are occupied by formations of the Pleistocene andHolocene age, lithologically constituted by

– Clay, sand, and silt of the Middle to Upper Pleistocene age– Sand and clay of the Lower to Middle Pleistocene– Clay and fine sand of the Pleistocene age– Aeolian sand of the Holocene age.

Hills are occupied by

– Deposits of the Oligocene to Pliocene formations, constituted bysand, clay, sandstone, and conglomerate

– Deposits of the Jurassic to Eocene age that are composed oflimestone and of alternations of limestone and marl.

The lithologic layer is prepared based on the lithologic map andgeotechnical tests based on the borehole data. The Jurassic to Eoceneage deposits are considered homogenous and compact soil and areclassified as the most suitable areas for construction. The Oligoceneto Pliocene deposits are considered as heterogeneous and compactformations and they are also classified as most favorable toconstruction. The Lower to Upper Pleistocene formations areheterogeneous and are assigned as the intermediate class. Sandyformations are the most favorable to liquefaction phenomena.Triassic formations, muddy soil, and Quaternary alluvium forma-tions are the least favorable to construction. The lithologic layer ofthe study area is given in Fig. 5a.

4.2. Topography

Geomorphological conditions are helpful in explaining the recenthistory of the development of the landscape and the processes active

ap of the study area.

Fig. 3. The digital elevation model (DEM) of the study area.

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in the landscape at the present time. Thus it is an essential part ofgeotechnical mapping and is often a decisive factor in the planning ofan investigation (Kleinhans, 2002).

Fig. 4. Liquefaction su

In its geological history, Tunis City has undergone tectonic andsedimentary events that have contributed to the creation of thecurrentmorphology of the site. Indeed the study area iswith a complex

sceptibility layer.

134 M. El May et al. / Engineering Geology 116 (2010) 129–138

morphology, composed of threemainmorphological elements that arereliefs, plains and depressions.

Topographical maps at a scale 1:25000 were first registeredaccording to the Universal Transverse Mercator (UTM) projectionsystem (Zone: 32 and Datum: 47 Carthage—Tunisia) and weredigitized. The study area covers about 1527 km2. The digitizedcontours were used for the elaboration of the Digital ElevationModel (DEM) of the study area (Fig. 2). The cell size of the DEM wasdetermined as 145 m; it is the most adequate cell size that betterrepresents the natural environment of the study area and gives lesserrors in interpolation.

Tunis City is surrounded by several mountains such as JebelAmmar and Nahli in the northwest side, Jebel Ain Krima, Jebel SidiSalah, and Jebel Mergueb to the southwest side and Jebel Boukornin-Ressas in the southeast part (Fig. 2). The study area is characterizedby coastal hills such as the Sidi Bou Saïd and Gammarth hills. The lowzones are occupied by the plains of Mannouba, Soukra, Choutrana,and Mornag. Three depressions are represented by Tunis Lake andthe sebkhas of Essijoumi and Ariana.

PERMANENT STREAM SUPPOSED FAULT STRIKE-SLIP FAULT

REVERSE FAULT NORMAL FAULT SURFACIAL FAULT

ARIANA SEBKHA

ESSIJOUMI SEBKHA

CHOUAT

SIDI THABET

MEDJE

RDA R

IVER

ARIANA

SOUKRA

EL AOUINA

TUNIS LAKE

GAMMARTH

SIDI BOU SAID

LA GOULETTE

HAMMEM LIF

TUNIS LA MEDINA

RADES MAXULA

OUED ELLIL

MIL

IANE R

IVER

MANNOUBA

MOST FRACTURETED AREA, AND

AREA WITH MANITUDE > 4 ON RICHTER SCALE

RECENT DEFORMATIONS

NE-SW J.Nahli-J.Ammar fault

JDAIDA FAULT NS

Ennasar fault

NS Mornag fault

NS M'hamdia

fault

NW-SE J.Nahli fault

TRIASSIC OUTCROPS

0 10 20Km

NO FRACTURATED AREA

0 20Km 10

TUNIS LAKE

ESSIJOUMI SEBKHA

ARIANA SEBKHA

ME

DIT

ER

RA

NE

AN

SE

A

Compact soil, considered as

homogeneous

Heterogeneous and compact

formation

Heterogeneous rock Herogeneous and loose soil

Sandy soil

Muddy soil Recent alluvium

ME

DIT

ER

RA

NE

AN

SE

A

Fig. 5. (a) Tunis City categorization based on lithologic data. (b) Study area categorization b(d) City categorization based on the flooding susceptibility map.

4.3. Slope layer

Slope is an important factor while considering the help ofengineering construction and susceptibility to landsliding (Dai et al.,2001; Süzen and Doyuran, 2004). There is a slope stability problemencountered in the study area (El May et al., 2009). Therefore, theslope layer will only contribute to the microzonation map to helpengineering constructions. The slope map was prepared (indegrees) using the DEM of the study area. The slope layer showsareas with cliffs and high slopes (from 7 to 90 degrees). It is given inFig. 5b.

4.4. Seismotectonic

In Tunis City, recent deformation indices have been observed andstudied (Dlala and Kacem, 2007). These indices are associated withmain active faults such as the NW–SE Jebel Nahli fault, i.e., shifts tosenestral strike–slip fault at the anticline of Jebel Nahli (Fig. 3). Itaffects the chalky encrustations, the Upper Pleistocene red silts and

STREAMNET WORK FLOODING-PRONE AREAS NON FLOOD AREAS

0 20Km 10

TUNIS LAKE

ESSIJOUMI SEBKHA

ARIANA SEBKHA

ME

DIT

ER

RA

NE

AN

SE

A

0 20Km 10

AREA WITH CLIFF AND HIGHT SLOPE

ARIANA SEBKHA

TUNIS LAKE

ESSIJOUMI SEBKHA

GAMMARTH HILLS

SIDI BOU SAID HILLS

RE

TID

EM

RA

NA

ES

NA

E

MIL

IANE R

IVER

ME

DJE

RD

A R

IVE

R

LA GOULETTE

HAMMAM LIF

ased on the slope layer. (c) Tunis City categorization based on the seismotectonic layer.

135M. El May et al. / Engineering Geology 116 (2010) 129–138

the shallow encrustations and pulverized deposits. The normal faultsnetwork with the NE–SW direction at Jebel Nahli-Jebel Amar affectthe most recent Quaternary deposits. El Mnihla Quaternary depositsare separated from the ElMenzeh and ElManar Pliocenemarly hills bythe N–S morphological ramp (Fig. 3). In this locality, a major faultappears on several kilometers affecting the most recent Quaternarydeposits. It is a senestral strike–slip fault where slip indices have beenobserved. In the southbound survey sector, the NS El M'hamdia faultaffects the red silts and the most recent chalky encrustations.

Recent deformation features confirm the importance of actualseismic activity that in spite of its weak magnitude affects recentand the most shallow deposits. This confirms the particularity ofthe Tunisian seismicity showed by Dlala et al. (1994). They provethat the Tunisia area is characterized by very superficial seismicfocuses. The seismotectonic context characterized by a regionalseismicity of relatively high ground acceleration (superior to 0.2 g)(Kacem, 2007) constitute a favorable field for the manifestation ofinduced effects or site effects in case of an earthquake. Soilliquefaction has already been mentioned in the Sidi Thabet zoneduring the 1970 earthquake with a magnitude of 5.1 Richter. Aprevious study (El May et al., 2009) has well demonstrated thatsoils are susceptible to liquefaction under a 0.2-g earthquakeacceleration.

Fig. 6. Water table depth m

4.5. Liquefaction susceptibility layer

The soil liquefaction potential has been evaluated for the studyarea because of its important socioeconomic aspect and its location.Liquefaction susceptibility mapping is carried out using a decisionalflow chart for evaluation of earthquake-induced effects based onavailable data such as paleoliquefaction, geological, groundwatertable depth, seismotectonics, sedimentary features and geotechnicalparameters of soils in particular laboratory testing like grain-sizeanalyses and state parameters. Survey results showed that some ofthese localities are considered as possible sites to soil liquefaction(Fig. 4). Indeed, Quaternary alluvium deposits, paleo beaches andrecent deposits that edge the lake and the sebkha represent the mostsusceptible soils to liquefaction. Quaternary deposits in the East andthe West parts of the studied area are less susceptible to liquefactiondue to groundwater-level deepening and to the relatively old age ofdeposits. Elsewhere sedimentary formations are classified as non-liquefiable as they are heavily compacted and old.

4.6. Flooding susceptibility layer

The flood susceptibility layer of the study area was examinedbased on the stream network amplitude combined with the elevation

ap of the study area.

136 M. El May et al. / Engineering Geology 116 (2010) 129–138

factor. The potential flood-prone areas considered are areas with aslope value less than 2° (Fig. 5d).

4.7. Water table depth map

Groundwater is one of the main factors affecting the stability offoundation excavations as well as the implementation of theexcavation works. In liquefaction assessment, the position of thewater table within noncohesive sediments should also be known. Thelithological units exposed in the study area formed shallowunconfinedaquifers due to the abundance of alluvium type soils. The static-water-level layer was prepared by considering the highest elevation of thestatic water levels from the available geotechnical boreholes andpiezometer data.

The groundwater level is less than 3 m in the recent Holocene areaoccupying the plain between Sebkhet Ariana and the Tunis Lake. Inthe Mannouba plain, the water levels vary from 0 m to 4 m in theSebkhat Sijoumi proximity (Fig. 6).

The depth of water table map is prepared based on the static levelgiven in the available geotechnical boreholes. The water table layer isestablished. Areas with shallow static level (0–5 m) are consideredthe least favorable, and areas with static level between 5 and 10 m areconsidered favorable. Areas with static levels deeper than 10 m arethe most favorable.

4.7.1. Suitable area for constructionTo create a suitability map to construction in Tunis City,

geotechnical zoning was carried out based on the previous presentedmaps. The approach consists of the superposition and combination ofparametric maps. The analysis is based on the impact of each factor onsoil stability. As an example, in the area with an important streamnetwork and flooding possibility, the suitability to a safe constructionis null. The area located between Sbkhet Ariana and the Tunis Lake,where the groundwater table depth is equal to 0, is not suitable for

Fig. 7. Bearing capacity m

safe construction. In the Ariana locality, the soil may significantlyliquefy in the case of a possible earthquake (Fig. 4), thus, the damagemay become higher in those regions. It is necessary to takepreventative measures in this region that is not appropriate for safeurban extension.

Combining all the cited stability factors affecting Tunis City, wedistinguish some zones of unsuitability caused by the structuralinstability and the lithological and seismotectonic aspects of the area.Careful analysis of the resulting map shows that perilous zones arelocated in the Quaternary formation and surrounding lakes. Thesezones are characterized by lower cohesion. The area with landslidingand important stream network can be exposed to the flooding effectand is hazardous for a safe urban construction.

The final stage of the geotechnical mapping involved theincorporation of the bearing capacity (Fig. 7), given from geotechnicaltests interpretation, to finalize the zonation maps in the investigatedarea.

The intention in assessing the bearing capacity is to add aquantitative aspect using the interpretation of geotechnical para-meters. In this case, zoning with respect to the results of thegeotechnical test analysis was accomplished. As can be clearly seenfrom the final geotechnical map (Fig. 8), results are consistent andappear to be more realistic.

The geotechnical zoning map was established considering spatialinformation from the field description and the susceptibility toliquefaction map. The geotechnical characteristics of Tunis City aremapped in terms of bearing capacity. The final geotechnical map isdivided into zones with suitability to a safe urban extension andsuitability to construction (Fig. 8).

4.8. Discussion

In the literature, there are cases of urban geology mapping fordifferent purposes (Campolunghi et al., 2006; Özsan et al., 2007). Such

ap of the study area.

Fig. 8. Final geotechnical zoning map of Tunis City.

137M. El May et al. / Engineering Geology 116 (2010) 129–138

cases do not include a lot of geological, seismotectonic and geotechnicaldata such as liquefaction susceptibility and bearing capacity.

The area of interest is especially a good example for a complexgeology area with important urban extension where planners anddecisionmakers need a useful map that shows geotechnical zoning fora safe urban extension. The available geotechnical data from bore-holes and geotechnical tests are presented in a georeferenced map toshow the most important factor, which is the bearing capacity.

The approach presented for urban geological mapping is based onthe GIS utility. The process applied consists of a multicriterionanalysis. The prepared zoning map is refined while superposing thezoning map to the depth of bearing capacity map to produce the finalgeotechnical mapping. Similar examples use other factors such as SPTresults (Goh and Goh, 2007; Papadimitriou et al., 2007), but in thiswork we integrate all factors that control the building protectionstarting with geology, lithology and closing with the flooding andliquefaction susceptibility. We also provide the application of the GIStechnique, which is a powerful tool for management of such a lot ofdata.

Preparation of layers needs data interpolation. This method givessome errors due to the lack of information. Because of that we tried tominimize error values by using adequate methods even in thepreparation of the DEM of the study area.

In our case of study, the liquefaction phenomenon becomes animportant factor to be mentioned for this disaster effect. Theliquefaction susceptibility map is prepared using all the ultimatefactors that induce this phenomenon. The liquefaction is signaled inmany urban areas (Lee et al., 2003; Krinitzsky, 2005) and is considered

a hazard phenomenon to be taken in consideration for safe urbanextension planning.

Previous discussions were focused in the process of urbangeological mapping and liquefaction susceptibility. There are howev-er, necessities for construction, such a simple zoning in order topreserve the environmental equilibrium in urban areas. A rationalplanning, for sustainable development, starts from this point. Thegeological urban environment mapping is a basis for all rational urbanplanning and development.

A final point is that the geotechnical zoning scale maps used in thisstudy and in similar cases are important to avoid big practical errors inplanning for the future urban extension. However, results of this studydo not lead to any loss of geotechnical mapping utility in safe urbanextensions. Before any local building project, a punctual specific studyis inevitable. The role of such a detailed geotechnical investigation iseither to provide some practice for safety urban project and to amplifythe building security. In addition, the accurate geotechnical testingdata can be used for the geotechnical microzoning of the study area,where a large amount of such data is available.

5. Conclusion

This study demonstrates the capacity of the GIS technique for thepreparation of the geotechnical zoning maps regarding the suitabilityof a safe construction. Advantages of using these tools are easy datamanagement, rapid and effective manipulation and analysis of data,data update and the possibility of producing various maps.

138 M. El May et al. / Engineering Geology 116 (2010) 129–138

In this study, all data are processed by using the multicriteriaanalysis by superposing georeferenced information layers. As aresult, the study area was categorized into four different zones as:(1) hillsides of potential surfacial perturbation risks; (2) a low flatzone with flooding risk; (3) a low zone with flooding and slidingrisk; and (4) a region with mud levels and settlement risk.

The maps prepared using this approach were found to beconsistent with the bearing capacity map. The geotechnical zoningmap prepared is recommended as the final map of Tunis City.

This final map, which is easy to use, is a helpful document tonongeologist planners and decisions makers for safe residential areaextension. Final results can be combined and contribute to thegeotechnical microzonation by adding more geotechnical test results.

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