The Use of Plaxis to Design an Earthdam in Laqlouq

An Engineering Design Project

Presented By

Rayan Abdel Baki

Alexandre Abi Aad

Hafeth Rafeh

Angela Sawan

To

The Department of Civil & Environmental Engineering

In Partial Fulfillment of the Requirements

For the degree of

Bachelor

In the Field of

Civil Engineering

NOTRE DAME UNIVERSITY

LOUAIZE, ZOUK MOSBEH

LEBANON

January 19, 2015

Approval Certificate

An Engineering Design Project

CEN 599

The Use of Plaxis to Design an Earthdam in Laqlouq

To The Department of Civil and Environmental Engineering

In Partial Fulfillment of the Requirements

For the Degree of Bachelor

Civil Engineering

Prepared By:

Alexandre Abi Aad

Rayan Abdel Baki

Hafeth Rafeh

Angela Sawan

Supervised By:

Naji Khoury, Ph.D., P.E.

Co-Supervised (Laboratory Testing) By:

Yara Maalouf, M.E.

Committee Members:

Dr. Jack Harb

Dr. Dima Jawad

Yara Maalouf, M.E.

ii

Acknowledgements

We would like to thank USAID’s Partnership for Enhanced Engagement in Research

(PEER) for their financial support and NSF for their support. We would like to express

our gratitude to Dr. Naji Khoury for his help and guidance. Special thanks go to Mrs.

Yara Maalouf and Mr. Elie Lahoud for their help in the laboratory and the field. We

would also like to acknowledge Dr. Dima Jawad and Dr. Jack Harb for their constructive

feedback.

iii

Disclaimer

This research was made possible through the generosity of both the American people and

the United States Agency for International Development (USAID). Neither the US

Government nor USAID is responsible for the contents of this paper, nor does it reflect

their views. The authors accept responsibility for the contents of this work.

iv

Abstract

Lebanon is known for its abundant water supply and the scarcity of usable water. The

problem lays in the lack of proper water management programs. Earthdams provide water

storage systems that allow better water management and distribution in remote areas. A

large percentage of these earthdams were built by private entities that lack the necessary

skills to construct and maintain earthdams. For this reason there are concerns about the

condition of Lebanon’s earthdams and whether they can meet the ongoing water needs of

communities.

At a preliminary phase, an assessment of the current conditions of an earthdam in the

Laqlouq region of Lebanon is conducted. Of course it is to be conducted following the

proper stages of assessment, of which included is laboratory testing on samples that are to

be acquired through site visits. Finally with these results in hand, conducting a proper

analysis of this earthdam and proposing a solution for its design using geomembrane is

now possible. Using Plaxis 2D the design can now be modeled and the targeted

parameters such as factor of safety and seepage rate can be attained.

v

Table of Contents

Acknowledgements ............................................................................................................. ii

Disclaimer .......................................................................................................................... iii

Abstract .............................................................................................................................. iv

1. Introduction ................................................................................................................... 9

2. Literature Review.......................................................................................................... 9

2.1 Electrical Resistivity Survey ................................................................................. 10

2.2 Geomembranes ..................................................................................................... 10

3. Objectives ................................................................................................................... 10

Methodology ............................................................................................................... 11

3.1 Visual Inspection .............................................................................................. 11

3.2 Sample collection ............................................................................................. 13

3.3 Laboratory Testing ........................................................................................... 14

3.4 Direct Shear Test .............................................................................................. 14

3.5 Permeability ...................................................................................................... 14

4. Software Analysis (Plaxis 2D) .................................................................................... 16

4.1 Design of a Full Dam. ........................................................................................... 16

4.1.1 Earthdam Design Without Geomembrane .................................................... 16

4.1.2 Earthdam Design with a Geomembrane on Its Surface ................................ 22

4.1.3 Earthdam Design with an Integrated Geomembrane. ................................... 23

4.1.4 Earthdam Design with Parallel Integrated Geomembranes. ......................... 25

4.2 Design of Half of a Dam ....................................................................................... 26

4.3 Comparision Between Full and Half a Dam Designs ........................................... 33

5. Conclusions ................................................................................................................... 33

6. References ..................................................................................................................... 34

vi

List of Figures

Figure 1 Laqlouq Lake with Geomembrane on Its Surface .............................................. 11

Figure 2 Satelite View of Lake I (GE 2014) ..................................................................... 12

Figure 3 British Columbia Check List .............................................................................. 13

Figure 4 Lake I Dimensions .............................................................................................. 12

Figure 5 Sample Collection .............................................................................................. 13

Figure 6 Shear Strenght Results - Soil to Soil .................................................................. 15

Figure 7 Shear Strength Results - Soil to Geomembrane ................................................. 15

Figure 8 Permeability Test Equipment ............................................................................. 16

Figure 9 Lake I Earthdam Without Geomembrane ........................................................... 17

Figure 10 General Clay Properties .................................................................................... 17

Figure 11 Cohesion Factor and Angle of Friction ............................................................ 18

Figure 12 - R Interface Factor ........................................................................................... 18

Figure 13 Water Rises to 4 meters .................................................................................... 19

Figure 14 Calculation Phases ............................................................................................ 19

Figure 15 Deformed Mesh Output .................................................................................... 20

Figure 16 Effective Stresses.............................................................................................. 20

Figure 17 Mean Stresses ................................................................................................... 21

Figure 18 Total Displacement ........................................................................................... 21

Figure 19 Surface Failure of The Earthdam ..................................................................... 22

Figure 20 Seepage Occurring in The Earthdam ................................................................ 22

Figure 21 Lake I Earthdam with Geomembrane on Its Surface ....................................... 23

Figure 22 Lake I Earthdamm with Geomembrane on Surface- Flow Field ..................... 23

Figure 23 Lake I Earthdam With an Integrated Geomembrane ........................................ 24

Figure 24 Lake I Earthdam With an Integrated Geomembrane- Flow Field .................... 24

Figure 25 Lake I Earthdam with Parallel Integrated Geomembrane ................................ 25

Figure 26 Lake I Earthdam with Parallel Integrated Geomembrane -Flow Field ............ 25

Figure 27 Lake I Earthdam with Parallel Integrated Geomembrane -Groundwater Head 26

Figure 28 Half of Dam- No Geomembrane ...................................................................... 27

Figure 29 Half of Dam- No Geomembrane- Flow Field .................................................. 27

Figure 30 Half of Dam- Geomembrane on Surface .......................................................... 28

vii

Figure 31 Half of Dam- Geomembrane on Surface-Flow Field ....................................... 28

Figure 32 Half of Dam- 44.8 º Inclined Geomembrane. ................................................... 29

Figure 33 Half of Dam- Inclined Geomembrane- Flow Field .......................................... 29

Figure 34 EarthDam with an 11 º inclined geomembrane. ............................................... 29

Figure 35 Half of Dam- Three Integrated Geomembranes ............................................... 30

Figure 36 Half of Dam- Three Integrated Geomembranes- Flow Field ........................... 30

Figure 37 Half of Dam- Two Integrated Geomembranes ................................................. 31

Figure 38 Half of Dam- Two Integrated Geomembranes- Flow Field ............................. 31

Figure 39 Half of Dam- Integrated Geomembrane beneath all surface ............................ 31

Figure 40 Half of Dam- Integrated Geomembrane beneath all surface-Flow Field ......... 32

Figure 41 Half of Dam- Integrated Geomembrane and One on Surface .......................... 32

Figure 42 Half of Dam- Integrated Geomembrane and One on Surface- Flow Field ...... 33

viii

List of Tables

Table 1 Soil Properties from (Khodor et al. 2014) ........................................................... 14

Table 2 Full Dam Results ................................................................................................. 26

Table 3 Half Dam Results ................................................................................................. 33

9

1. Introduction

Lebanon is noted for its rich water supply. There is, however, a problem with the

management of this water. The lack of proper water networks, distribution systems and

adequate storage has led to a water scarcity for the people of Lebanon. Earthdams are a

common solution to this problem. There are over 500 earthdams in Lebanon, some of

which are supervised and maintained by the Ministry of Water and Energy. Others are

funded by local community members who might lack the skills and experience needed to

properly and effectively maintain the dams. Dam maintenance is vital to the life and

economy of a community since failure of a dam can cause serious injury and death as

well as loss of crops, livestock and businesses. This study looked at an earthdam (Lake I)

in the Laqlouq area of Lebanon. Field trips were conducted to visually assess Lake I. and

collect soil samples that were shipped to the laboratory for testing (i.e., sieve analysis,

liquid limit, plastic limit, standard Proctor, direct shear, permeability). Plaxis 2D, a finite

elements software, was used to design earthdam scenarios and determine the stability and

seepage with and without geomembranes. The results were used to identify the optimum

design for the earthdam at Lake I.

2. Literature Review

The first part of this study determined if the current design is adequate. The second part

determined the presence of internal seepage or erosion in Lake I. Internal seepage and

erosion occur when water seeps through upstream boundaries and leaks through to the

outside of the downstream boundaries. Seepage can be visible or below the surface

where it cannot be seen. This study also looked at spillway capacity, the amount of

surplus water that is channeled over or around a dam. If spillway capacity is exceeded it

will eventually cause failure. Over flowing basically sets the ultimate water level of this

dam, the reason being that the maximum level that the water can reach is the bottom of

the spillway. A study has shown that even the smallest sustained seepage or overflow can

remove thousands of tons of soil from the mass of the dam within hours (Sinha, 2008).

Visual inspection, based on a checklist by the British Colombia Check, and laboratory

testing were used to determine the presence of seepage or overflow at Lake I.

10

2.1 Electrical Resistivity Survey

There has been increasing interest in the use of non-intrusive geophysical techniques

(e.g., temperature measurement, self-potential, electrical resistivity, seismic methods) to

facilitate early detection of anomalous seepage, piping, and internal erosion. Internal

erosion is the second largest cause of earthdam failure worldwide and often requires

expensive remediation (Sheffer and Lum, 2005). Electrical resistivity consists of injecting

electric current into the ground using a pair of electrodes in order to measure the electric

potential distribution in the ground. Fluctuations in pool levels, seasonal temperatures,

and total dissolved solids affect the electrical properties of the dam embankment,

particularly its electrical resistivity (Sheffer and Lum, 2005). Internal erosion and

irregular seepage cause changes in the dam’s soil conditions and, therefore, in the

electrical resistivity. This study looked for seepage and internal erosion by monitoring

changes in the dam’s electrical resistivity. .

2.2 Geomembranes

Geomembranes are widely used in current dam design. When building an earthdam using

geomembranes it is important to provide three layers within the embankment. The first

layer, the basal support layer, supports the geomembrane and the above soil. The second

layer, the geomembrane, prevents water from seeping around and through the surface

area that it has been placed. (see Figure 1). The third layer consists of the geomembrane

barrier and protective cover. The primary purpose of this layer is to protect the

geomembrane from damage. Proper design of this cover is the most important aspect of

earthdam design. A study has shown that the interface between layers two and three, the

geomembrane and the wedge shaped cover, becomes the plane of failure, or the weak

point of the dam. This is due to the low shear strength that has been created along the

interface of layers one and three.

3. Objectives

The main objective of this study was the assessment and design of an existing earthdam

in Laqlouq. The specific tasks were:

1) visual inspection of the earthdam using British Columbia checklist

11

2) determining physical and mechanical properties of soil used in the selected

earthdam.

3) designing earthdams using Plaxis 2D with and without geomembranes.

Figure 1 Laqlouq Lake with Geomembrane on Its Surface

Methodology

This section shows the steps conducted to assess the current health status of Lake I.

Figure 2 shows a satellite view of Lake I whose coordinates are 34°08’ 29.69” N and

35°53’43.65” E.

3.1 Visual Inspection

A field trip was conducted to visually assess Lake I for any deficiencies in accordance

with the British Colombia checklist. The dimensions of the lake were also determined

(see Figure 3) for use in the analysis and design part of the study. Results of the visual

assessment are shown in Figure 4. It is evident that Lake I had some deficiencies (i.e.,

cracks on the crest, settlement on the crest).

12

Figure 2 Satellite View of Lake I (GE 2014)

Figure 3 Lake I Dimensions

13

Figure 4 British Columbia Check List

3.2 Sample collection

Bulks samples were collected during the site visit (see Figure 5) for testing at NDU.

Figure 5 Sample Collection

14

3.3 Laboratory Testing

A series of tests were conducted on the collected bulk samples in accordance with ASTM

standard tests. Sieve analysis, liquid limit, plastic limit, and standard Proctor test results

are summarized in Table 1.

Table 1 Soil Properties from (Khodor et al. 2014)

Test Results

Sieve Analysis CL- Clay

Liquid Limit 34%

Plastic Limit 17%

Plasticity Index 17%

Maximum Dry Unit Weight 19 kN/m3

Optimum Moisture Content 22%

3.4 Direct Shear Test

Shear Strength tests were conducted according to the ASTM D 6528-07 standard test

method. Two sets of samples were prepared at approximately optimum moisture content

and maximum dry unit weight.

(1) Soil-to-Soil samples were used to determine the shear strength of soil to soil.

Specimens had an average diameter of 6.33 cm and an average height of 4.11 cm.

(2) Soil-to-Geomembrane samples were used to determine shear strength of soil to

geomembrane. Specimens had an average diameter of 6.33 cm and an average

height of 2.74 cm.

Shear strength (soil-to-soil) results are shown in Figure 6. c' (cohesion factor) = 38 psi

and Ø' (angle of friction) = 27˚. Figure 7 shows the shear strength results soil to

geomembrane. Cohesion factor (c') was equal to 0 psi and an angle of friction (Ø ') was

equal to 18˚.

3.5 Permeability

A permeability test was conducted in accordance with the ASTM D 5084–03 standard

test model using a triaxial cell panel as shown in Figure 8. The specimen was

consolidated at 5 psi to a maximum of 25 psi and a 2 psi pressure head was used to

determine permeability. Results showed the average permeability k was 3.45*10-5

m/d.

15

Figure 6 Shear Strenght Results - Soil to Soil

Figure 7 Shear Strength Results - Soil to Geomembrane

16

Figure 8 Permeability Test Equipment

4. Software Analysis (Plaxis 2D)

Plaxis 2D, a geotechnical engineering software based on finite elements, analyzes the

deformation and stability of geotechnical structures such as foundations, slope stability,

and, in our case, earthdams.. Design of the earthdams using geomembrane is based on a

pre-constructed earthdams. Also different applications of these geomembranes both on

the surface and integrated of Lake I where considered. Two types of studies were

designed: (1) full dam and (2) half dam.

4.1 Design of a Full Dam.

Four scenarios for full dams were selected: (1) earthdam without geomembrane, (2)

earthdam with a geomembrane on its surface, (3) earthdam with an integrated

geomembrane, (4) earthdam with parallel integrated geomembranes.

4.1.1 Earthdam Design without Geomembrane

Figure 9 shows the model of Lake I, an earthdam without geomembrane, in Plaxis.

Material properties are shown in Figures 10 and 11, where γunsat = 15.2 kN/m3, γsat =

17

18.55 kN/m3. The interference was assumed to be 0.7 (see Figure 12). Figure 13 shows

the water level prior to analysis.

Figure 9 Lake I Earthdam without Geomembrane

Figure 10 General Clay Properties

18

Figure 11 Cohesion Factor and Angle of Friction

Figure 12 - R Interface Factor

19

Figure 13 Water Rises to 4 meters

Plaxis requires different phases for calculations. Figure 14 shows the three phases created

for this study: (1) initial phase having total multipliers as loading input, (2) water level

rises to 4 meters, (3) factor of safety (FOS) analysis.

Figure 14 Calculation Phases

20

Results are shown in Figures 15-20. FOS and seepage were found to be 4.43 and 17.08

x10-6 m/d respectively. Figure 17 shows the mean stresses. It should be noted that the

mean stress value equals -114.48 kN/m2. Figure 20 shows the flow field.

Figure 15 Deformed Mesh Output

Figure 16 Effective Stresses

21

Figure 17 Mean Stresses

Figure 18 Total Displacement

22

Figure 19 Surface Failure

Figure 20 Seepage

4.1.2 Earthdam Design with a Geomembrane on Its Surface

The design and analysis of this scenario consisted of four phases. It is important to note

that an additional phase (geomembrane) was added to the procedure described in Figure

23

14. Figure 21 shows the Plaxis model. Findings showed that FOS was 4.41 and seepage

0 m/d. Figure 22 shows the flow field.

Figure 21 Lake I Earthdam with Geomembrane on Its Surface

Figure 22 Lake I Earthdam with Geomembrane on Surface- Flow Field

4.1.3 Earthdam Design with an Integrated Geomembrane.

In this design a geomembrane was integrated beneath the dam surface. Figure 23 shows

the Plaxis model for an earthdam with an integrated geomembrane. The FOS was 4.52

and the seepage 16.66 x10-6

m/d. Figure 24 shows the flow field.

24

Figure 23 Lake I Earthdam with an Integrated Geomembrane

Figure 24 Lake I Earthdam with an Integrated Geomembrane- Flow Field

25

4.1.4 Earthdam Design with Parallel Integrated Geomembranes.

In this design two parallel integrated geomembranes were added as shown in Figure 25.

The FOS was 4.52 and the seepage 14.26 x10-6

m/d. Figures 26 and 27 show the effective

mean stresses and the ground water head respectively.

Figure 25 Lake I Earthdam with Parallel Integrated Geomembrane

Figure 26 Lake I Earthdam with Parallel Integrated Geomembrane - Flow Field

26

Figure 27 Lake I Earthdam with Parallel Integrated Geomembrane - Groundwater Head

The results show that design 2 had the lowest seepage value as shown in Table 2.

Table 2 Full Dam Results

LAKE I DESCRIPTION FACTOR OF SAFETY SEEPAGE ( m/d)

1) without geomembrane 4.43 17.08 x

2) with geomembrane on surface 4.41 0

3) with an integrated geomembrane 4.52 16.66 x

4) with two parallel geomembranes 4.52 14.26 x

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4.2 Design of a Half Dam

Five scenarios were selected and the models and results are shown in Figures 28-42. The

design procedures used for the half dams were the same as those used for the full dams.

27

Table 3 summarizes the results of the five models. The flow field results are shown in

Figures 29, 31, 33, 36, 38, and 40. It is obvious that model 1 (Figure 28) had the highest

FOS, 4.715, and models 2 (Figure 30) and 4 (Figure 35) had the lowest seepage 0 m/d.

Figure 28 Half Dam - No Geomembrane

Figure 29 Half Dam - No Geomembrane - Flow Field

28

Figure 30 Half Dam - Geomembrane on Surface

Figure 31 Half Dam - Geomembrane on Surface - Flow Field

29

Figure 32 Half of Dam - 44.8 º Inclined Geomembrane.

Figure 33 Half Dam - Inclined Geomembrane - Flow Field

Figure 34 Half Dam with 11 º Inclined Geomembrane.

30

Figure 35 Half Dam - Three Integrated Geomembranes

Figure 36 Half of Dam - Three Integrated Geomembranes - Flow Field

31

Figure 37 Half Dam - Two Integrated Geomembranes

Figure 38 Half of Dam - Two Integrated Geomembranes - Flow Field

Figure 39 Half Dam - Integrated Geomembrane Beneath the Surface

32

Figure 40 Half Dam - Integrated Geomembrane Beneath the Surface - Flow Field

Figure 41 Half Dam - Integrated Geomembrane and One on Surface

33

Figure 42 Half Dam - Integrated Geomembrane and One on Surface - Flow Field

Table 3 Half Dam Results

DESCRIPTION FACTOR OF SAFETY SEEPAGE ( m/d)

Case 1 No geomembrane 4.715 17 x

Case 2 geomembrane on surface 4.69 0

Case 3 single geomembrane inclined

44.8º 4.7 14.64 x

11º 4.7 87.34 x

26º 4.7 61.22 x

Case 4 multiple geomembrane

two geomembranes 4.7 218 x

three geomembranes 4.69 0

Case 5 with different integrated geomembranes

5.1 4.7 118.62 x

5.2 4.712 34 x

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4.3 Comparision Between Full and Half Dam Designs

The results of this study showed that the design and analysis of half dams had a higher

FOS and almost the same seepage as those of full dams. Seepage in both designs was 0

m/d when the geomembrane was at the earthdam’s surface.

5. Conclusions

The objectives of this study were to design an earthdam with the highest possible FS and

lowest possible seepage that was easy to implement. The community members’ solution

34

to insuring a low seepage rate and adequate FS was to build an earthdam with the

geomembrane placed on the surface which led to a FOS of 4.41 and seepage of 0 m/d. A

geomembrane on the surface is subject to animal activity and changing weather

conditions which minimize its service life. The results of this study showed that the

optimum design, while not considering economic factors and service life of

geomembranes, was the placement of a geomembrane on the earthdam’s surface. This is

because it had almost zero seepage. Another possible solution to insuring a low seepage

rate and adequate FS is using an integrated geomembrane at a minimum depth of 2

meters beneath the surface in order to increase the service life of the geomembrane. Lake

I’s current design, no geomembrane, had a low seepage value, 17.08x10-6

m/d, and a high

FOS 4.43. Therefore it is preferable not to integrate a geomembrane since the stability

and seepage are acceptable and the cost of a geomembrane for this size lake is relatively

high.

5. References

Artières, O, Oberreiter, K. and Aschauer, F. (2011), Geosynthetic Systems For Earth

Dams Retrieved on the 18th

of December, 2014 from

www.fao.org/docrep/012/i1531e/i1531e.pdf

Dowers, B (2012), Ohio Department of Natural Resources, Division of Soil and Water

Resources. Retrieved on the 12th

of December from http://ohiodnr.gov/soilandwater.

Jansen, B, R, (2013), Advanced Dam Engineering For Design, Construction and

Rehabilitation, Retrieved on the 5th

of January, 2014 from

www.bvsde.paho.org/bvsamat/48smallear.pdf

Oswald, M, R (2013), ASCE library Embankment, Dam and slope failure. Retrieved on

the 12th

of December, 2014 from http://libguides.ndu.edu.lb/library.

35