geosynthetics engineering: in theory and … 54.pdf · geosynthetics engineering: in theory and...

GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE

Prof. J. N. Mandal

Department of Civil Engineering, IIT Bombay, Powai , Mumbai 400076, India. Tel.022-25767328email: [email protected]

Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay

Module-12LECTURE- 54

DESIGN OF GEOSYNTHETICS FOR LANDFILLS


Introduction

Landfill classification

Some engineered solutions for landfill

Low permeability clay liner in landfill system

Geosynthetics in landfill system

Wet Landfilling (bioreactor landfills)

Leachate detection, collection and removal systems


Recap of previous lecture…..

SLOPE STABILITY OF SIDE LINER

Various landfill lining systems to obtain a stable slope areshown below.

Slope stability of landfill lining system with unreinforced cover soil


Reinforced cover soil

Needle punching of GCLsProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay

Slope stability of landfill lining system with Anchor trench


Laying Geomembrane on SlopeProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay

DESIGN OF LANDFILL LINERS In India, there is no proper computer program available forlandfill design and its stability analysis.

Across the world, Hydrologic Evaluation of LandfillPerformance (HELP) (Schroeder et al., 1994) model isavailable for this purpose which is developed by UnitedStates Environmental Protection Agency (USEPA).

However, HELP model has several limitations such as itrequires several trials to get the desired solution.

HELP model does not give any response during executionsuch as suggestion for minimum required tensile value ofreinforcement to stabilize the cover soil, response againstchoice of wrong values.


HELP model cannot perform the estimation of tensilestress in geosynthetic which is necessary to design theanchor trench, stability analysis in seepage conditions andwaste mass failure analysis.

Different theories and design models along with designequations proposed by various researchers (Koerner andSoong, 2005; Giroud, 1989; Giroud, 2000; Jain andMandal, 2005; Koerner and Hwu, 1991; Koerner andSoong, 1996; Koerner and Soong, 1998; Koerner, 1997;Qian et al., 2002; Qian, 2003; Richardson, 1987;Richardson, 2000; Richardson, 2002; Thiel, 1998 andUSEPA, 1993) have been used to prepare a new softwarenamed as Landfill’s Slope Stability Model (LSSM).


The ‘LSSM’ software was developed using Visual Basic,.NET in C# language (also termed as advanced Clanguage).

It may widely be used by consultants working in thesolid-waste field in India and around the world (Soni,2008).

The inclusion of graphical interface in LSS-Model withdetailed diagrams makes this software much moreeffective, attractive and user friendly as compare tonormal C/ C++/ FORTRAN program.


Landfill’s Slope Stability Model (LSSM) can deal with thefollowing problems:

1. Stability of cover soil for infinite Slope

2. Veneer Slope Stability Analysis (without reinforcement)

3. Veneer Slope Stability Analysis (with reinforcement)

4. Veneer Slope Stability Analysis for unreinforcedTapered Cover Soil

5. Veneer Slope Stability Analysis for reinforced TaperedCover Soil

6. Seismic Analysis for Veneer Slope Stability

7. Seismic Analysis for Veneer Slope Stability (Withreinforcement)


8. Seepage Force Analysis for Veneer Slope Stability Seepage is built up horizontally Seepage is built up parallel to the slope

9. Designing of run out length and Anchor trench Only run out length Run out length with Rectangular anchor trench Run out length with V-shaped anchor trenches

10. Application of GCLs as liquid containment liners Geometric consideration Thickness consideration


1. Stability of cover soil for infinite Slope :

Forces involved in Limit Equilibrium Analysis of infinite slope (After Koerner and Soong, 1998)

W = Weight of cover soil, β = Slope angle, and δ = Friction angle between the GCLs and cover soil


Resisting ForcesFS =Driving Forces

N.tanFS =W.sin

W.cos .tanFS =W.sin

tanF S =tan

From the above equation, it is clear that GCLs having highinterface shear strength will exhibit high stability i.e. highFS value.


Design Example (Stability of cover soil for infiniteslope):

Properties of side slope and materials used in the LSS-Model and Help-Model are mentioned below. Factor ofsafety is found out against the sliding of cover soil.

Cover soil slope (β) = 18.4°

Interface friction angle between GCLs/GM and cover soil(δ) = 23.4°


Input data format in LSS Model:


Output data format in LSS Model:


Results of LSS-Model were found to be similar ascompared to Help-Model.

Case Help-Model LSS-ModelInfinite length slope

stability analysisNo such options

is available FS = 1.301


2. Veneer slope stability for uniform thicknessunreinforced cover soil

Limit equilibrium forces involved in a finite length slope analysis for a uniformly thick cover soil (After Koerner and

Soong, 1998)Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay

2 0.5b (b 4.a.c)FS2.a

A A

A A A a P2

A a

a (W N .cos ).cosb [(W N .cos ).sin . tan (N . tan C ).sin .cos (C W .tan ).sin ]

c (N . tan C ).sin . tan

WA = total weight of the active wedge,WP = total weight of the passive wedge,NA = effective force normal to the failure plane of the activewedge,NP = effective force normal to the failure plane of thepassive wedge,


Design Example (Veneer slope stability for uniformthickness unreinforced cover soil):

Determine the factor of safety against cover soil slidingusing LSS model.

Cover soil slope (β) = 18.4°,

Length of slope (L) = 35 m,

Thickness of the cover soil (h) = 0.3 m,

Unit weight of the cover soil (γ) = 17.5 kN/m3,

Cohesion of the cover soil (c) = 0,

Adhesion between cover soil and GCLs (ca) = 0,

Internal friction angle of cover soil () = 30°,

Interface friction angle between GCLs and cover soil (δ) = 15°


Case Help-Model LSS-ModelUniform thick

unreinforced cover soilFS = 0.842 FS = 0.842

Comparison of LSS-Model with Help-Model:

Design curve for FS-Value of uniform thick unreinforced cover soil


3. Veneer slope stability for uniform thickness reinforcedcover soil:

Forces involved in a finite length slope analysis for a uniformly thick cover soil with reinforcement (After Koerner

and Soong, 1998)Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay

2 0.5b (b 4.a.c)FS2.a

A A

A A A a P2

A a

a (W N .cos Tsin ).cosb [(W N .cos Tsin ).sin .tan (N .tan C ).sin .cos (C W .tan ).sin ]

c (N .tan C ).sin .tan


Design Example (Veneer slope stability for uniformthickness reinforced cover soil):

Cover soil slope (β) = 18.4°, Length of slope (L) = 35 m,


Unit weight of the cover soil (γ) = 17.5 kN/m3,


Adhesion between cover soil and GCLs (Ca) = 0,


Interface friction angle between GCLs and cover soil (δ) = 15°,

Tult = 100 kN/m, and RF = 4Determine the factor of safety against cover soil sliding.


Design chart for uniform thickness reinforced cover soilProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay

Case Help-Model LSS-ModelReinforced

Case

(Tult = 100 kN/m

RF = 4)

FS = 1.506 FS = 1.507

(Also provides us the choice to include the

seismic force)

Reinforced Case

(Tult = 500 kN/m

RF=4)

FS = -0.701

(It gives a negative value,

practically which is not possible)

(This software suggests that we have selected a

reinforcement having higher tensile strength

than required, so choose a geogrid\geotextile with lower tensile strength)

Comparison of the results from LSS-Model and Help-Model


4. Seismic analysis in Veneer slope stability for uniformthick cover soil (without reinforcement)

Limit equilibrium forces involved in pseudo-static analysis(After Qian et al., 2002)


2 0.5b (b 4.a.c)FS2.a

S A A S P2

S A A A a P

A a

a (C .W N .sin ).cos C .W .cos

b [(C .W N .sin ).sin .tan (N .tan C ).cos (C W .tan ).cos ]c (N .tan C ).sin .cos .tan

CS = average seismic coefficient


Design Example (Inclusion of seismic force in veneerslope stability analysis for unreinforced case):

Cover soil slope (β) = 15°, Length of slope (L) = 35 m,


Unit weight of the cover soil (γ) = 18 kN/m3,


Adhesion between cover soil and GCLs (Ca) = 0,


Interface friction angle between GCLs and cover soil (δ) = 18°,

Average seismic coefficient (CS) = 0.1 Determine the factor of safety against cover soil sliding.


Case HELP-Model LSS-ModelUnreinforced

caseFS = 0.905 FS = 0.905

(Since the FS value is lessthan 1.00, the design is notstable. So it gives us the twosolutions for this particularcase to increase the FS value

Comparison of LSS-Model with Help-Model:


Please let us hear from you

Any question?


Prof. J. N. Mandal

Department of civil engineering, IIT Bombay, Powai , Mumbai 400076, India. Tel.022-25767328email: [email protected]


geosynthetics engineering: in theory and … 54.pdf · geosynthetics engineering: in theory and...

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