final report_sakthy selvakumaran

7/26/2019 FINAL REPORT_Sakthy Selvakumaran

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Low Cost Structures inMicro-Hydroelectric Power

Generation

by

Sivasakthy Selvakumaran (SID)

Fourth-year undergraduate project in Group D

2009/2010

" I hereby declare that, except where specif icall y indicated, the work submi tted herein i s

my own ori ginal work."

Signed:

Date:


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Low Cost Structures in Micro-Hydroelectric Power Generation

i

Sivasakthy Selvakumaran Sidney Sussex

Technical Abstract

Motivations and fieldwork

This report performs an analysis, evaluation and systemisation of techniques used in the

construction of micro-hydroelectric systems by Practical Action in Cajamarca, Peru. The systems

make use of the potential energy of water as it flows across the mountainside. Water is diverted

by an intake structure, from a stream or river into a channel. The gravitational force of falling

water drives a water turbine and generator. Practical Action work in some of the most

mountainous regions of Peru. The geography of the area provides many challenges, including

transportation, limited infrastructure and energy supply, and both drought and flooding events.

My time in Cajamarca was split between desk-based research at the Practical Action offices in

Peru, and field survey at micro-hydroelectric system sites. Project objectives and evidence of the

problems that exist were discussed with engineers and local people. A sample of micro-

hydroelectric power sites were analysed including systems of different sizes, and projects under

construction as well as in operation. Work was undertaken in Spanish.

Analysis and conclusions

It was proposed to undertake an analysis of a micro-hydroelectric scheme with a focus on the

intake structure and the channels. The proposed improvements to the civil works would increase

some costs but these are offset through efficiencies in reduced material usage and reduced costs

over the lifecycle of the schemes.

Issues identified with the intake included lack of reinforcing steel combined with low strength

concrete. A program was developed to provide an efficient design of the intake structure and to

check suitability in terms of stability, sliding and other design considerations. The design

program is simple and easy to understand; it allows design variations and provides estimated

costs for the system components. Design options for the intake structure, other than a

reinforced concrete, have been assessed, and developed into alternative solutions for

consideration by Practical Action. These include low cost structures of gabion or masonry walls

that are considered as appropriate technology. These walls have a dual function of acting as the

intake structure and aiding with stability of the adjacent slopes.

Issues identified with the channels included some cracking and the collapse or destruction of the

channels due to slope failure. The channels conveying water to the turbines in micro-


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hydroelectric systems are relatively small and do not pose any significant structural issues. For

these types of systems, the primary purpose of the channel lining is to avoid losses of water by

infiltration and to protect the base and the slopes of the channel against erosion caused by the

flow velocity. In terms of the structural integrity, the thickness of the wall can be reduced to the

minimum within the practical functionality and without running significant risks.

As a direct result of the observations and analysis it was concluded that the slope stability was a

major influence in the effectiveness of both the intake structure and the channels. It was

therefore decided to investigate the behaviour of the surrounding slopes and their effect on the

infrastructure of concern.

Various means to improve slope stability were investigated and some design guidelines are

suggested for appropriate solutions (such as gabion walls). A significant factor contributing to

the slope instability is the build up of water pressures in the soil which exert significant forces on

the structures as well as interrupting the water supply to the turbine. To address this risk, the

provision of land drainage features was considered.

Further work

An important part of this research is the transfer of knowledge. This means that approved and

improved technology can be developed and implemented, avoiding potential risks and improving

reliability of the systems.

A practical guide is being developed in order to clearly illustrate the issues identified through this

research project and mitigation measures. This covers potential design options, early warnings

and prevention of failures from the point of view of the local people who are responsible for the

building and maintenance. A technical report is being produced, in Spanish, with the results of

this research and suggestions for improvements to the micro-hydroelectric works.

If any of the suggestions made prove to be appropriate or worthwhile to Practical Action to trial

or implement, I would be interested in returning to Cajamarca, Peru to help with the

development and assess how the effective they prove to be.


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Acknowledgements

Firstly, I would like to thank ITDG: Soluciones Prcticas (Practical Action) in Peru for hosting

me and providing me with the opportunity to carry out this research. I am especially thankful toGilberto Villanueva for guiding me whilst I was in Cajamarca and to Javier Coello for his

support. I am grateful to all those who welcomed and accompanied me in the various villages

and to the site visits in Cajamarca. I am most grateful for those who hosted and allowed me to

attend their meetings and functions and provided their valuable input.

I owe an enormous debt of gratitude to the staff at PREDES (Centro de Estudio y Prevencin

de Desastres) who hosted me and provided assistance and advice during my entire stay in Peru.

This research was financially supported by an Engineers Without Borders UK (EWB-UK)

research bursary and later affiliated to the EWB-UK Research Programme.

I would like to thank staff at Hyder, Mott MacDonald, Grontmij, and EWB-UK for their advice,

suggestions and input. Also thanks are due to Maccaferri Ltd. (England), GVC and Oasys for

allowing me free usage of their software as part of the analysis carried out under the study.

At Cambridge University, I would like to thank my supervisor, Mr McRobie, for his support,

encouragement and guidance throughout the project.

To everyone who has supported me, in Peru and back home, and to those I met along the way

that I have not specifically mentioned, I am also very thankful.


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Table of Contents

Technical Abstract ........................................................................................................................................i

Acknowledgements .................................................................................................................................... iii

1. INTRODUCTION ................................................................................................................................ 1

2. BACKGROUND ................................................................................................................................... 1

2.1 The Work of Practical Action in Cajamarca ................................................................................ 1

2.2 Objectives .......................................................................................................................................... 2

3. FIELDWORK ........................................................................................................................................ 3

3.1 Methodology ..................................................................................................................................... 3

3.2 Design Issues .................................................................................................................................... 4

4. DESIGN REVIEW AND CONSIDERATIONS ........................................................................... 5

4.1 Total Energy Demand ..................................................................................................................... 6

4.2 Hydraulic Energy Potential ............................................................................................................. 7

4.3 Design of Civil Components to Meet Turbine Flow Requirements ........................................ 8

4.3.1 Intake Structure ........................................................................................................................ 8

4.3.2 Channels .................................................................................................................................... 9

4.3.3 Settling Tank ........................................................................................................................... 11

4.3.4 Pipeline..................................................................................................................................... 12

5. INTAKE STRUCTURE ..................................................................................................................... 13

5.1 Existing Intake Structure .............................................................................................................. 13

5.1.1 Advantages over Complete Reinforced Concrete ............................................................. 15

5.2 Field Observations ......................................................................................................................... 16

5.3 Analysis ............................................................................................................................................ 17

5.3.1 Existing Structures ................................................................................................................. 17

5.3.2 Efficient Design ...................................................................................................................... 18

5.3.2.1 Detailed Design of Larger Scale Intake Structure .......................................................... 18


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5.3.2.2 Development of Analysis Program .................................................................................. 19

5.4Conclusions on Intake Structure .................................................................................................. 20

6. CHANNELS ......................................................................................................................................... 22

6.1 Existing Channel Types ................................................................................................................ 22

6.1.1 Advantages over Traditional Design and Construction ................................................... 23

6.2 Field Observations ......................................................................................................................... 24

6.2.1 Problems Observed with Channels ..................................................................................... 24

6.2.2 Risks ......................................................................................................................................... 24

6.2.3 Infrastructure Vulnerability ................................................................................................... 24

6. 3 Analysis ........................................................................................................................................... 26

6.3.1 Larger Channels ...................................................................................................................... 26

6.3.2 Optimisation of Size .............................................................................................................. 27

6.3.3 Effect of Rapid Drawdown .................................................................................................. 28

6.4 Conclusions on Channels ............................................................................................................. 29

6.5 Recommendations ......................................................................................................................... 29

7. SLOPE STABILITY............................................................................................................................ 30

7.1 Typical Geological Conditions and Problems Encountered ................................................... 31

7.2 Slope Analysis and Rapid Drawdown ......................................................................................... 33

7.3 Potential Solutions to Slope Failure Issues ................................................................................ 34

7.3.1 Reinforced Soil ....................................................................................................................... 34

7.3.2 Soil Nailing .............................................................................................................................. 35

7.3.3 Masonry walls .......................................................................................................................... 35

7.3.4 Gabion walls............................................................................................................................ 36

8. STRUCTURAL DESIGN SOLUTIONS ........................................................................................ 38

8.1 Drainage Considerations ............................................................................................................... 38

8.2 Potential Design Options. ............................................................................................................ 39


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8.2.1 In-situ and Pre-Cast Reinforced Concrete Headwall ........................................................ 39

8.2.2 Sheet Piled Headwall.............................................................................................................. 39

8.2.3 Fabric Formwork ................................................................................................................... 40

8.3 Use of Gabion Walls for an Intake Structure ............................................................................ 40

8.4 Masonry Structures ........................................................................................................................ 41

8.4.1 Mass Gravity Side Walls for Intake Structure .................................................................... 42

8.5 Re-Design of an Intake System .................................................................................................... 44

9. PRACTICAL GUIDE ......................................................................................................................... 46

10. FURTHER WORK ........................................................................................................................... 47

RISK ASSESSMENT RETROSPECTIVE ......................................................................................... 48

BIBLIOGRAPHY .................................................................................................................................... 49

Books ..................................................................................................................................................... 49

Articles and Papers ............................................................................................................................... 49

Internal Reports .................................................................................................................................... 49

Standards ............................................................................................................................................... 50

Images and Websites ............................................................................................................................ 50


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1. INTRODUCTION

Practical Action (known as ITDG: Soluciones Prcticas in Peru) work in some of the most

mountainous regions of Peru promoting micro-hydroelectric power schemes to provideelectricity to these remote populations. In their work Practical Action take the view that, for this

scale of work, it is not necessary to use the same techniques or standards of safety for the civil

engineering structures that are required on large hydroelectric stations. Some innovations have

been permitted to reduce costs. Practical Action would like to better develop the engineering

analysis for the solutions that they have been using so that any potential risks can be minimised.

They are evaluating the themes of safety versus investment under a theme known as, obras

civiles de bajo costo para microcentrales" (low cost civil works for micro-hydro schemes). This

research project investigates civil infrastructure in micro-hydroelectric power schemes built by

Practical Action in the region of Cajamarca.

2. BACKGROUND

The cities and villages of Peru have electricity but the communities that live in the mountains

have few facilities and limited access to services. The geography of the area provides many

challenges. These include difficulties in transportation as well as challenges in how to provide

infrastructure and supply energy in a region prone to both drought and to flooding. Whilst

rainfall in the region is favourable to a wide variety of agricultural production, the lack of access

to electricity severely restricts economic development. A source of electricity could provide

power for domestic lighting and cooking needs; for refrigeration for vaccine storage; for school

classes; and for womens groupsto meet in the evening. It could be used directly for income-

generating activities such as agricultural processes or light industry.

2.1 The Work of Practical Action in Cajamarca

Practical Action promotes small-scale hydroelectric power schemes that generate up to 500

kilowatts of power. Their 'run-of-the-river' micro-hydro systems do not require a dam or storage

facility to be constructed. Instead, water is diverted from the stream or river, channelled it in to a

valley and 'dropped' in to a turbine via a pipeline, which converts the kinetic energy of flowing

water into electricity. These schemes provide low-income communities in rural areas with an

affordable, easy to maintain and long-term solution to their energy needs. This type of


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hydroelectric power generation avoids the adverse social and environmental impacts that larger

hydroelectric schemes cause.

Large projects use a dam and reservoir to smooth out the effects of water level fluctuations

throughout the year. The major adverse impacts of hydroelectric power are social (forced

displacement of people) and environmental (destruction of natural habitats). Large dams raise

underground water levels near the reservoir, which has a significant effect on the surrounding

flora and fauna. Millions of people have also lost their land and livelihoods, and have suffered

because of downstream impacts and other indirect impacts of large dams. Large dams have been

suggested as a major factor in the rapid decline of riverine biodiversity worldwide. This is why

such larger hydroelectric schemes incorporating storage dams are not considered as a potential

solution in this project.

Practical Action require a technical analysis of specific aspects of their system for improved cost

efficiency and reliability. They are interested in the analysis, evaluation and systemisation of the

techniques and criteria that they have developed and adapted over the years.

2.2 Objectives

This research project is based on two civil components of the micro-hydro system, the intakestructure and the water channels. The objectives of this research project are:

1. To better develop the analysis of the solutions implemented, analysing the safety and

integrity of the structures and assessing current designs against technical standards.

2. To analyse the design, materials, dimensions, etc., to determine whether the design being

implemented uses the best available technology.

3. To evaluate the safety and integrity of the designs against costs based on risk & value

assessment.


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3. FIELDWORK

3.1 Methodology

Before leaving the UK to conduct the site works, a health and safety risk assessment was

conducted and an assessment on the nature of the work to be investigated were undertaken (as

outlined in a proposal document I wrote prior to travel). All of the works carried out for this

project have been conducted in Spanish and therefore I have translated all existing documents,

technical information and drawings as required. The final findings produced will be translated to

Spanish before being issued to Practical Action.

Time in Cajamarca was split between desk-based research at the Practical Action offices in Limaand Cajamarca, and fieldwork at micro-hydroelectric system sites; working with local people and

engineers. The sites included both those under construction and those already in operation. A

summary of the sites investigated is provided in Table 1 below:

Table 1: Information on hydro-electric systems investigated in Cajamarca.

Name Location Power(kW)

FamiliesBenefiting

District Province Region

MicrocentraleYanacancha

La

Encaada

Cajamarca Cajamarca 40 110

MicrocentraleChontabamba

Paccha Chota Cajamarca 22 80

Microcentrale ElRegalado

Tumbadn San Pablo Cajamarca 12 40

Central Hidroelctrica

Chicce

Baos del

Inca

Cajamarca Cajamarca Unknown Town of

Cajamarca

The sites were chosen in conjunction with the Practical Action engineer responsible for the

projects. Transport and safety were limiting factors in the range of projects chosen for site visits.

A sample was selected to show systems of different sizes, and to show projects ranging from

those in operation for some years to projects still in the process of being constructed. In

addition to visiting micro-hydro systems designed by Practical Action, a site visit was conductedto look at two intake structures which had been built by the Local Government for comparison


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purposes. These provided examples of larger structures, more expensive and more sophisticated

technically than those designed by Practical Action.

The visit to Peru was followed up in the UK by working with engineering consultancies to study

water retaining structures and construction considerations in the UK.

A few issues of concern in existing systems as well as those in the process of being constructed

were observed. Some of the significant issues have been identified, investigated and suggested

solutions developed throughout this project.

3.2 Design Issues

Following the fieldwork, the general design considerations were determined. Irrespective of the

type of structure chosen for the application, calculations must demonstrate:

Overall stability The whole structure should not sink, float, overturn, slide, or impose

unacceptable loads on the ground.

Element design The reinforced concrete walls/bases should have sufficient reinforcement

and adequate thickness; timber/masonry/other materials should be of adequate strength.

MaterialsConsiderations of whether materials are adequate and appropriate. For example,

whether the concrete strength is adequate and whether materials are appropriate for the given

environmental conditions.


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4. DESIGN REVIEW AND CONSIDERATIONS

It was decided that a complete analysis would be conducted for the civil works for a micro-

hydroelectric scheme. This was done in order to develop a more efficient and appropriatetechnology for the remote regions of Cajamarca. The civil works which are a significant and

critical component have the potential for design improvements and cost savings based on a risk

and value approach (Owen 2009). In broad terms this looks at impacts and the probability of

occurrence and assessing solutions, costs and residual risks.

Figure 1 depicts the outline of a typical micro-hydroelectric scheme constructed by Practical

Action in Cajamarca.The review would start at the intake structure where water is diverted from

its natural course and carried along channels up to the settling tank (which helps to remove

sediment that could harm the turbine). It then flows through a pipeline down the mountainside

to drive a turbine. This in turn drives a generator to produce electricity.

The purpose of this study is to have a better understanding of the current designs with a view to

achieving cost efficiency and system security. The review of the parts of the micro-hydroelectric

scheme would include the adequacy of the water conveyance system and improvements to the

system design based on geotechnical, structural and hydraulic considerations.

Figure 1: Diagram illustrating a typical micro-hydroelectric power scheme designed byPractical Action (image courtesy of Practical Action UK).


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The components of interest are the intake structure and channels. These were investigated and

analysed for all of the sites visited in Peru. The analysis in this section of the report concentrates

on two systems designed by Practical Action: one larger scheme and one smaller, the

Chontabamba 22 kW Pelton Turbine and the Yanacancha 40 kW Pelton Turbine, respectively.

The following outlines the process of checking the suitability of the system for Chontabamba

Micro-Hydroelectric Power System. The system at Yanacancha was also analysed in a similar

way (but not included in the body of this report as the methodology is the same).

4.1 Total Energy Demand

Before a system can be designed, the estimated energy requirement must be determined. All of

the energy demands of these rural villages will not need to be satisfied simultaneously, for

example the industrial consumption for a zone of little development will have a usage of nearly

nil during the nights. Practical Action have developed calculation methods in order to make this

estimation. The procedure is outlined in a Practical Action manual for the construction of

micro-hydroelectric power schemes (Davila, C. et al. 2009).

The following excerpt (Table 2) is taken from the technical profile produced for the

Chontabamba Micro-Hydro System which is a document produced by Practical Action (ITDG2009) to calculate the size of the turbine, and hence size of the overall system, required to serve

the Chontambamba population.

Table 2: Determining the energy requirement of Chontabamba.

TIPO DE DEMANDA(TYPE OF DEMAND)

Demanda Diurna(Daily Demand)

Demanda Nocturna(Nightly Demand)

f.s. f.u. f.s. f.u.

DOMSTICA (DOMESTIC) 16.00 kW 0.20 0.50 1.60 0.80 0.70 8.96ALUMBRADO PUBLICO

(PUBLIC LIGHTING)1.28 kW 0.00 0.00 0.00 1.00 1.00 1.28

INSTITUCIONAL(INSTITUTIONAL)

6.00 kW 0.70 0.70 2.94 0.40 0.50 1.20

INDUSTRIAL (INDUSTRIAL) 8.00 kW 0.80 0.80 5.12 0.40 0.40 1.28

9.66 12.72

f.s = factor de simultaneidad (simultaneity factor)

f.u = factor de uso (usage factor)

Mxima demanda diurna actual: 9.96 kW (Maximum actual nightly demand)Mxima demanda nocturna actual: 12.72 kW (Maximum actual daily demand)


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These figures were calculated by estimating the daily and nightly demand separately and

comparing which of the two values is the maximum energy demand. Practical Action anticipate a

10% loss in energy in the electrical transmission system, and a 40% increase in predicted usage

requirements over the 15 year design life for this particular scheme. Hence, the demand

becomes: 12.72 x1.10 x1.40 = 19.59 kW.

4.2 Hydraulic Energy Potential

At Chontabamba, the Ramosmayo stream passes through the right side of the town, at an

approximate distance of 200 metres and the Suro stream passes through the left side at a greater

distance. The Suro stream eventually joins the Ramosmayo stream. The streams originate in thewilderness at altitudes between 2600 and 3200 metres.

The consistency of the flow of the streams throughout the year has been measured by Practical

Action engineers by procedure as outlined in a Practical Action manual (Davila, C. et al. 2009).

This ensures that the system is able to function throughout the year. Flow measurements made

by Practical Action indicated that the flow rate during the months of lowest flow is between 0.06

and 0.08 m3/s, with an average during the months of low flow of 0.07 m3/s.

Electrical power is generated as the water falls from the settling tank down to the turbine house

in the pipeline, converting the drop in potential energy into kinetic energy to drive the turbine.

The approximate height of elevation through which the water needs to fall was found to be 58

metres, using the inclinometer and measuring tape instrumental method; thus the electric power

to be generated is 22.33 kW as estimated using Equation 1:

= ....

where = hydraulic efficiency of the turbine; = density of water (1000 kg/m3);

g = acceleration due to gravity (9.81 m/s2); Q = volume flow rate passing through the turbine

(m3/s); H = effective pressure head of water across the turbine (m).

This generated power of 22 kW is assumed to be available at all times with a flow rate of

0.07 m3/s passing through the turbine. Consequently the structures, pressure pipe and the

mechanical and electrical components would be designed to achieve this design capacity. This isa suitable design value as it is above the power requirement of 19.59 kW with a factor of safety

(Equ. 1)


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but not too far above so as to require enlarged structural, mechanical and electrical components,

which would incur unnecessary additional costs. With the conditions of flow rate and elevations

defined, the components of the micro-hydro system would have to be designed with

characteristics of acceptable efficiency to achieve the required power of 22 kW. In this example,

the components would be designed such that the minimum water flow rate is 0.07 m3/s.

4.3 Design of Civil Components to Meet Turbine Flow Requirements

4.3.1 Intake Structure

The inlet structure should be located in the appropriate location of Ramosmayo River (just after

the Suro River merges with it to access maximum available flow), and should consist of raising

the necessary barrier height to permit the bypass of the design flow towards the transmission

channel to the settling tank by constructing a barrier perpendicular to the direction of flow.

Flow diverted into the channel would be controlled by a hand-wheel operated weir or sluice gate

to regulate the required flow of water. The specified sluice gate size of 0.50m x 0.40m was

determined to be appropriate.

As part of this initial investigation the discharge over rectangular, trapezoidal and V-notch weirs

were undertaken. Figure 2 below summarises these shapes used for which I calculated flow rate,

Q, based on the value of water depth, h(as marked on Figure 2). Flow rates were estimated using

data for the Practical Action systems.

The size and dimensions of this intake structure are dependent on the width of the river bed and

the slope conditions either side of the intake structure. These specific issues are investigated in

greater depth in Section 5.3 of the report.

Figure 2: Weir Shapes


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4.3.2 Channels

The dimensions and hydraulic characteristics of the channels need to be such that the channels

are able to conduct a specified minimum flow. This flow requirement at Chontabamba has been

calculated as 0.70 m3/s. The calculation of the net elevation head is also required. This is the

vertical distance between the discharge level at the settling tank and the micro-hydroelectric

station at which point the diverted flow is returned to the stream/river.

The normal water level in the channel (the water level at some distance downstream of the intake

structure) will depend on the flow and the properties of the channel. The key properties of the

channel will include slope, roughness coefficient and the geometric properties. The design of the

intake structure will maintain a reservoir for continuous feed to the micro-hydroelectric powerstation via the channels.

The water in the channel is being carried along the side of the mountain slopes, though flows at

a gentler slope from the intake to reach the settling tank before it flows downhill to drive the

turbine and rejoin the main flow. Power is derived from the kinetic energy as it flows downhill.

Figure 3: Open channel parameters as specified on a trapezoidal channel.

The Mannings equation for uniform flow in an open channel (Chow 1959) is written as:

= 5

3 .

.2

3 = 1

where Q = design discharge of the channel (m3/s); A = cross sectional area of flow (m2); S0=bed

slope of the channel; n = roughness coefficient of the channel section; P = wetted perimeter

(m); = energy correction factor.

Mannings Equation (Equation 2) is used to calculate the normal depth of flow in the inlet

channel (where the water depth does not change in the direction of the flow) which would

(Equ. 2)


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facilitate the determination of the sluice/weir gate level required at the intake structure and hence

the overflow into the channel to the settling tank.

In view of the channel bed material (sandy soil) any critical areas are required to be lined with

concrete (or other impermeable material). This is to prevent loss of water by infiltration and to

avoid the channel being undermined due to water washing away the sand. In some critical zones

PVC pipe of 300 mm diameter would be utilised.

Though there are regions of rectangular shaped channels and below ground pipelines, the

majority of channel sections installed are trapezium in shape, lined or unlined earth channels.

The parameters for rectangular, triangular and trapezoidal open channels (as defined in Figure 3

for a trapezoidal channel) were substituted into Mannings equation, re-written in terms of flow,and the heights versus the flow rate plotted to gain an appreciation of the open channel

behaviour and the suitability of existing designs.

Figure 4: Graph to plot how the discharge rate is affected by water level depth in atrapezoidal channel.

This is calculated based on Mannings equation. For this particular example, the channels weremodelled to be concrete lined

(n=0.015), with a channel depth of 2.5 m and side slopes of 1 in 1. The slope of the channel was modelled with the average

slope value at Chontabamba of 6 in 1000.

Calculated and plotted by author.

0.000

10.000

20.000

30.000

40.000

50.000

60.000

0.00 0.50 1.00 1.50 2.00

Flow

Rate,Q(m3

/s)

Water Depth, h (m)


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4.3.3 Settling Tank

Typical settlings tanks in these schemes (example pictured in Figure 5) are of concrete

construction that would receive the water flow at the required rate and allow any sediment to

settle at the bottom in order to avoid any damage to the mechanical equipment in the turbine

house. The discharge pipe is set a level higher than the base slope to allow the sediment to be

retained. Routine maintenance would be required to clean the settling tanks. The inlet and

outlet would be designed to minimise turbulence and to avoid any air entrainment and sediment

into the pipeline.

Figures 5 and 6: System at Chontabamba

Figure 5: Settling Tank Figure 6: Pipeline leading from settling tank intoturbine house, held in place by thrust block.

Further to consultation with Practical Action staff, it was decided that this structure would notbe examined in detail during this research project. Engineering design guidance has already been

produced on this structure by Practical Action. This allows for a focus in greater detail within

this project on the two elements of greatest interest to Practical Action, the intake structures and

the water channels.


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4.3.4 Pipeline

This pipe carries the water from the settling tank down to the turbine. The pipeline and the

thrust block (used to prevent the movement of the pipe) can be seen adjacent to the building

that houses the mechanical and electrical equipment in Figure 6.

The following profile was developed from information collected for this project with the

assistance of other Practical Action engineers. It models the slope from the settling tank to the

turbine house and shows the pipeline along the longitudinal section.

Figure 7: Slope and pipeline between settling tank and turbine house at Chontabamba.

Profile drawn by author.

Pipe calculations were also undertaken based on the existing tube profile to see that the flow at

the bottom end of the pipe would be sufficient to drive the turbine. This included estimating

friction losses and using hydraulic design charts. It was found that the pipeline currently in placeat Chontabamba was fit for purpose.


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5. INTAKE STRUCTURE

5.1 Existing Intake Structure

The intake structure is used to divert water from the main river flow into channels that carry

water along a different path to that of the natural flow. As the natural river course flows

downhill, this diverted water can be maintained at a higher level, creating a height potential.

The region experiences periods of both drought and heavy rain. In the case of drought, the

construction of a barrier of appropriate height is necessary to secure the supply of water for

power generation. However, a barrier that provides this requirement but can also support the

passage of water in the river during rainy periods would result in a large infrastructure. This wasconfirmed by my calculations.

Practical Action make use of an intake structure of concrete and wood mixed type barrier. This

can be implemented at a low cost. The intake structure consists of two side walls either side of

the stream. The required quantity of the water is diverted into a channel by means of a manually

operated sluice/weir gate on one of these side walls and by a series of wooden stoplogs that can

be placed perpendicular to the flow of the river to hold water back.

The removable stoplogs are used to maintain water level upstream of the barrier in rivers with

moderate slopes between 1 and 2%. For small streams, the stoplogs are slotted in rebates

perpendicular to stream in concrete walls either side of the stream and base, forming the opening

(see Figure 8 below). In large streams and rivers, structures are much wider with the opening

formed by concrete columns and base.

In the dry season, it functions with the interlocking stoplogs in place up to a height to allow

sufficient build up of water level for draw-off, and during the rainy season the wooden stoplogs

can be removed as necessary so that the flow can be regulated. This system also facilitates the

removal of material accumulated at the barrier as the stop logs can be removed, and material

transported down the river without the need to dredge upstream.

This technology consists of designing channelling walls and base made of concrete of 140 to 175

kg/cm2(approximately 1417.5 MPa) strength, which is made up of cement, sand and gravel,

mixed with 25 - 30% large stone by volume.


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Figure 8: Overhead schematic drawing by ITDG of Chontabamba

intake structure with translations annotated.

No stoplogs are in place in these intake structures.

Figure 9: Photograph of Chontabamba

intake structure (one of the smaller

intake structures).

Figure 10: Photograph of Yanacancha

intake structure. Height of side wallseither side of intake is about 2m above

ground.

Figures 8, 9 and 10: Intake Structures


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5.1.1 Advantages over Complete Reinforced Concrete

Reduces the cost by well over 35%, both in terms of materials and workmanship with

reduced concrete, formwork and reinforcement.

Facilitates cleaning of the accumulated sediments. The wooden stoplogs that make up

the barrier are simply removed and the river flow is used to remove the sediments.

It is a simple technology, easy to maintain and can be operated by the rural population.

Reduces transport requirements, due to reduced use of cement and aggregates.

In case of flooding, the wooden stoplogs serve as a fuse. When a flood occurs, it is possible that

this action will break the wooden boards of the barrier allowing the river to move freely,

preventing flooding or damage to the intake and other components of the micro-hydro system.Alternatively, the stoplogs can be removed as required during rainy season.

Figure 11: Intake structure at Chontabamba

Despite being only a couple of years old, there is quite significant concrete damage. The effects of scour are already visible at

the bottom edges of the structure.


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5.2 Field Observations

A large number of cracks were observed in the intake concrete structures (as well as other

structures and components such as channels, settling tanks, etc.). A number of factors could be

contributing to this problem, including differential settlement, ground movements, thermal

cracking due to inadequate reinforcement and the use of low strength concrete (14-17.5 MPa)

for structures.

Some of the issues with the current design

have been identified from the site visits,

and several others are evident from the

detailed review of the design drawings.One noticeable feature is the distinct lack

of reinforcement. There is more

reinforcement present in the larger intake

structures; however this is still insufficient

by British Standards. Furthermore, the

reinforcement used is not placed in the

appropriate locations for the

reinforcement to be effective. For

example, by placing it at the bottom of the

structure with little or no cover means that

the reinforcement will be subjected to the

effects of the environment and are likely to

corrode. If this occurs, it will not serve

any useful function as part of the structural

member in providing adequate strength or

preventing any thermal cracks.

Based on the initial review, it was determined that the design of the intakes structure and

channels would be carried out in accordance with the British Standards (primarily BS8110 for

concrete, and BS8007 for water retaining structures and earth pressures acting on below ground

structures in accordance with BS8002) to have an understanding of what considerations are vital

in carrying out the design of such structures. Furthermore the regulations and standards of

safety practised in the UK would be considered together with any specific requirements that

Figure 12: Severe cracking in structures in

Chontabamba.

British Standard BS8007 states that water retaining structures

should have a maximum crack width of 0.2mm. Cracks seen

here were measured at around 1mm.


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apply in Peru. There is no guidance available at present to aid in the design of such intermediate

technology of small scale with alternative design and construction methods. The challenges and

concerns present in designing larger scale structures are greater, making the design of intake

structures and channels for larger scale hydroelectric power schemes worth examining closer in

order to avoid any potential for catastrophic failures.

5.3 Analysis

5.3.1 Existing Structures

Analysis of the structures in use in Peru has been undertaken based on guidance provided by

British Standards, technical notes and design manuals currently used by practising engineers.

The designs currently used by Practical Action have tried to use more innovative designs and

construction methods and low cost technology. They had not, prior to this investigation, been

designed or checked to ensure their safety and security. Having analysed the structures it can be

seen that they have not been adequately designed to cope against certain failure mechanisms, for

example, overturning, flotation and sliding.

Hand calculations determined that the overturning stability was satisfactory and the risk of

failure by sliding was very low in all cases. There were, however, several other concerns to

address (as outlined in Section 5.2).

Limitation of thermal and shrinkage crack widths (typically to 0.2mm as stated in BS8007) would

need to be undertaken. This could be done by providing small bar reinforcement at small

regular centres. In this country, meshes are considered good for this type of use (e.g. A252 or

A393 meshes) though the availability and costs are likely to mean that this is not a suitable

option for the micro-hydroelectric schemes built by Practical Action in

Cajamarca. Reinforcement is generally needed in both faces for sections greater than 250mm

thick. Minimum cover to reinforcement should typically be 40mm, subject to the environmental

conditions.

In the UK, virtually every reinforced concrete structure designed uses a cement mix of 50% each

of Ordinary Portland Cement (OPC) and Ground Granulated Blast Furnace Slag (GGBFS). This

is cheaper that 100% OPC, has better sulphate resisting properties and means that less

reinforcement is needed to prevent thermal cracking, the only downside being that the concrete


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has to be cured for longer while it gains strength. This would be recommended if GGBFS is

available at an economical cost in Peru.

There should be a limit on water/cement (w/c) ratio of 0.55. A lower water-cement ratio leads

to higher strength and durability but may make the mix more difficult to work with. Too much

water will result in settling and segregation of the water, cement, sand and aggregate

components. Also, mix with too much water will experience more shrinkage as the excess water

leaves, resulting in internal cracks and visible fractures which will reduce the final strength.

Problems are commonly caused by site workers ignoring this limit and putting more water in the

mix to make the handling easier. Concrete should be mixed and cast on a low permeability

surface, otherwise some water will leach out. In this case, more water will need be added to make

the mix workable and control over the w/c ratio would be lost. This is another possible cause

for the cracks mentioned in Section 5.2.

5.3.2 Efficient Design

The existing structural designs of concrete intake structures appear to be practical, although they

could certainly be improved in order to achieve an acceptable standard based on a risk and value

approach. This improvement is likely to increase the cost of the project by increasing the amount

of reinforcement and increasing concrete quality. However this could potentially be offset by a

more efficient design to reduce material costs and lessen maintenance requirements (lower life

cycle costs). This includes, for example, making thinner walls so that a smaller volume of

concrete is required. This would allow the costs saved to be used for other items such as more

reinforcing steel.

5.3.2.1 Detailed Design of Larger Scale Intake Structure

The full detailed design was carried out for a potential intake structure. This design was in

accordance with the British Standard design and resulted in a much greater amount of

reinforcing steel than those used in the schemes designed by Practical Action in Peru. The

structure was designed to be about 1.5m above the river bed and extending into the slope and

below river bed level. This will then be the same scale as the Practical Action designed intake

structures.


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Figure 13: Drawings produced for the detailled design of a reinforced concrete intakestructure.

These drawings are some examples and include the corner reinforcement detail, a plan view and a schematic explaining someof the reinforcement.

Drawings by author.

5.3.2.2 Development of Analysis Program

Using the considerations highlighted in the detailed design, it was decided to develop a program

that could very quickly design parts of the intake structure and check their suitability in terms of

stability, sliding and other design considerations. Though there are software packages readily

available on the market to design such features, the program that I developed has the Practical

Action micro-hydro schemes in mind and focuses on creating side walls of the intake structure

that also act as retaining walls to stabilise the slopes (parts of which can be seen in Figure 14

below).

The main advantage is that this program allows the user to input and vary the dimensions, and

rather than simply presenting an output as to whether or not the input meets various structural

safety criteria, it has the equations and calculations used to provide this output quite clearly


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displayed. Thereby the user is able to follow these calculations through and develop an

understanding of how the design process works.

The program then displays the rough estimated cost of components of the system. A key part of

the focus of this program is to develop structures of low cost, and I created a section of the

program to be used to see whether a structurally stable yet economic solution can be achieved

when using reinforced concrete compared to the current designs.

The initial idea was to use a general quantity surveyors guide to provide a cost estimate; however

the costs of materials in the local area of the Province of Cajamarca will of course vary from the

pricing and materials used here in the UK. Instead it was thought to be appropriate to pro rata

the data for the Practical Action structures that have been reviewed. This feature has beenincluded in the program, but will need to be reviewed by Practical Action Engineers in

Cajamarca to provide more accurate cost values based on what is available in the local region.

5.4 Conclusions on Intake Structure

Many of the problems identified in these structures were as a result of insufficient amount and

poor usage of reinforcing steel combined with low strength concrete. The costs of the structure

could be reduced by designing the structures to require less concrete.

It was found that, in most cases, the resulting costs still ended up being higher than those of the

intake structures actually constructed. It should be kept in mind that the cost values calculated

are not confirmed. Despite the decrease in concrete required by a more efficient design, the

increase in cost tends to come from the increase in reinforcement steel that these structures

should have.


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Figure 14: Example sections of intake design program developed by author.


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6. CHANNELS

6.1 Existing Channel Types

The method of the trusses described below for the lining the channels with concrete permits an

important reduction of costs. This is due to the decrease of the thickness of the lining down to

between 5cm and 7.5cm, depending on the cross section of the channel. This permits an

important saving in materials in that there is less concrete and no formwork. The method

consists of placing trusses at required intervals, with straight runs at long intervals and curved

zones at short intervals. The channels are then lined with concrete.

These trusses are then removed and the gaps are filled with materials suitable to serve asexpansion joints, thereby avoiding formation of cracks in the channels. Currently, many

governmental and private organizations are using these lined channels for other uses, mainly in

small and medium irrigation works.

This technique used to construct channels is simple and permits the employment of unskilled

labour as compared to the more skilled labour required for using formwork.

Figures 15 and 16: Channels

Figure 15: Trusses being placed beforeconcrete is applied to earth channels.(Photograph courtesy of Practical Action).

Figure 16: Complete concrete lined channels.(Photograph courtesy of Practical Action).


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6.1.1 Advantages over Traditional Design and Construction

The advantages of the trusses over formwork (shuttering) for this scale of channel is summarised

below:

Allows more flexibility and facilitates the work in curved and straight sections with ease.

Allows concrete thickness down to 50mm, thereby reducing the amount of concrete

required. With the formwork method, a minimum thickness of 150mm is required to

vibrate the concrete for adequate compaction. As there is no electricity available in these

remote regions, a portable generator would be required which would incur further

expenses, and so a process that does not involve electrical processes is ideal.

Reduces the quantity of wood by approximately 80% as the need for formwork is

eliminated.

Reduces the use for materials for the placement of expansion joints (asphalt, sand) by

50%.

Eliminates the labour requirement for fixing and removal of formwork.

Allows the finishing of the slopes and the floor (fair, rough or rip-rap finish) on the same

day or almost immediately without having to remove the formwork.

Good workmanship can be achieved with the local labour available with better efficiency.

Allows reduction of raw materials (concrete, stone, sand, timber and other), resulting in

lower cost of transportation of materials and reduces efforts for its attainment, especially

in remote areas with difficult access.


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6.2 Field Observations

6.2.1 Problems Observed with Channels

Based on observation, only the critical sections are lined with concrete due to a lack of availablefunding. Therefore, there is a potential for loss of water by infiltration into the ground due to

channel cracking and channel erosion. The existing concrete channel sections have no provision

for pressure relief to avoid issues with for flotation.

Several large cracks were noticed in existing channels. Though this poses no risks to structures

of such small size in terms of safety and structural integrity, these cracks lead to significant losses

in water being transported towards the turbine.

6.2.2 Risks

It has been established that the structural integrity of the channels themselves is not a key

concern for small works; however they are subjected to other risks.

Landslides, soil creep and to a lesser extent the falling blocks of earth are the main risk factors

that threaten the channel. These phenomena occur more frequently during the rainy season. It is

also the case that the local population, intent on improving their pastures, have a tendency to

over-irrigate the land. This exacerbates these risk factors as they further saturate the soil during

the rainy season.

6.2.3 Infrastructure Vulnerability

The channel is the structural the most vulnerable structural component as channels often run up

to a few kilometres through fields of various unstable conditions, susceptible to phenomena ofinstability such as landslides and soil creep, as well as erosion of slopes by river scour.

The Yanacancha Micro-Hydro system can be used to highlight typical problems encountered

with channels. In the first 50 metres from the intake, the channel does not present any major

problems. Then, as it crosses a stream with a culvert the channel has accumulated sediment and,

in combination with the falling earth from adjacent slopes, has a reduced channel capacity. In

this same section the ground appears to be saturated with water, resulting in an increased lateral

pressure of soils on the channel side. This causes sliding as well as a moment which may exceed

the available moment resistance capacity, resulting in channel collapse. This prospect is even


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more likely to occur when these channels are not reinforced concrete, which would have a

greater moment resistance capacity. The steel bars in the reinforced concrete would be able to

carry some of the tensile forces which mass concrete is unable to carry, as well as some of the

compressive and bending forces.

In the next 150m of the channel, it is affected by landslides and soil creep. These phenomena

generate a lateral thrust on the channel, the effect of which can be seen by the breaking and

settling of the channel in several sections. Some of these breaks have been repaired very simply

with a patch, however this is not a long-term solution as the soil sliding process continues and is

likely to become acute in the following rainy season (Figure 17). Where then channel has

developed cracks, a significant quantity of water is lost by seepage into the ground. These

conditions are aggravated by an accelerated ground settlement process.

The figures above illustrate some of these failure examples and critical conditions. For example,

the yellow dotted lines in Figure 18 show the presence of faults in the slope due soil creep. This

results in blockage and in some cases destruction of the channel.

Figures 17 and 18: Slope Movement

Figure 17: Indication of movement of slope

into channel path A cover over the open

channel has been added to prevent the flow in

the channel from being cut off.

Figure 18: The movement of sections of the

slope material towards channel is visible here,

as marked by the dotted lines.


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Figures 19 and 20: Channels

Figure 19: Slope collapse at Yanacancha. Thislead to the destruction of channels.

Figure 20: Slope collapse at Yanacancha. Newchannels were dug into the earth and covered

pipe put into place at critical sections. Pipecould not be installed along the entire length ofchannel due to lack of funding.

6. 3 Analysis

6.3.1 Larger Channels

The full detailed design was carried out for a large channel section to have a good understanding

of the design concept prior to looking at the relatively small channels that are part of the

Practical Action micro-hydroelectric power schemes. The channel was designed to be five to ten

times larger than those used in these micro-hydro schemes (about 3m in depth) with reinforced

concrete and without any drainage features. It was noted that a large amount of concrete would

be required to negate the flotation effects caused by ground water pressure. To overcome the

flotation, a means to relieve these pressures must be considered in the channel design. However,

the flotation was found not to be an issue with such small channels as those used in these micro-

hydro schemes.


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6.3.2 Optimisation of Size

The size channels without any reinforcement or other structural considerations will be limited.

For example, a large trapezoidal unreinforced channel with unstable side slopes can collapse,

thereby resulting in the channel being destroyed and the system security will not be guaranteed.

The graph below (Figure 21) shows the results of the examination of a general unreinforced

concrete trapezoidal channel. It shows how large a concrete channel can be at given channel

depths, varying the angle of the trapezoids side slopes and the thickness of channel wall. This is

calculated irrespective of pressure relief, drainage, or other devices.

It was assumed that the channel is built on top of a porous material (e.g. sand backfill) and that

that the trapezium is hinged at the base corners. An analysis has been carried out to determine

the maximum height that an empty channel can have such that the channel does not float away

or have the sides collapse.

Each line on the graph depicts a different depth, d, and the thickness of the channel is plotted

against the angle, , of the side slopes to the horizontal.


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Figure 21: The relationship of varying channel depth, side wall angle and thickness for anunreinforced concrete trapezoidal channel for different channel depths.

Graphs produced by author using Matlab software.

6.3.3 Effect of Rapid Drawdown

A key concern with water-carrying structures is the build up of excess pore water pressure. This

can cause an uplift pressure in channels resulting in failure during rapid drawdown of the

channel water level.

Embankments may become saturated by seepage during prolonged high water levels. If

subsequently the reservoir pool is drawn down faster than the pore water can escape, excess pore

water pressures and reduced stability will result. For the purposes of this analysis it is assumedthat drawdown is very fast, and no drainage occurs in materials with low permeability.

Calculations were also undertaken to model the limits of a general trapezium channel with

varying channel depth as affected by rapid drawdown.

Channel depth

increasing from 0.5m

to 10m


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6.4 Conclusions on Channels

In small works, as is the case of the micro-hydroelectric systems and especially in those where

the flow rates will be relatively small, the channels do not bear significant ground forces or the

forces imposed are negligible for the calculation of resistance. For these types of systems, the

primary purpose of the channel lining is to avoid losses of water by infiltration and to protect the

base and the slopes of the channel against erosion caused by the flow velocity. In terms of the

structural integrity, the thickness of the wall can be reduced to the minimum within the practical

functionality and without running significant risks.

The channel is one of the most vulnerable parts of the system as it is at high risk of collapsing

due to the effects of landslides, soil creep and lateral displacement by saturated and waterlogged

terrain sliding downhill.

6.5 Recommendations

Based on the evaluation of the channels, my recommendations can be summarised as follows:

1. Implement land drainage to reduce the build up of groundwater that appears to make the

ground unstable. This is generally achieved by excavating selected regions of ground and

filling it with drainage material such as gravel such that it leads to a pipe to carry the

water away. As this may involve constructing drainage trenches and ditches on land

owned by the local population, any implementation would require discussion with them

and could only take place with their consent and understanding of the benefits.

2. Adjust the irrigation system in pasture lands with appropriate technology such as drip

irrigation or spraying, so that the moisture content of soils can be controlled.

3. Evaluate the options for changing the channel liner or designing an alternative water

conveyance system for the critical sections such as a flexible structures/ pipes that adapt

to deformations of the soil. Use of PVC pipe for conveyance of water is an option. Thisis currently implemented in sections where slope failure has occurred, but the use of

open section channels with design modifications could also be considered.

4. Design the works to implement security and protection for the channel at critical

locations. In order to prevent collapse, the slopes affected by erosion of the river could

be stabilized. For lands affected by landslides, land drainage can be implemented.


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7. SLOPE STABILITY

The brief from Practical Action was to

investigate the structural considerations ofthe intake structure and channels; however,

from the work carried out it can be

concluded that these factors could not be

isolated form the effect of slope behaviour

(examples of which can be seen in Figures 23

and 24 below). It was therefore decided to

investigate the behaviour of the surrounding

slopes and their effect on the infrastructure

of concern.

Slope failure has the potential to destroy

parts of the channel system following a

period of heavy rainfall. There is evidence of slopes failure and the concrete lined channels

being destroyed. Threats to existing structures exist in many regions due to the susceptibility of

the landscape to erosion and landslide. For example, in Figure 22 of the intake structure, slope

erosion and past slope failure can be seen. Similar slope failure in the future could potentially

lead to damage to the intake structures.

The channel and intake structures are some of the most vulnerable components of the micro-

hydro system, due to exposure to various hazards that occur in this region such as landslides, soil

creep, scour due to the river and erosion of slopes that support the channel and falling rocks.

The risk of failure of the micro-hydroelectric system is high due to likely break-up, settlement

and collapse of the channels and intake in such sections.

These factors could result in collapse and total disruption of hydroelectric power for all users. In

some cases it also affects farmlands that depend on these transmission channels to provide water

for agricultural activities.

Figure 22: Chontambamba intake

Notice adjacent slope erosion.


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Figures 23 and 24: Slope Failure

Figure 23: Slope failure that affects the base of

the channel. Earth is fractured in blocks.

Figure 24: Channel path affected by slope

failure.

7.1 Typical Geological Conditions and Problems Encountered

Similar issues with regards to slope stability are encountered throughout the Cajamarca region.

Though the specific geological conditions are subject to some variation, they are quite similar in

most cases and the study on Yanacancha Micro-Hydro system is provided here to illustrate a

typical scenario where problems occur. This information was collected in the field, from

speaking with local people and Practical Action engineers and from looking at previous studies

(Rengifo, J. M. 2008).

In terms of morphology, the area is located in the middle-lower part of the river basin which

drains the Llaucano River. The valley presents mountainsides of moderate slope that are more

rugged towards the lower regions and provided with good vegetation cover consisting primarily

of grasses and some pine, eucalyptus and queuales.

The area where the micro-hydro structures are located is predominantly land dedicated to

livestock grazing and related agricultural production. Towards the downstream end of the system

where the settling tank and the turbine house is located, the terrain becomes more rugged with

steep slopes. There are no signs of increasing instability, except in the first few hundred metres

of channel leading from the intake structure. This section of channel passes through pasture

land that is oversaturated with water due to lack of proper drainage. This leads to the ground

becoming unstable.

Settling tank, turbine house,

etc. further along


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Figure 25: Aerial view of part of the Yanacancha River Basin.

Photograph courtesy of Google Maps. Annotations by author.

The geological characteristics of the land are varied; the zone around the intake structure is

composed primarily of sedimentary rock formation composed of limestone, marl and fine clays.

The composition leads to fairly unstable soils that slide easily under the influence of excessive

moisture from infiltration and uncontrolled flood irrigation applied to the grasslands.

On the other hand, the predominant climate conditions in the area are divided into two marked

stages. Between the months of November to April there is high humidity and intense rain, the

rivers reach substantial levels and lands are prone to sliding. Between the months of May to

October there are periods of drought, during which there is a reduction in the water supply

available to drive the turbine to produce electricity.

The geotechnical instability is associated with changing environmental conditions, above all the

saturated ground conditions during the rainy season. Currently there is very little being

implemented in order to address this issue of slope stability, apart from a few minor rock walls

put in place after problems have arisen. This highlighted the need for further investigations of

slope behaviour and design work to suggest alternative methods, in order to prevent such failure.


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7.2 Slope Analysis and Rapid Drawdown

While the development of deep-seated failure surfaces is possible, the more common sight

viewed in site survey was in the form of relatively shallow slope failures. If left unattended these

lead to the gradual deterioration of the slopes either side of the river channel, and eventually the

collapse of the channel.

The rapid drawdown scenario is one of the most severe loading conditions that earth slopes can

experience. Flood events can leave water levels high in rivers and drainage channels for

significant periods of time and then drop relatively rapidly once the floodwaters recede. The

effect of this inundation on the soil in the slope, both prior to and subsequent to drawdown, is

the idea of the rapid drawdown loading condition. To model this loading condition, the soil

strength and pore pressure development in the slope must be considered.

It was determined that the slopes for the systems being investigated would be analysed using the

Simplified Bishop Method. Oasys Slope (produced by the software house of Arup) was used to

analyse and understand some of the slope behaviour in the Cajamarca region (an example of

such analysis using Oasys Slope can be seen in Figure 26). Oasys Slope performs two-

dimensional slope stability analysis to study circular or non-circular slip surfaces. The program

uses the method of slices and offers a variety of established methods for calculating interslice

forces.

The calculations used in this programme were backed up by carrying out a hand calculation of

the Bishop Circle Method for failure, method of which is outlined in the U.S Army Corps of

Engineers Engineering and Design Manual for the Structural Design of Concrete Lined Flood

Control Channels (EM 1110-2-2007).

The slopes input used in the example were generalised versions of the slopes that exist adjacent

to intake structures in Cajamarca, and a geotechnical survey of much greater depth would need

to be undertaken before a more accurate model could be produced. It was found that there were

some variations between the hand calculations and the computer generated model. This was to

be expected as the computer generated model goes through calculations with a greater degree of

accuracy, though the outputs were within reasonable margins (19.7% difference between the

factor of safety value generated) considering the factor of safety values are within 1.00 and 4.00.


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Figure 26: Example of Oasys Slope analysis of slope failure using the Bishop Circle Methodfor failure.

7.3 Potential Solutions to Slope Failure Issues

Where the soil or ground is not inherently stable it will be prone to failure and so the

performance of the existing soils needs to be improved. There are many ways to achieve

reinforcement of the soils within embankments or slopes. Some of these potential methods are

outlined in this section.

7.3.1 Reinforced Soil

Reinforced soil is the technique whereby fill material (frictional or cohesive) is compacted in

successive layers onto horizontally placed sheets or strips of geosynthetic or metallic

reinforcement. The considerations involved in designing this type of slope reinforcement have

been developed into a programme called GCV Reactive.

Some examples of soil reinforcement schemes for the sites investigated were produced. To verify

the suitability of the output of the programme hand calculations were used, following those

contained in HA 68/94: Design Methods for the Reinforcement of Highway Slopes by

Reinforced Soil and Soil Nailing Techniques guide produced by the Highways Agency.


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This method cannot be considered for protecting soil slopes and repairing soil slope failure

throughout the entire system but the option may be considered in critical zones, such as those

slopes which are adjacent to key pieces of infrastructure which are difficult or costly to replace.

7.3.2 Soil Nailing

HA 68/94 defines soil nailing as the technique whereby in situ ground (virgin soil or existing fill

material) is reinforced by the insertion of tension-carrying soil nails. Soil nails may be of either

metallic or polymeric material and either grouted into a predrilled hole or inserted using a

displacement technique. They will normally be installed at a slight downward inclination to the

horizontal.

Soil nailing has the advantage over the reinforced soil method in that that slope would not have

to be rebuilt and compacted; however the this technique requires specialised equipment and the

costs incurred would be prohibitive. With such limited and difficult transport routes, it would be

far too difficult to access the site with the required heavy machinery.

7.3.3 Masonry walls

Masonry walls have been used on a small scale to repair damaged zones that provide furtherthreat to the infrastructure. This idea is developed in further detail in Section 8.4, which

provides potential solutions to the issue of infrastructure vulnerability due to slope failure.

Figure 27: Section of channel which has been broken, rebuilt and protected by a masonrywall.


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7.3.4 Gabion walls

Gabions are wire mesh baskets that are filled on site with stone or rock to form larger building

modules. Usually, gabion baskets of different sizes are stacked in a stepped formation, with

larger units at the base, to provide stability. Gabions have some advantages over loose stone

material because of their modularity and ability to be stacked in various shapes; they are also

resistant to being washed away by moving water. Gabions have advantages over more rigid

structures because they can conform to ground movement, dissipate energy from flowing water,

and drain freely.

Maccaferri LTD. (England) granted a license to the gabion design programme GawacWin 2003.

This enabled the possibility of investigating many different designs and design parameters in

order to discover what characteristics would yield the optimum structure.

Although flush faced walls can be used where space is limited, it is not recommended to

construct vertical faced walls as gabions are a flexible structure and movement can occur during

backfilling. This may cause instability and give the appearance of the wall leaning forward. To

overcome this effect, and to improve stability, gabions are often designed at a slight inclination

from the vertical.

In determining the stepping arrangement it is not advisable to have a unit which overhangs the

unit below at the rear of the wall. This is because the poor backfilling behind the wall will result

in further soil movement later.

Figure 28: Example of a gabion design input into GawacWin.

This particular structure was used in the investigation to see the effect of flush-faced front walls. (Design 2 in Table 3).


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The validity of the results generated from running this software was checked by carrying out a

hand calculation, which provided a result within reasonable agreement of the computer

simulated run.

There were a few conclusions reached as a result of the investigations carried out using this

software package. An excerpt from one of these investigations can be seen in Table 3 below and

the full set of results was used to conclude that flushed faced walls facing the river generally give

higher toe bearing pressures. In Table 3 below, the same size gabion baskets are used in each

level, but with their position relative to one another altered. Another conclusion was that it may

be possible to reduce the bearing by spreading the load over a greater area using a gabion as a

founding course.

Table 3: Analysing the effect of flush-faced walls.

Stability Checks 1(Flush Front

Wall)

2(SymmetricFigure 28)

3(Stepped

Front Wall)

4(Stepped

Front Wall)

Sliding Safety Coefficient 2.72 3.22 2.86 2.47

Overturning Safety Coefficient 3.78 4.96 4.35 3.62

Overall Stability Safety Coefficient 1.57 1.60 1.58 1.53

Base Normal Stress (left) kN/m2 84.66 48.66 67.25 67.65

Base Normal Stress (right) kN/m2 29.90 49.34 40.11 34.27

Max Allowable Stress kN/m2 183.69 187.85 184.72 179.14


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8. STRUCTURAL DESIGN SOLUTIONS

This report thus far has identified the problems that occur with some of the structures in the

micro-hydroelectric power systems constructed by Practical Action, and investigated some of thecauses of these problems. This section of the report aims to address some of these problems

with potential solutions.

8.1 Drainage Considerations

The back faces of the retaining walls that form either side of the intake structure are likely to be

subjected to hydrostatic forces from groundwater. A means of providing a more structurally

stable system, whilst only slightly modifying the existing design, would be to consider theprovision of drainage features.

These hydrostatic forces could be reduced by the provision of a drainage path at the face of the

wall. Such a drain could be provided by a layer of gravel, rubble or porous blocks with pipes to

collect and remove the accumulated groundwater. This type of system is illustrated in Figure 29

below which is based on a typical design currently in use but modified to make use of such

features.

Figure 29: Proposal to modify existing intake structures to include drainage features.

Technical drawing produced by author. Drawing based on an original structure design by Practical Action. The

proposed modifi

final report_sakthy selvakumaran

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