PREDICTION OF CONSOLIDATION TIMES FOR SHEAR STRENGTH TESTING OF GEOSYNTHETIC CLAY LINERS USING CS2 MODEL

ABSTRACT

The objectives of this study were to investigate the consolidation behavior and predict consolidation times for shear strength testing of geosynthetic clay liners (GCLs).  These objectives were achieved by performing a series of numerical simulations using the numerical model CS2 (Fox and Berles 1997; Fox and Pu 2012).  GCL consolidation was assessed for three values of initial overburden stress (10 kPa,

100 kPa and 1000 kPa), six load increment ratios (LIRs) (0.25, 0.5, 0.75, 1.0, 1.25, 1.5), double-drained (DD) and single-drained (SD) conditions, and two values of specific gravity (1.0 and 2.21).   Constitutive relationships for GCL compressibility and hydraulic conductivity were taken from experimental data published by Kang and Shackelford (2010).

Values of consolidation times ₅₀, ₇₀, ₉₀, ₉₅, and ₉₈ are presented for various conditions and plotted versus LIR.  Consolidation times for both DD and SD conditions decrease as LIR increases for a given initial overburden stress, and these times decrease as initial overburden stress increases at a given LIR.  The longest predicted time required for 98% GCL consolidation and SD conditions is 32.5 h, which corresponds to low initial stress and low LIR.  Thus, for a direct shear test, the recommended consolidation time for a GCL prior to the start of shearing is 48 h for single-drainage.  The longest predicted time required for 98% GCL consolidation and DD conditions is 8.1 h, which also corresponds to low initial stress and low LIR.

Thus, the recommended consolidation time for a GCL prior to the start of shearing is

24 h or overnight for double-drainage.

This study found that the ratios of consolidation times for DD to SD conditions are approximately equal to 4.0 in all cases, which is consistent with classical consolidation theory.  Numerical solutions obtained for = 1 are nearly identical to corresponding solutions obtained for = 2.21.  This indicates the effect of selfweight of GCL solids is negligible, which is consistent with GCLs being very thin materials.

 

Table of Contents

List of Figures ……………………………………………………………………………………………….. vii

List of Tables ………………………………………………………………………………………………. viii

Acknowledgements …………………………………………………………………………………………. ix

Chapter 1 Introduction ……………………………………………………………………………………… 1

1.1 Geosynthetic Clay Liners …………………………………………………………………………. 1

1.2 Numerical Modeling Approach …………………………………………………………………. 3

1.3 Research Objectives ………………………………………………………………………………… 4

1.4 Outline …………………………………………………………………………………………………… 5

Chapter 2 Literature Review ……………………………………………………………………………… 6

2.1 GCL Products …………………………………………………………………………………………. 6

2.2 Direct Shear Test Procedure ……………………………………………………………………… 8

2.3 Direct Shear Device ………………………………………………………………………………… 9

2.4 Effect of Consolidation ………………………………………………………………………….. 11

Chapter 3 CS2 Model Description ……………………………………………………………………. 14

3.1 Model Geometry …………………………………………………………………………………… 14

3.2 Constitutive Relationships ………………………………………………………………………. 15

3.3 Model Formulation ………………………………………………………………………………… 17

3.4 Computational Procedures ……………………………………………………………………… 23

3.5 Model Performance ……………………………………………………………………………….. 26

Chapter 4 GCL Consolidation Times for Shear Strength Testing …………………………. 29

4.1 GCL Consolidation Properties ………………………………………………………………… 29

4.2 Model Input Data and Simulation Arrangements ………………………………………. 32

4.3 Consolidation Times for GCLs ……………………………………………………………….. 35

4.4 Ratio of Consolidation Times for DD and SD Conditions …………………………… 43

4.5 Comparison of Solutions for = 1 and = 2.21 ………………………………… 47 Chapter 5 Conclusions and Future Research ……………………………………………………… 49

5.1 Conclusions ………………………………………………………………………………………….. 49

5.2 Future Research …………………………………………………………………………………….. 50

References …………………………………………………………………………………………………….. 52

Appendices ……………………………………………………………………………………………………. 59

Chapter 1  Introduction

1.1 Geosynthetic Clay Liners

In the last 25 years, geosynthetic clay liners have been widely utilized as hydraulic barriers in a large number of waste containment facilities and other engineering applications.  As part of a composite liner, GCLs are typically placed at the bottom of landfills, as shown in Fig.1-1, or in cover system.  A primary concern for such applications is the static and seismic stability of slopes that incorporate GCLs.  This is because bentonite, the major component of a GCL, has low shear strength after hydration.  GCLs also display variability in shear strength due to the variability of component materials and changes in GCL design and testing over years.  As a result, internal and interface shear strengths of GCLs must be measured on a routine basis and detailed technical specifications have been developed for such tests

(e.g., ASTM D 6243).

 

Figure 1-1. GCL installation on side slope of a bottom liner system (from http://d6cbwp89cp4qo.cloudfront.net/images/products/640/61.jpg)

Extensive shear strength tests of GCLs have been conducted in the laboratory, mostly using the direct shear test (Fox and Stark 2015).  Such tests are typically drained because excess pore pressures within GCLs in the field are usually considered to be small (Gilbert et al. 1997).  A conditioning stage is needed for shear strength tests of GCLs before the shear force is applied, including hydration and consolidation of a GCL specimen, since the bentonite moisture content can significantly affect measured shear strength (Daniel et al.1993; Zelic et al. 2002).

In the ideal case, a GCL specimen is hydrated under low normal stress and slowly consolidated to the expected final normal stress in the field prior to shear.  If the hydration normal stress matches expected field conditions, shearing can begin once the GCL is fully hydrated.  However, the field normal stress often increases after hydration, and the shear strength at a higher normal stress condition is required.  In this case, consolidation of a GCL test specimen to the desired normal stress is necessary to obtain this shear strength.  As with natural soils, GCL shear strengths are a function of the effective normal stress acting on the failure plane, and a GCL specimen that is not fully consolidated prior to shearing may yield low shear strength due to the presence of excess pore pressures (McCartney et al. 2009; Fox and Stark 2015).

Many studies have been conducted to investigate internal and interface shear strengths for GCLs; however, the long time required for GCL consolidation often influences the practicality of such tests.  Some laboratories use accelerated schedules to avoid long testing times, with the GCL consolidation period as short as a couple of hours.  The consolidation behavior of natural soils has been widely studied, but almost no similar work has been conducted for GCLs.  This thesis presents a numerical investigation of the consolidation behavior for GCLs and provides predictions and recommendations for consolidation times for shear strength testing of GCLs.

1.2 Numerical Modeling Approach

Soil consolidation has been one of the oldest analysis methods in geotechnical engineering and a large number of theories and numerical models have been developed.  Fox and Berles (1997) first published a piecewise-linear model for 1D large strain consolidation, called Consolidation Settlement 2 (CS2).  Fox and Pu (2012) published an updated and enhanced version of CS2.  The CS2 modeling approach has been extensively used and validated since these original publications, and thus, CS2 was utilized to conduct numerical simulations of GCL consolidation behavior in this study.

In the CS2 method, all variables pertaining to geometry, material properties, fluid flow, and effective stress are updated at each time step with respect to a fixed coordinate system.  This method is a Lagrangian approach that follows the motion of the solid phase throughout the consolidation process.  In subsequent studies, the method has been adapted to accommodate accreting layers (Fox 2000), radial and vertical flows (Fox et al. 2003), compressible pore fluid (Fox and Qiu 2004), highgravity conditions in geotechnical centrifuge (Fox et al. 2005), and coupled solute transport (Fox 2007a; Fox and Lee 2008).  The original CS2 model takes into consideration “vertical strain, soil self-weight, general constitutive relationships, relative velocity of fluid and solid phases, and changing hydraulic conductivity and compressibility during the consolidation process.  Fox and Pu (2012) then incorporated several new capabilities into the model, including time-dependent loading, an external hydraulic gradient acting across the consolidating layer, and unload/reload effects.”

1.3 Research Objectives

The primary objectives of this research study were to investigate the consolidation behavior and provide recommendations for consolidation times for shear strength testing of GCLs, by numerically simulating the consolidation process.

The CS2 model was utilized to achieve these research objectives.

1.4 Outline

This thesis is divided into five chapters.  Chapter 1 presents the background information, research objectives and thesis outline.  Chapter 2 summarizes the existing literature associated with shear strength testing of GCLs and the CS2 numerical model.  Chapter 3 describes the CS2 model, including assumptions, main procedures, capabilities, and verification checks.  Chapter 4 presents an analysis of numerical simulations for GCL consolidation under various conditions.  Chapter 5 presents conclusions and recommendations from this study and suggestions for future research.

PREDICTION OF CONSOLIDATION TIMES FOR SHEAR STRENGTH TESTING OF GEOSYNTHETIC CLAY LINERS USING CS2 MODEL

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