Performance of soil-bentonite slurry walls : flow rates and contaminant containment

다국어 초록 (Multilingual Abstract)

Slurry wall (or slurry trench wall) is one of the most popular constructions for environmental protection since the public attention to ground water contamination arose. In this dissertation, the soil-bentonite slurry wall is the subject of the study. Several practical factors have effects on not only the performance of the slurry wall but also the characterizing tests for the slurry wall backfills with the conventional methods such as the bentonite cake on the wall boundary, the well skin, the construction defects, the variability in hydraulic conductivity of the walls, etc.
The bentonite cake plays an important role in stabilizing excavation trench surfaces. The very low hydraulic conductivity and the position of the bentonite cake, moreover, significantly influence the overall hydraulic performance of the soil-bentonite slurry wall. Therefore, there is a concern with measuring the hydraulic conductivity of the bentonite cake in the laboratory, which is formed on the excavation surface at a soil-bentonite slurry wall. The modified fluid loss test seems to be a reliable method to evaluate the hydraulic conductivity of a bentonite cake. In this study, the Ruth (1935) approach, which has been popularly used in the filtration industry, is also applied to calculate the hydraulic conductivity of a bentonite cake. The Ruth (1935) method takes into account the filter medium resistance that is disregarded in the Chung and Daniel (2008) method in describing the filtration process.
On the way of performing the modified fluid loss test, in this study, the filter paper is replaced with a layer of Korean standard sand in order to reproduce the bentonite cake in laboratory to resemble as closely as possible the in-situ condition. The hydraulic conductivity of the bentonite cake deposited on the sand is then estimated employing the filtration theory. The obtained values are compared with that of the bentonite cake deposited on the filter paper. In addition, the hydraulic conductivity of the bentonite cake under the effect of salinity is measured for the both types of filter medium (i.e., filter paper and Korean standard sand).
The role of a bentonite cake in slug test analyses to evaluate the hydraulic conductivity of the slurry wall backfill needs to be considered carefully. The numerical simulations of the slug test in slurry trench wall are carried out using the expanded Slug_3D model in which the bentonite cake with the typical properties obtained from the laboratory experiments including hydraulic conductivity, specific storage, and thickness was modeled. Based on the analysis of the simulation results, the effect of bentonite cake on analyzing practical slug test results in the slurry trench wall is evaluated. A case study is reanalyzed using the originally proposed approaches by Choi and Daniel (2006a) (i.e., the type curve and modified line-fitting methods) with the assumption of existing real bentonite cake. The case study results are compared with the previous studies to demonstrate the bentonite cake effect and the validity of the modified line-fitting method in the case of existing real bentonite cake.
The performance of the soil-bentonite slurry walls can be evaluated based on the flow rate and contaminant transport through the walls. The contaminant transport through the soil-bentonite slurry walls were studied experimentally, analytically, and numerically. However, a study on the hydraulic performance of the soil-bentonite slurry walls with the presence of the bentonite cake has never been carried out. In this study, the effect of the bentonite cakes on the performance of soil-bentonite slurry walls is examined using numerical models. Two popular numerical programs (i.e., MODFLOW and MT3DMS) are employed to simulate the flow rates and contaminant mass flux through the slurry wall with consideration of the bentonite cake. The analytical solutions for one dimensional solute transport through the slurry wall with various boundary conditions are also compared with numerical analyses which consider the existence of the bentonite cakes on the wall boundary. Based on the results of the simulations, it is shown that the bentonite cake formed at the interface significantly influences the performance of the soil-bentonite slurry walls.

목차 (Table of Contents)

• Abstract I
• Acknowledgements III
• Dedication IV
• Table of Contents V
• List of Figures VII
• List of Tables XI
• Chapter 1. Introduction 1
• 1.1 Overview 1
• 1.2 Organization 6
• Chapter 2. Literature Review 8
• 2.1 Slurry Walls 8
• 2.2 Hydraulic Performance of Slurry Walls 15
• 2.2.1 Hydraulic Conductivity of Soil-Bentonite Backfill Used for Slurry Walls 15
• 2.2.2 Flow Rates through Soil-Bentonite Slurry Walls 25
• 2.3 Contaminant Containment of Slurry Walls 28
• 2.3.1 Contaminant Transport through Slurry Walls 28
• 2.3.2 Analytical Evaluation Methods 32
• 2.3.3 Numerical Evaluation Methods 35
• 2.4 Practical Factors Influencing the Performance of Soil-Bentonite Slurry Walls 36
• 2.4.1 Bentonite Cake on Natural Soil-Slurry Wall Interface 36
• 2.4.2 Construction Defects 39
• 2.4.3 Heterogeneity in Hydraulic Conductivity of Slurry Walls 40
• Chapter 3. Properties of Bentonite Cake 42
• 3.1 Hydraulic Conductivity of Bentonite Cake 43
• 3.1.1 Modified Fluid Loss Test 43
• 3.1.2 Application of Filtration Theory 48
• 3.1.3 Hydraulic Properties of Bentonite Cake deposited on Filter Paper 55
• 3.1.4 Hydraulic Conductivity of Bentonite Cake deposited on Real Soil Medium 74
• 3.1.5 Hydraulic Conductivity of Bentonite Cake deposited on Korean Standard Sand under Effect of Salinity 103
• 3.2 Parameters of Bentonite Cake Influencing Contaminant Containment 111
• 3.2.1 Porosity 111
• 3.2.2 Bulk density 111
• 3.2.3 Molecular Diffusion Coefficient 112
• 3.2.4 Distribution Coefficient 114
• 3.2.5 First-Order Rate Constant for VOCs 115
• 3.2.6 Thickness of Bentonite Cake 115
• Chapter 4. Slug Test for Soil-Bentonite Slurry Wall with Consideration of Bentonite Cake 117
• 4.1 Current Methods for Analyzing Slug Test Data of Slurry Wall with Consideration of Bentonite Cake 117
• 4.2 Numerical Model of Slug Test with Consideration of Bentonite Cake 118
• 4.3 Bentonite Cake Effect in Slug Test Analysis for Slurry Trench Wall 123
• 4.4 Reanalysis of Previous Case Study considering Real Bentonite Cake 138
• 4.5 Proposed Methods for Analyzing Slug Test Data with Presence of Bentonite Cake 145
• Chapter 5. Bentonite Cake Effect on Performance of Soil-Bentonite Slurry Walls 147
• 5.1 Numerical Models for Flow through Soil-Bentonite Slurry Walls 147
• 5.2 Results of Numerical Flow Simulations 153
• 5.3 Numerical Model for Contaminant Transport through Soil-Bentonite Slurry Walls 160
• 5.4 Results of Numerical Contaminant Transport Simulations 165
• Chapter 6. Summary and Conclusions 175
• Appendixes 183
• References 225
• Vita 236