a new coupled consolidation and contaminant transport ...825/fulltext.pdfa new coupled consolidation...

A NEW COUPLED CONSOLIDATION AND CONTAMINANT TRANSPORT DEVICE TO TEST A REACTIVE CORE MAT FOR REMEDIATION OF CONTAMINATED,

SUBAQUEOUS SEDIMENTS

A THESIS PRESENTED BY

DOGUS MERIC

to

GRADUATE SCHOOL OF ENGINEERING

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS

FOR THE DEGREE OF

Master of Science

In

Civil and Environmental Engineering

IN THE FIELD OF

Geoenvironmental Engineering

NORTHEASTERN UNIVERSITY

BOSTON, MASSACHUSETTS

APRIL, 2010

NORTHEASTERN UNIVERSITY

Graduate School of Engineering

Thesis Title: A New Coupled Consolidation and Contaminant Transport Device to Test A Reactive Core Mat for Remediation of Contaminated, Subaqueous Sediments

Author: Dogus Meric

Department: Civil and Environmental Engineering

APPROVED FOR THESIS REQUIREMENT OF THE MASTER OF SCIENCE DEGREE

THESIS ADVISOR DATE

THESIS READER DATE

DEPARTMENT CHAIR DATE

GRADUATE SCHOOL NOTIFIED OF ACCEPTANCE:

DIRECTOR OF THE GRADUATE SCHOOL DATE

2

A NEW COUPLED CONSOLIDATION AND CONTAMINANT TRANSPORT DEVICE

TO TEST A REACTIVE CORE MAT FOR REMEDIATION OF CONTAMINATED,

SUBAQUEOUS SEDIMENTS

by DOGUS MERIC

Submitted to the Department of Civil and Environmental Engineering on April 20, 2010 in

partial fulfillment of the requirements for the degree of Master of Science in Civil and

Environmental Engineering.

ABSTRACT

This thesis describes a laboratory testing program to assess the efficacy of a reactive core mat

(RCM) for the remediation of contaminated, subaqueous sediments.

The RCM is a 1.25 cm (0.5 in) thick sheet that consists of a reactive layer confined within

geotextile filtering layers. The reactive layer is composed of needle-punched fabric impregnated

with one or more reactive and/or adsorbing materials (e.g., organoclay, activated carbon, etc.)

depending on the contaminant and aqueous environment type.

To test the efficiency of the RCM, a new bench scale testing device was designed and fabricated,

the Integrated Contaminated Sediment Testing Column Apparatus (ICSTAC),that physically

models the bio-geo-chemical behavior of the contaminated sediment, RCM overlay and the so-

called biogeneration zone where new biota could be expected to develop. The device consists of

an acrylic column (20.3 cm diameter, 40.6 cm height), which serves as both the vertical process

column for the testing and as a guide and sealing cylinder for the loading piston to travel

through. Two independent pressurized water cylinders, actuated by deadweight hangers, provide

flow and pressure through the column. Sampling ports enable the monitoring of dissolved

3

contaminants within the testing column throughout the experiment. In the ICSTAC tests, the

biogeneration zone above the RCM is clean sand mixed with 3% organic material. Sediment

placed in the testing column is backpressured and then vertically loaded incrementally. Vertical

deformations are monitored and sediment pore fluid samples are collected during loading. At the

completion of the consolidation test, overlying sand is collected and exposure tests on tracer

worms (Nereis virens) in the sand are performed for 28 days. A comprehensive experimental

study was carried out, including 7 conventional consolidometer tests and 16 ICSTAC tests.

Sediment used in this research was sampled from the Neponset River, Milton, Massachusetts,

and used either in its natural state or after spiking with 250 ppm of naphthalene.

In addition, the CS2 large strain consolidation model (Fox and Berles, 1997) was adapted to

predict the experimental consolidation behavior.

Results indicate success in design and implementation of the device. Regarding the consolidation

test results on the sediment, the ICSTAC tests with non-spiked sediment shows stiffer behavior

compared to ICSTAC tests with spiked sediment. Further, although the ICSTAC test results

show decreasing incremental strains with increasing stress increment, conventional tests showed

more or less the same incremental strain for all of the stress increments. Comparisons to the CS2

large strain consolidation model indicate that such a model will be useful for predicting field

performance and linking consolidation to contaminant transport. Finally, contaminant transport

data indicates that the RCM prevented the breakthrough of contaminants to overlying layers,

which supports the hypothesis that the RCM can be used as thin isolation barrier even in high

advective flow conditions.

4

Thesis Supervisor : Dr. Thomas C. Sheahan

Title : Professor of Civil and Environmental Engineering

Thesis Co-Supervisor : Dr. Akram N. Alshawabkeh

Title : Professor of Civil and Environmental Engineering

5

ACKNOWLEDGEMENTS

The work described in this paper is supported by the National Institute of Environmental Health

Sciences under grant number R01ES16205. Any opinions, findings, and conclusions or

recommendations expressed in this material are those of the authors and do not necessarily

reflect the views of the NIEHS.

My appreciation goes to the following:

Professor Thomas C. Sheahan, for his endless motivating support and full trust in my decisions

throughout the research, for his extra patience during my learning process of experimental study,

for the limitless supportive information that he provided in this research, and for being more than

an advisor.

Professor Akram N. Alshawabkeh, for his supportive attitude towards me, for his willingness and

trust on me about modeling part, and for invaluable information that he taught me about the

research.

Dr. James Shine, for teaching me integration of chemistry science into engineering application

and guiding me for bio-chemical references throughout the research.

Dr. David Whelpley, who is the laboratory director of the Civil and Environmental Engineering

Department, for his efforts to teach me variety of machining skills and handling the machining

himself for some critical parts.

Finally, my family, who had supported me throughout my life and provided me the best life

possible ever and teach me how to be good person besides being a good professional.

6

TABLE OF CONTENTS

Title Page 1

Page

Abstract 2

Acknowledgements 5

Table of Contents 6

List of Tables 11

List of Figure 12

1. INTRODUCTION 18

1.1 Significance of the Problem 18

1.2 Sediment Remediation Techniques 18

1.2.1 Conventional Sediment Remediation Techniques 18

1.2.2 Potential Materials for Active Cap Use 19

1.3 Reactive Core Mat Approach for Sediment Remediation 20

1.4 Scope of Work 21

1.5 Organization of Thesis 22

2. BACKGROUND 27

2.1 Introduction 27

2.2 Prior Development and Use of Large Strain Consolidation Devices 27

2.2.1 Incremental Loading Devices 28

2.2.2 Seepage-Induced Consolidation Devices 29

7

2.2.3 Self-Weight Consolidation Devices 30

2.2.4 Constant Rate of Strain (CRS) Devices 31

2.2.5 Summary of Large Strain Consolidation Devices 32

2.3 Large Strain Consolidation Theories for Data Interpretation and Modeling 33

2.3.1 Introduction to Large Strain Consolidation Theories 33

2.3.2 Early Large Strain Consolidation Models 35

2.3.3 Evaluation of Analytical Approaches on Large Strain Consolidation 36

Problem

2.3.4 Piecewise-linear Large Strain Consolidation Theories by Fox and 37

Co-workers

2.3.5 Summary of Large Strain Consolidation Theories 38

2.4 Coupled Contaminant Transport Studies 39

2.4.1 Introduction to Coupled Contaminant Transport 39

2.4.2 Initial Work to Link Contaminant Transport to Gradient-Induced 40

Advection

2.4.3 Theoretical and Experimental Approaches on Consolidation Coupled 41

Contaminant Transport

2.4.3.1 Small Strain Consolidation Coupled Contaminant Transport 41

2.4.3.2 Large Strain Consolidation Coupled Contaminant Transport 43

2.4.3.3 Piecewise-linear Approach to Coupled Contaminant Transport 44

2.4.4 Summary and Need for Further Work 46

3. DESCRIPTION OF THE NEW DEVICE 66

3.1 Introduction 66

8

3.2 Design and Fabrication of the New Device 66

3.2.1 Mechanical Design and Fabrication 66

3.2.1.1 Computer Aided Design of the Components 67

3.2.1.2 Mechanical Fabrication of Device Components 68

3.2.2 Measurement Instrumentation and Software Design 71

3.3 ICSTAC Testing Procedure 73

3.3.1 Pre-Test Procedures 73

3.3.2 Testing Phase Procedures 74

3.3.3 Post-Test Procedures 76

3.4 Proof Testing 77

3.5 Conclusion 78

4. TESTING OF CONTAMINATED SEDIMENTS USING THE ICSTAC 95

DEVICE

4.1 Introduction 95

4.2 Sediment Sampling 95

4.2.1 Neponset River, Milton, Massachusetts 97

4.2.1.1 Site Information and Background of Contamination 97

4.2.1.2 Neponset River Sediment Physical and Chemical Properties 97

4.2.2 Hocomonco Pond, Marlborough, Massachusetts 98

4.2.2.1 Site Information and Background of Contamination 98

4.3 Sediment Spiking 99

4.3.1 Collection of Reference Sediment 101

4.3.2 Spiking Compound and Spiked Sediment Concentration 101

9

4.3.3 Calculation of Sediment and Naphthalene Compound Required 101

for ICSTAC Tests

4.3.4 Spiking Method and Procedure 102

4.4 Basic Results 103

4.4.1 Conventional Consolidation Tests (CONV) Results with Neponset 104

Sediment

4.4.1.1 Description of CONV Tests 104

4.4.1.2 Discussion of CONV Tests Results 104

4.4.2 Large Strain Consolidation Coupled Contaminant Transport Tests 105

(ICSTAC) Results with Neponset River Sediment

4.4.2.1 Description of ICSTAC Tests 105

4.4.2.2 Discussion of ICSTAC Consolidation Results 106

4.4.2.3 Comparison of CONV and ICSTAC Test Consolidation Results 107

4.4.2.4 Discussion of ICSTAC Contaminant Transport Results 108

4.5 Conclusion 109

5. ANALYSIS OF RESULTS 153

5.1 Introduction 153

5.2 CS2 Model Prediction 153

5.2.1 CS2 Piecewise Linear Large Strain Consolidation Model 153

5.2.2 Formulation of the CS2 Model 154

5.2.3 Procedure for CS2 Model Application to CONV and ICSTAC Data 161

5.3 Comparison of Experimental Results and CS2 Model Prediction 162

5.4 Discussion of Analysis Results 162

10

5.5 Conclusion 164

6. SUMMARY, CONCLUSIONS AND RECOMMENDATIONS 192

6.1 Introduction 192

6.2 Summary and Conclusions 192

6.2.1 Design and Fabrication of New Testing Device 192

6.2.1.1 Design Process 192

6.2.1.2 Final Design of the ICSTAC Device 193

6.2.1.3 Fabrication of the New Device 194

6.2.2 Testing Protocols Developed for ICSTAC Testing 194

6.2.2.1 Sediment Sampling and Spiking 194

6.2.2.2 Testing Procedures 195

6.2.2.3 Other Tests and Procedures 196

6.2.3 Experimental Study 196

6.2.3.1 Conclusions on Consolidation Behavior 197

6.2.3.2 Conclusions on Contaminant Transport Results 199

6.2.4 Conclusions on CS2 Model Prediction of Experimental Data 200

6.3 Recommendations for Further Research 201

REFERENCES 204

APPENDIX A : Source Code for Data Filtering Program DM-CDE 218

APPENDIX B : Adapted CS2 Large-Strain Consolidation Model 225

11

LIST OF TABLES

Table 2.1 Summary of theoretical large strain consolidation studies 47

Page

Table 3.1 Materials used for various ICSTAC components 79

Table 4.1 Chemical analysis result of PCB contaminated Neponset River sediment 110

Table 4.2 ICSTAC experiment grid 111

Table 4.3 ICSTAC & CONV tests comparative data 112

Table 5.1 CS2 predictions k-factor values 165

12

LIST OF FIGURES

Figure 1.1 : Schematic (left) and photo (right) of reactive core mat 24

Page

Figure 1.2 : Schematic of reactive core mat application in field 25

(courtesy of CETCO™)

Figure 1.3 : Reactive core mat application during Anacostia River 26

pilot project (courtesy of CETCO™)

Figure 2.1 : Slurry consolidometer used to prepare samples of illitic 49

slurry (Sheeran and Krizek, 1971)

Figure 2.2 : Large strain consolidation device to test sedimented Bridgewater 50

clay followed by incremental loading consolidation (Lee, 1979)

Figure 2.3 : Slurry consolidation cell to test dredged sludge material (Carrier 51

and Keshian, 1979)

Figure 2.4 : Slurry consolidation device to prepare reproducible clay samples 52

for cone penetration tests (Kurup, 1993)

Figure 2.5 : Slurry consolidometer to obtain consolidation characteristics of 53 Madison Metropolitan wastewater sludge (Aydilek et al., 1999)

Figure 2.6 : Seepage induced consolidation device to test phosphatic waste clay 54

consolidation characteristics (Abu-Hejleh et al., 1996)

Figure 2.7 : Self-weight consolidation column (2 m high) used to test Combwich 55

Estuarine mud (Been and Sills, 1981)

Figure 2.8 : Self-weight consolidation column to obtain the mechanical behavior 56

of oil sand slurry (Scott et al., 1986)

Figure 2.9 : Comparison between large strain consolidation models and linear 57

and nonlinear small deformation models (McVay et al., 1986)

Figure 2.10 : Comparison of settlement versus time from large strain consolidation 58

theory and field results (McVay et al., 1986)

Figure 2.11 : Comparison of experimental average degree of consolidation 59

versus time data from slurry consolidometer and conventional

consolidometer, and CS2 model estimations (Aydilek et al., 1999)

13

Figure 2.12 : Comparison of settlement versus time in Madison Metropolitan 60

Sewage sludge field test data and CS2 model estimation

(Aydilek et al., 1999)

Figure 2.13 : Schematic of capped sediment section used as base model in finite 61

element analyses (after Potter et al., 1994 and Loroy et al., 1996)

Figure 2.14a : Model of subaqueous sediment section used for contaminant 62

transport modeling (Alshawabkeh et al., 2005)

Figure 2.14b : Breakthrough curve of contaminant in 25cm thick sand cap 62

without consolidation, retardation factor =10

(Alshawabkeh et al., 2005)

Figure 2.14c : Breakthrough curve of contaminant in 25cm thick sand 62

cap with consolidation, retardation factor = 10

(Alshawabkeh et al., 2005)

Figure 2.15 : Saturated soil layer configuration, a) before loading b) after 63

loading, for piecewise-linear approach to coupled contaminant

transport (Fox, 2007a)

Figure 2.16 : Comparative settlement versus time curves (left) and 64

contaminant breakthrough concentrations with time (right)

curves for CST1 Model and Peters and Smith (2002) models

(Fox, 2007b)

Figure 2.17 : Comparison of contaminant concentrations in a consolidating 65

layer at different times using the CST1 model (Fox, 2007b)

Figure 3.1 : Schematic representation of the new ICSTAC device 80

Figure 3.2 : Three-dimensional design stage of the testing device in 81

SolidWorks™ (left); photo of the ICSTAC sediment testing column

after assembly (right)

Figure 3.3 : Loading piston and piston shaft drawing (left) and photo (right) 82

Figure 3.4 : ICSTAC vertical loading mechanism with 4:1 advantage pulley 83

system and pressure cylinders details

Figure 3.5 : Top sealing plate and precision bush bearing drawing (left) and 84

photo (right)

Figure 3.6 : Bottom plate with porous stone and O-ring attached 85

Figure 3.7 : Long stem LVDT used to measure displacement 86

14

(from Collins Corp.)

Figure 3.8 : National Instruments NI-USB 6008 DAQ box 87

Figure 3.9 : Algorithm for the DAQ software for ICSTAC device 88

Figure 3.10 : DM-CDE software interface 89

Figure 3.11 : Tie-rod guiding blocks for sealing at the column’s bottom cap 90

Figure 3.12 : Top water inlet, top sampling port, quick release shaft 91

clamp and top plate

Figure 3.13 : Bottom water inlet and bottom sampling port 92

Figure 3.14 : Chamber used for exposure tests at Harvard School 93

of Public Health

Figure 3.15 : Deformation-time curve for ICSTAC proof test with Neponset River 94

sediment

Figure 4.1 : Ekman sediment sampler (produced by Wildco® 113

from Ben Meadows, Co. 2010)

Figure 4.2 : Neponset River flow path from Neponset reservoir in Foxborough, 114

MA to Dorchester Bay MA (top); close-up of sampling location at the

Tilestone & Hollingsworth Dam (bottom) (Google Earth Images)

Figure 4.3 : Neponset River sampling location behind Tilestone & Hollingsworth 115

(T&H) Dam

Figure 4.4 : Sediment sampling from Neponset River behind T&H Dam 116

Figure 4.5 : Hocomonco Pond, Westborough, MA (Google Earth Image) 117

Figure 4.6 : Conventional consolidometer (left) and modified consolidometer 118

(right)

Figure 4.7 : CONV-NEP-1 consolidation curves: a) 5 kPa, b) 10 kPa, c) 25 kPa, 119

d) 55 kPa

Figure 4.8 : CONV-NEP-2 consolidation curves: a) 5kPa, b) 10 kPa, c) 25 kPa, 120

d) 55 kPa


d) 55 kPa


d) 55 kPa


d) 55 kPa

15


d) 55 kPa


d) 55 kPa

Figure 4.14 : CONV-NEP Tests Comparative consolidation curves: a) 5 kPa, 126

b) 10 kPa, c) 25 kPa, d) 55 kPa

Figure 4.15 : ICSTAC-NEP-1 consolidation curves: a) 20 kPa, b) 40 kPa, c) 100 kPa 127


Figure 4.17 : ICSTAC-NEP-3 long term test consolidation curves (10 kPa) 129













Figure 4.30 : ICSTAC-NEP consolidation curves comparison: a) 10 kPa 142

non-spiked b) 10 kPa c) 25 kPa spiked d) 25 kPa non-spiked e) 55 kPa

spiked f) 55 kPa non-spiked

Figure 4.31 : Comparison of ICSTAC-NEP-6&7 (non-spiked sediment), 143

ICSTAC-NEP-11&12 (spiked sediment), CONV-NEP-5&6

(Non-Spiked Sediment) compression curves: a) 10 kPa, b) 25 kPa,

c) 55 kPa

Figure 4.32 : ICSTAC-NEP-8 normalized contaminant transport data: a) 10 kPa, 144

b) 25 kPa, c) 55 kPa




16





b) 25 kPa, c) 55 kPa Figure 4.37 : ICSTAC-NEP-14 normalized contaminant transport data: a) 10 kPa, 149






Figure 4.40 : Comparison of ICSTAC contaminant transport data: duplicate 152

tests with RCM and trout chow, and no RCM with trout

chow a&b) 10 kPa, c&d) 25 kPa, e&f) 55 kPa

Figure 5.1 : Geometry for CS2 model: initial configuration (left), configuration 166

after the application of additional surcharge load (right) (from Fox

and Berles, 1997)

Figure 5.2 : Constitutive relationship for the model: void ratio vs. vertical 167

effective stress (left) and void ratio vs. permeability (right)

(from Fox and Berles, 1997)

Figure 5.3 : Inter-elemental fluid flow (from Fox and Berles, 1997) 168

Figure 5.4 : Algorithm of the CS2 model (from Fox and Berles, 1997) 169

Figure 5.5 : CONV-NEP-1 experimental and CS2 prediction comparison: 170

a) 5 kPa, b) 10 kPa, c) 25 kPa, d) 55 kPa


a) 5kPa, b) 10 kPa, c) 25 kPa, d) 55 kPa

Figure 5.7 : CONV-NEP-3 experimental and CS2 prediction comparison 172






a) 5kPa, b) 10 kPa, c) 25 kPa

17


a) 5kPa, b) 10 kPa

Figure 5.12 : ICSTAC-NEP-1 (non-spiked sediment) experimental and CS2 177

prediction comparison: a) 20kPa, b) 40kPa, c) 100kPa


prediction comparison: a) 10 kPa, b) 25 kPa, c) 55 kPa Figure 5.14 : ICSTAC-NEP-3 (non-spiked sediment) (single increment, 10 kPa, 179

long term test) experimental and CS2 prediction comparison


prediction comparison: a) 10 kPa, b) 25 kPa, c) 55 kPa


prediction comparison: a) 10 kPa, b) 25 kPa





Figure 5.19 : ICSTAC-NEP-8 (spiked sediment) experimental and CS2 184













prediction comparison



18

1. INTRODUCTION

1.1 Significance of the Problem

Contaminated, subaqueous sediments have been recognized as one of the most serious national

and world-wide environmental problems. In the United States alone, 10% or 1.2 billion cubic

yards of the sediment underlying the nation’s surface water is contaminated enough to pose

significant health risks (U.S. EPA, 2004). Currently 545 out of 1279 Superfund sites contain

contaminated sediments (U.S. EPA CERCLIS Database, 2010). Hydrophobic organic

compounds and heavy metals are the major contaminants of concern in Superfund sites and

polycyclic aromatic hydrocarbons (PAH) are the primary risk determinant at one-fifth of the

contaminated sediment sites (U.S. EPA, 2010). These sediment pose ongoing risks to complex,

aquatic biosystems, and ultimately to humans through a combination of possible pathways.

1.2 Sediment Remediation Techniques

1.2.1 Conventional Sediment Remediation Techniques

The U.S. EPA currently uses the following remediation techniques for contaminated sediments:

environmental dredging, in situ passive capping, in situ active capping, and natural monitored

recovery. Dredging is the most widely used method for EPA remediation projects simply

because removal of contaminants and treating the contaminants ex situ allows for more control

of remediation that seems inherently more preferable (Reible et al., 2003). Major disadvantages

of environmental dredging are known as the “four R’s”: potential resuspension during

implementation, associated release of disturbed contaminants, residual contamination after the

completion of dredging, and resulting health risks (Bridges et al., 2008). Capping the

contaminated sediments with a clean sand and/or clay layer is another commonly used

19

remediation alternative, and is a subaqueous adaptation of terrestrial landfill capping. The

objectives of such so-called “passive capping” are to contain and stabilize contaminants, and

reduce their interactions with the biologically active zone, usually the top 5-10 cm of the

sediment-water boundary (Reible et al., 2003). In some applications, passive capping material is

mixed with reactive materials (e.g., activated carbon, organoclay, apatite, zeolite, Aquablok®) to

increase the remediation effectiveness and chemical retardation ability of the cap, thus reducing

the cap’s breakthrough time (Barth et al., 2008). However, due to limited information on the

many mechanical and chemical processes that can affect the long term stability of such caps,

uncertainty exists over the efficacy of this approach (Olsta et al., 2006). Natural monitored

recovery relies on the intrinsic processes of bio- and chemical transformation, dispersion, and

additional sedimentation to reduce the future risks associated with contaminated sediments.

Following site characterization, the contaminated sediment area is monitored over time to verify

the effectiveness of these natural processes. Because such processes operate over a large

timescale and require stable sediment, natural monitored attenuation is a less desirable

remediation approach for most contaminated sediments (Magar et al., 2009).

1.2.2 Potential Materials for Active Cap Use

Active capping is a promising emerging strategy for in situ sediment remediation. Active caps

are similar to passive caps in that they isolate the contaminated sediments from the water column

with a physical barrier. However, active caps make use of a reactive layer that can sequester or

degrade the contaminants, thus retarding contaminant breakthrough (Yin et al., 2009). As such,

an effective barrier with an active cap can be achieved with a much thinner layer as compared to

passive caps, and has been shown effective in specialized applications, such as contaminant hot

spots in upwelling areas, which typically exhibit significant integrity risk to passive caps (Barth

20

et al., 2008). Several reactive materials have been tested for their ability to retard and contain

contaminants for their potential use in capping projects. For example, coking coal, or coke, is a

low cost petroleum byproduct agent that can sequester some organic compounds (Murphy et al.,

2006). Activated carbon (AC) is another potential active cap material that is generated from

physically and/or chemically treated carbon material, and can adsorb and degrade large amounts

of hydrophobic organic compounds very effectively compared to traditional sand caps and coke

(Zimmerman et al., 2004). Zeolite is a low cost natural aluminosilicate mineral that is capable of

demobilizing cationic contaminants (e.g., heavy metals). Additionally, zeolite can be used as

adsorbent for organic compounds and anionic contaminants when its surface is pretreated with

surfactants while leaving the porous structure untreated (Jacobs and Forstner, 1998; Jacobs and

Waite, 2004). Organoclay is a bentonite originated element modified with quaternary amines that

changes the surface cation of bentonite with organic molecules (Olsta and Darlington, 2009).

Knox et al. (2008) showed that organoclay can remove heavy metals 100% in fresh water and

75% to 100% in salt water. Also, organoclay has a significantly higher partitioning coefficient

for PAHs compared to other sequestering agents. Even under upwelling flow conditions,

organoclay have a significantly higher partitioning coefficient for PAH’s as compared to other

sequestering agents.

1.3 Reactive Core Mat Approach for Sediment Remediation

The use of reactive core mats (RCM) represents a new class of sediment remediation strategy.

The RCM, shown schematically and in a photo in Fig. 1.1, consists of a reactive layer that is

confined between permeable geotextile layers. Due to the introduction of the core reactive

material, the RCM not only isolates but also has the potential to adsorb and neutralize target

21

contaminants, resulting in the retardation of the contaminant breakthrough to the overlying

biologically active zone and overlying water column.

In the field, the RCM is applied by rolling it onto the sediment surface (Figs. 1.2 and 1.3) from

rolls that typically contain 4.5 m (15 ft) wide by 30 m (100 ft) long sheets. Each applied section

overlaps the adjacent section to ensure continuous coverage. Ends of each section are anchored

at the shoreline to provide higher stability and tensile strength. After application, the RCM is

typically covered by at least 15 cm (6 in) of sand to ensure its stability and to provide a new

habitat for benthic organisms. This application process prevents any potential sediment

disturbance and minimizes the contaminant resuspension into the overlying water column as

observed in dredging and some capping applications.

While the concept of the RCM appears to be an important advance in managing contaminated,

subaqueous sediments, there is surprisingly little data on how RCMs impact contaminants in the

sediment as well as their efficacy in sequestration of the sediment from the overlying water

column.

1.4 Scope of Work

This thesis describes experimental and numerical modeling research conducted to test the

efficacy of the reactive core mat as new means of subaqueous sediment remediation. The

following tasks were undertaken and are reported in this thesis: a new testing device, the

Integrated Contaminated Sediment Testing Column Apparatus (ICSTAC) was designed and

fabricated, and testing protocols developed for the new device; proof testing of the ICSTAC was

performed to ensure that the device is capable of working based on design criteria, and

supporting software developed for data acquisition and data reduction; natural sediment was

22

sampled from designated areas and chemical testing performed to assess contaminant levels prior

to testing; a comprehensive experimental study (7 conventional consolidometer tests and 16

ICSTAC tests) was conducted, including tests on natural sediment as well as on sediment spiked

with naphthalene; and the CS2 numerical model was modified and implemented to compare the

test results with the model prediction, and evaluate the model’s ability to predict the

experimental results.

1.5 Organization of the Thesis

Chapter 2 provides information on previous research conducted on large strain consolidation

devices (Section 2.2), large strain consolidation theories (Section 2.3), and consolidation coupled

contaminant transport experimental and theoretical studies (Section 2.4).

Chapter 3 describes the new testing device. Computer aided and mechanical design and

fabrication of the new testing device is explained in detail (Section 3.2), testing protocols, from

pre-test to post-test processes, are given (Section 3.3) and proof testing is described (Section

3.4).

Chapter 4 contains information about sediment collection and preparation and provides test

results. Information about sediment sampling locations are provided and sampling procedures are

explained (Section 4.2), the procedures for spiking sediment with reactive chemicals are defined,

conventional and ICSTAC tests are described, and the data obtained from experimental study

provided (Section 4.3).

Chapter 5 is dedicated to the numerical analysis of the experimental data with a computer model.

Formulation of the model and the procedure applied to predict experimental data are explained in

23

detail (Section 5.2), experimental data versus model prediction are given for all tests (Section

5.3) and findings from the comparison are discussed (Section 5.4).

Chapter 6 gives brief summary of thesis and its findings and conclusions and provides

recommendations for future research.

24

Figure 1.1 Schematic (left) and photo of reactive core mat (right)

25

Figure 1.2 Schematic of reactive core mat application in field (courtesy of CETCO™)

26

Figure 1.3 Reactive core mat application during Anacostia River pilot project (courtesy of CETCO™)

27

2. BACKGROUND

2.1 Introduction

This chapter provides a synopsis of previous research in three important aspects of contaminant

transport combined with consolidation of very soft sediments. Section 2.2 presents prior efforts

to develop experimental devices to measure large strain consolidation. In Section 2.3, large strain

consolidation theories for data interpretation and modeling are presented and evaluated. Finally

in Section 2.4, the theories and experimental approaches for coupled consolidation and chemical

transport are presented.

2.2 Prior Development and Use of Large Strain Consolidation Devices

To measure the consolidation behavior of soft, highly compressible soils such as very soft

sediments, slurries, mine tailings and organic soils, conventional consolidation devices are not

well-suited because of difficulties associated primarily with the large volume changes and

potential extrusion of soil around conventional loading platens that occur during consolidation.

Further, testing of high water content soil using conventional consolidation devices limits control

of boundary and applied conditions that are critical for both accurate experimental data, and use

of the results in constitutive relations. In addition to soil extrusion, use of conventional devices

allows measurement of only a very limited amount of strain before the results become unreliable

(Scott et al., 1986). A number of approaches have been developed to address these difficulties

and obtain laboratory data that have some similitude to the field problems being addressed.

These include large strain versions of conventional devices such as incrementally loaded and

constant rate of strain (CRS) consolidometers, seepage-induced consolidation tests, consolidation

tests performed in the geotechnical centrifuge, and self-weight consolidation tests.

28

2.2.1 Incremental Loading Devices

The incremental loading device is used to apply a static, constant load during each consolidation

increment. Perhaps the first attempt to measure large strain behavior in the laboratory using an

incremental loading device was initiated by Vernon (1961) on kaolinite and montmorillonite

slurries with hydraulic gradient-induced consolidation. Leonard and Altschaeffl (1964)

investigated the consolidation characteristics of artificially sedimented clay with a piston and

loading system. A piston that seals against the consolidometer cylinder wall was found to be

desirable since it prevents extrusion of the soil around the loading platen, a problem in

conventional consolidometers. Sheeran and Krizek (1971) developed a more advanced device

(Figure 2.1) for consolidation of an illitic slurry (initial water content, w = 240%), which was

used to obtain homogeneous, reproducible, consolidated samples such that specimens could be

trimmed for follow-on laboratory tests. Their device consisted of three parts: the consolidometer,

the control panel and an automatic load application device that had a capacity of 1725 kPa (250

psi). Features of this device included the use of a sealed soil loading piston that prevented soil

extrusion, and an equipment configuration that allowed the application of backpressure for

specimen saturation during consolidation. Reports of other efforts to produce uniform soils for

laboratory testing by sedimentation and large strain consolidation include the long-time

production of resedimented Boston blue clay at MIT (e.g., Germaine, 1982; initial w = 100%) as

well as at Northeastern University (Sheahan and Watters, 1997, initial w = 100%).

Lee (1979) developed an innovative large strain consolidation device (Figure 2.2) to test

Bridgewater Bay silt (initial w = 330%) in which self-weight sedimentation was followed by

incremental piston loading from below, with the goal of minimizing disturbance and achieving

high quality stress control; strains from 50% to 66% were obtained. This innovative device thus

29

“loaded” the soil vertically downward under self-weight during initial sedimentation (durations

of one or two days), followed by vertical loading upward to apply high stress levels to achieve

the final consolidation state and water content. Carrier and Keshian (1979) developed a large

strain consolidation cell (Figure 2.3), to test the behavior of dredged sludge material, capable of

measuring strains up to 80%, and also had the capability to test the slurry’s hydraulic

conductivity after each increment. This was achieved by the application of very low hydraulic

gradients between increments to avoid seepage-induced deformations. When consolidation under

a load increment was completed in this device, a loading piston was locked in place and a

constant head permeability test performed. Another interesting aspect of their procedures

provided for the applied stress to be reduced automatically, based on electronic pressure

measurements during permeability test phases to prevent seepage-induced consolidation. Kurup

(1993) developed another version of the slurry consolidometer (Figure 2.4) to prepare

homogeneous, high water content slurry samples of two different fine sand and kaolin clay

mixtures. These were to be used for penetration tests under different boundary conditions. To

minimize possible sample disturbance during specimen extrusion, the consolidation cylinder

consisted of a split PVC tube that was disassembled after consolidation, exposing the resulting

soil sample. Aydilek et al. (1999) developed an incremental loading slurry consolidometer with

sealed piston loading system (Figure 2.5). The device was developed to obtain the consolidation

characteristics of Madison (Wisconsin) Metropolitan wastewater sludge (initial w = 305%)

contaminated by 50 ppm PCBs, the use of which was proposed for a landfill capping project. To

obtain data for a consolidation and contaminant transport constitutive model, the device could be

used to perform a hydraulic conductivity test at the end of each increment.

2.2.2 Seepage-Induced Consolidation Devices

30

To investigate consolidation behavior of high water content soils immediately after

sedimentation, a testing device must be capable of accurately measuring consolidation

characteristics, including stress versus void ratio dependence at very low effective stresses. In a

seepage-induced consolidation device, consolidation is initiated using a hydraulic gradient

applied vertical through the specimen; such a method is effective for soils with low initial

effective stress states. Imai (1979) built a seepage-induced consolidation testing device to

provide a downward seepage consolidation stress superimposed on stresses due to specimen self-

weight. This accelerated the sedimentation process for their soil, Osaka Bay mud (initial w =

1000%), and enabled direct measurement of consolidation constitutive relations (effective

vertical stress-void ratio and permeability-void ratio relationships) without the need for

theoretical interpretation, such as that required for the CRS-based large strain consolidation

approach, even for very low effective stress ranges. The test duration for their tests, even with

the combination of self-weight and seepage gradients, was excessively long, even by the

standards for fine-grained soils. Abu-Hejleh et al. (1996) developed a modified version of Imai’s

device (Figure 2.6) that allowed for seepage-induced consolidation testing at low stress levels

and incremental loading tests for larger stress levels with direct permeability measurement

capability. They sedimented phosphatic waste clay (initial void ratio, eo, ranging between 13 to

32), to study the mechanical behavior of phosphatic waste clay ponds and to develop a more

accurate approach for estimating consolidation parameters.

2.2.3 Self-Weight Consolidation Devices

To observe the mechanics of soft soil formation in an aqueous environment (sediment deposition

and self-weight consolidation), consolidation behavior of slurries has been studied using tall

column, self-weight consolidation tests. After sedimentation, these tests simply rely on self-

31

weight consolidation of the slurry for effective stress development. These tests have been

particularly relevant for measuring the consolidation behavior of mine tailings, as well as

consolidation of very soft subaqueous sediments being used as foundation soils in land

reclamation projects. For example, Imai (1981) reported on sedimentation and self-weight

consolidation tests on Osaka Bay Mud (in this case, the initial water content was report as wi =

2000%) used to obtain the time-dependent water content distribution through the column. Been

and Sills (1981) built a 2 m high, self-weight consolidation column (Figure 2.7) for soft soils

(Combwich Estuarine mud, initial density ρi = 1.02-1.15). They wanted to obtain experimental

data to validate their theoretical study on sedimentation and consolidation of soft soils. Scott et

al. (1986) used a 10 m high self-weight consolidation column (Figure 2.8) to obtain the self-

weight consolidation characteristics of mine tailings in disposal lagoons (an oil-sand slurry with

typical wi = 500%) with the goal of estimating lagoon capacity over time. However, as with

Imai’s (1981) tests, while these self-weight tests provide similitude to the field situation, the

duration of such self-weight tests make their use highly impractical in most situations (e.g., Scott

et al., 1986 reported test durations exceeding one year).

2.2.4 Constant Rate of Strain (CRS) Devices

Constant rate of strain tests (CRS) are performed to analyze the consolidation of very soft soils

with high water content due to difficulties faced during piston or loading platen actuated loading

tests during their initial phases. They are particularly useful for capturing deformation

measurements at early loading times, and this was especially important before the introduction of

computer automated data acquisition systems. Among those using mechanically driven CRS test

devices and Rowe consolidation cells (hydraulically driven consolidation cells) for large strain

consolidation, Berry and Poskitt (1972) tested amorphous granular and fibrous peat (eo = 7)

32

using a Rowe cell, recording a maximum vertical strain level of 40%, to generate data for

modeling peat consolidation. Zen and Umehara (1986) developed a CRS consolidation device

for testing Honmoku clay, Tokyo Bay mud and Minamata Bay mud (initial water contents

ranging from wi = 200 to 230%). The use of the CRS decreased the testing duration significantly

from other methods (e.g., incremental loading), and provided the ability to preload the specimen,

enabling a well-defined initial stress state for reference. However, even the more efficient CRS

testing required very slow rates of deformation at low stress-strain states, resulting in longer

testing durations during those phases.

2.2.5 Summary of Large Strain Consolidation Devices

In summary, in the testing of very soft soils in large strain consolidation, while incremental load

devices enable fairly short testing durations compared to other methods, special precautions are

required to prevent soil extrusion. Further, difficulties arise in the observation of the initial rate

of consolidation (especially the use of a sealed loading piston rather than a floating loading

platen). Seepage-induced consolidation tests are used to examine the very low effective stress

consolidation conditions on highly compressible soils with hydraulic gradient application. Data

from such tests directly provide void ratio-hydraulic conductivity relationships at very low stress

ranges, but can be complex to implement and can be prone to interior sidewall leakage due to

preferential flow paths. Self-weight column devices are well-suited for tests to mimic

sedimentation and very low stress consolidation of high water content materials, but experiment

durations can be extremely long due to use of only gravity driven stresses. CRS devices facilitate

evaluation of data with the associated ease of defining the stress state of the specimen throughout

the consolidation process, but experiment durations can be lengthy due to very low strain rates

required at low stress levels.

33

2.3 Large Strain Consolidation Theories for Data Interpretation and Modeling

2.3.1 Introduction to Large Strain Consolidation Theories

Large or finite strain consolidation behavior presents a daunting modeling challenge since it

typically involves a material that is highly compressible, has a high water content, and an

associated high initial void ratio. The stiffness and pore volume change significantly during

consolidation, and there is a large amount of associated pore fluid flow. It is truly a dynamic

environment in which critical modeling parameters vary both spatially and temporally. This

section provides an overview of efforts to address this modeling need over the past 50 years.

The basis for the models begins with Terzaghi’s (1925) conventional consolidation theory. As is

well-known, this theory provides an estimate of the pore pressure dissipation and time rate of

settlement for a specific loading condition by considering the fundamental physics of soil

behavior. Terzaghi’s theory is a small or infinitesimal strain theory in which the applied load

increment causes only small strains in the compressible layer. This simply indicates both

coefficient of compressibility (av, Eq. 2.1) and the hydraulic conductivity (k) is constant

throughout the load increment, and thus a unique relationship between void ratio and effective

stress is obtained:

(2.1)

For the primary consolidation derivation the key processes are the incompressible pore fluid flux

governed by k, and excess pore pressure, u, which varies over time, t, and depth in the

consolidating layer, z (Holtz and Kovacs, 1981):

34

(2.2)

where ρw is the density of water and g is the gravitational constant. Volume change due to

change in effective stress can be computed,

(2.3)

By equating Eqs. 2.2 and 2.3, the governing equation of Terzaghi’s consolidation is obtained:

(2.4)

After rearranging Eq. 2.4, the commonly used version of Terzaghi’s consolidation theory is

obtained:

(2.5)

where the coefficient of consolidation, cv, is

(2.6)

However, for very soft soils undergoing large strains (or what are are referred to as finite

strains), these parameters can change significantly in a given stress increment (e.g., Gibson et al.,

35

1967; Duncan, 1993). There have been several approaches to address this issue. For example,

one can adopt variable parameters (e.g. permeability, compressibility) within conventional

Terzaghi one-dimensional consolidation theory (Davis and Raymond, 1965; Barden and Berry,

1965), but still under the assumption of small strains.

2.3.2 Early Large Strain Consolidation Models

A milestone in the development of a more realistic, large strain consolidation theory was the

definition of a material coordinate system (McNabb, 1960), which basically considers a small

sublayer, with specified datum and boundaries, that is part of the larger consolidating layer. This

approach set up the solution of large strain problems using an iterative, numerical analysis

method. It is noted that until the mid- to late 1970s, when digital computers were introduced,

such numerical solutions could not commonly be adopted.

Mikasa (1965) developed a one-dimensional, large strain consolidation theory that accounts for

self-weight consolidation, and also considered variable permeability and compressibility in a

manner that parallels Terzaghi theory since it assumes no particle movement. Mikasa’s theory

was later modified for evaluating constant rate of strain (CRS) test data by Zen and Umehara

(1986). Gibson et al. (1967) combined the work of McNabb (1960) and Mikasa (1965) to

develop a seminal model used for most of the large-strain consolidation theories developed since

that time. They considered finite strain, the relative velocity of pore fluid to soil particles within

Darcy’s flow law, variable permeability and compressibility, non-homogeneity of soil, and the

compressibility of both the pore fluid and the soil particles. Poskitt (1969) developed a large

strain theory using the perturbation series and power functions for the solution of the governing

differential equation, as follows:

36

(2.7a)

(2.7b)

where A is the parameter needed to be obtained, is the approximation of the full solution, Ao is

the known solution of the exactly solvable initial problem, and A1, A2, … are higher order terms

that can be obtained iteratively. Berry and Poskitt (1972) accounted for secondary consolidation

within a large strain consolidation theory, a development particularly important for high

plasticity and high organic content soft soils. Monte and Krizek (1976) developed a new large

strain consolidation theory that considers the initial, “stress-free” state (just before the start of

self-weight consolidation), and set the initial strain at the point when effective stresses start to

develop between particles at the end of sedimentation. They used semi-empirical power

functions to solve the governing equation. Schiffman (1980) described a new large strain theory

similar to the work of Gibson et al. (1967) but with Lagrangian coordinates.

2.3.3 Evaluation of Analytical Approaches on Large Strain Consolidation Problem

As the problem of mine tailings disposal required more exact analyses, the application of large

strain consolidation theory became more widely used for that application. Cargill (1984)

developed solution charts based on Gibson et al. (1967) theory in linear form; however, in a

discussion to this paper, Carrier (1984) asserted that linearized parameters cannot reflect the true

behavior of soft clay. Mikasa and Takada (1986) developed three different approaches: a

“standard” method with a primary consolidation ratio correction; a method for finite strain in

which cv is constant; and, a method for finite strain with a variable cv, modified using either

linear and non-linear approaches. Later, McVay et al. (1986) compared published experimental

37

and theoretical predictions of soft soil consolidation and concluded that both linear and non-

linear small deformation models underestimate the consolidation behavior compared to large

strain consolidation models (Figure 2.9). Figure 2.10 shows that all of the large strain

consolidation models compared were able to successfully estimate the field settlement values;

however, they failed to estimate the rate of settlement at early stages of consolidation. To solve

the problem of seepage-induced consolidation in sedimented slurries, Huerta et al. (1988)

developed a one-dimensional large strain consolidation model. Kiousis et al. (1988) developed a

computational method based on an advanced elasto-plastic large strain formulation to examine

the deformations taking place during the cone penetration, with results that fit well with the soil

response during the penetration process. However, due to the introduction of large strains,

questions were raised about the reliability of the constitutive relationships. Morris (2002)

published an analytical model based on Gibson et al. (1967) theory that can account for the

consolidation of unconsolidated, remolded soil.

2.3.4 Piecewise-linear Large Strain Consolidation Theories by Fox and Co-Workers

The work of Fox and his co-workers represents the next generation of large strain consolidation

models. Starting with Fox and Berles (1997), who introduced the first of these models (CS2),

these models had as their basis a piecewise-linear, large strain model that considers the soil layer

as a set of thin sublayers with soil characteristics that vary with both depth and time based on

constitutive relationships (i.e., void ratio-effective stress, void ratio-hydraulic conductivity). One

feature of this model is that it is much easier than other formulations in the literature to apply and

modify with different initial and boundary conditions. Piecewise-linear models differ from

analytical large strain consolidation theories such as Gibson et al. (1967) in their numerical

implementation of constitutive relationships that enables the simultaneous solution of parameters

38

by iterations in time and space until the system reaches equilibrium without the need for a

complex set of differential equations. The CS2 model results were compared with experimental

results (Fig. 2.11) and field test results (Fig. 2.12) performed in Madison Metropolitan Sewerage

District site (initial water content, wi = 305%) reported by Aydilek et al. (1999), with the

conclusion that the CS2 results closely matched those from the field test, and showed a fair

match with experimental data. Fox (1999) developed solution charts based on CS2 that enable

estimation of large strains by basic hand calculations. Fox (2000) developed a piecewise-linear

model called CS4 for one-dimensional accreting soil layers by using a fixed Eulerian coordinate

system. Fox and Qiu (2004) developed a piecewise model that can also account for

compressibility of the pore fluid in addition to the consolidation mechanics predicted by the CS2

model. Although compressibility of pore fluid is negligible for most engineering problems, it

may be important for high stress loadings on stiff materials, deep deposits with high pore

pressure, and consolidation of materials in which the pore fluid is a gas or a mixture of gas and

liquid. Fox et al. (2005) published the CC1 model that accounts for the large strain consolidation

under centrifuge-induced consolidation stresses. The CC1 model is capable of including

variation in the acceleration factor with depth, in addition to incorporating properties of the CS2

model.

Table 2.1 summarizes the primary finite strain models, and provides an overview of the

evolution of relevant work.

2.3.5 Summary of Large Strain Consolidation Theories

Numerous attempts have been made to model the mechanics and unique compressibility

characteristics of high water content, soft soils. Starting with Gibson et al. (1967), the variability

39

of hydraulic conductivity and compressibility has been taken into consideration and there was

steady progress toward defining the problem (i.e., gradual rather than step-wise load application,

compressible pore fluid, etc.). Current large strain consolidation models are capable of defining

and simulating the compression response of the high water content soil by accounting for several

variability and boundary conditions. However, since the recent studies include most of the major

soil variables within their model, introducing more variables with decreasing emphasis on

compression behavior will probably be the next phase in large strain consolidation theory

development.

2.4 Coupled Contaminant Transport Studies

2.4.1 Introduction to Coupled Contaminant Transport

As contaminant remediation and waste disposal projects gained importance in the United States

and world-wide, isolation of contaminants either in situ or ex situ (e.g., storage of waste slurries

in engineered landfills) became an important research field to fully understand the mechanical

behavior of the matrix, and the fate and transport of contaminants during remediation and

storage. The conventional dispersion-advection equation for non-deformable porous media has

been used to describe the contaminant transport phenomena (e.g., Bear, 1972; Freeze and Cherry,

1979). However, while these approaches accounted for advection due to conventional hydraulic

gradients and diffusive transport due to concentration gradients, they did not consider

consolidation-induced advective flows. As one example of the impact of this additional, often

significant phenomenon, breakthrough time in landfill liners can be overestimated depending on

the direction and rate of hydraulic gradient induced advection (Alshawabkeh et al., 2005; Fox

40

2007a). This section provides background on research efforts to characterize coupled

consolidation and contaminant transport behavior.

2.4.2 Initial Work to Link Contaminant Transport to Gradient-Induced Advection

With the widespread use of clay liners for waste repositories, there was a growing need to

understand how compression of those liners in the presence of contaminants could lead to

contaminant transport through those materials. Barbour and Fredlund (1989) were apparently the

first investigation on the influence of pore water chemistry on mechanical behavior of

compressible clay layers and induced contaminant transport. They developed an analytical

explanation of the interacting phenomena, and used experimental studies to show that changes in

pore fluid chemistry can significantly affect the compressibility of the clays. Shackelford and

Daniel (1991a) defined the parameters for diffusion in saturated soils and Shackelford and

Daniel (1991b) performed an experimental study on compacted clay barriers to define diffusion-

dominant contaminant transport and the factors affecting the process. Closed form and numerical

computer model solutions were compared to the experimentally obtained effective diffusion

constant (D*), which defines the molecular diffusion ability within the soil media, and they

found that retardation factor has a significant impact on the determination of D*:

(2.6)

where D is the free molecular diffusion coefficient in water and τ is the tortuosity that accounts

for the irregular porous structure of the soil media.

Similarly, Rowe (1994) investigated diffusive transport of contaminants through clay liners and

compared field data and different computer models. He quantified the intuitive conclusion that

41

diffusive transport of contaminants accelerates the breakthrough of contaminants through these

liners, and should be considered in landfill liner design.

2.4.3 Theoretical and Experimental Approaches on Consolidation Coupled Contaminant

Transport

The effect of consolidation on contaminant transport was first considered by coupling small

strain consolidation and contaminant transport theories, which increased the accuracy of

modeling contaminant transport phenomena in consolidating layers compared to the use of non-

deformable soil media in such models. After this, large or finite strain models were adopted for

more accurate depictions of the compressibility behavior and subsequent impacts on the

advection properties such as; effect of deforming porous media, considering relative velocity of

contaminants moving with pore fluid egress to moving with solid particles.

2.4.3.1 Small Strain Consolidation Coupled Contaminant Transport

Potter et al. (1994) developed the first analytical approach to consider the effect of consolidation

on conventional advective/dispersive contaminant transport from a waste layer. They used the

centrifuge to conduct their experiments to take advantage of the both the time and dimensional

scaling in such tests. An experimental capping section (Figure 2.13) was modeled as

contaminated mineral slurry being covered by a non-reactive sand and gravel cap and underlain

by a permeable contaminated stratum. It was observed that following the cap placement, high

excess pore pressures developed in the contaminated slurry and caused high pore fluid flow

through the cap, accelerating contaminant breakthrough from the cap layer. Loroy et al. (1996)

developed a finite element analysis approach to model the contaminant transport within the

capped sediment layer of Potter et al. (1994), and provided a comparison between concentrations

42

in the cap with and without consolidation. As one would expect, there was a significant

difference between the two scenarios. Potter et al. (1997) performed centrifuge tests simulating

the long term behavior of waste disposal sites and corresponding discharge of contaminated

water due to consolidation of capped mineral waste. They developed a finite element model that

was validated by the centrifuge test data, but this underestimated the total pore pressure in waste

clay layer throughout the process and could not estimate the contaminant concentration within

the system precisely. Those errors were credited to the high water content of the waste slurry and

the assumption of small strain consolidation. Peters and Smith (1998) developed a coupled

contaminant transport model with small strain consolidation to investigate the observation that

premature volatile organic compound (VOC) breakthrough was occurring through a

geocomposite liner. Their model considered contaminant transport occurring under 3 conditions:

a) only diffusion through non-deforming media; b) advection plus diffusion through non-

deforming media; and c) advection plus diffusion through deforming media. Again, as would be

expected, their results indicate that when advection induced by consolidation is considered, it

significantly accelerates the contaminant breakthrough. Such was the state of research at this

point that investigators were simply trying to quantify a phenomenon that was logical but not

well documented.

Van Impe et al. (2002) included chemically and osmotically induced effects in a model that

considered consolidation-induced contaminant migration. Mazzieri et al. (2002) carried out

column tests on flexible permeameters to determine the effect of consolidation on contaminant

transport parameters. It is noted that the retardation factor is insignificantly influenced by

effective stress, and due to numerous factors, hard to predict.

43

(2.7)

In Eq. 2.7, Rd is the retardation factor, which is the ratio of transport time of the reactive tracer to

the transport time of non-reactive tracer from same path, ρd is the dry density of media, n is

porosity and Kd is the partitioning coefficient of the contaminant with given media.

Tang et al. (2004) performed an experimental study on consolidation-induced transport of non-

reactive contaminants using a modified consolidometer. Results of this experimental study

revealed that consolidation-induced advection is a significant component of contaminant

transport only at the initial stage of consolidation. Alshawabkeh et al. (2005) developed a

numerical model for contaminant flux in capped sediment under consolidation, assuming small

strain consolidation and a corresponding constant relationship between hydraulic conductivity

and porosity during the load increment. Figure 2.14 shows the obtained breakthrough curves

with and without consolidation and these clearly demonstrate the effect of consolidation on

contaminant transport.

2.4.3.2 Large Strain Consolidation Coupled Contaminant Transport

There are important cases in which contaminant transport takes place through high water content,

highly compressible materials. As a result, large strain consolidation must be considered for

more accurate estimation of contaminant transport in these soils and sediments. As discussed in

Section 2.4.3.1, accounting for small strains in such materials can lead to underestimating

consolidation rates, which in turn leads to underestimating contaminant transport rates.

Gibson et al. (1995) combined large strain consolidation theory (Gibson et al., 1967) and

contaminant transport in a finite element model for field predictions with higher accuracy.

Smiles (2000) examined solute transport through unsaturated bentonite conducting experimental

44

study and comparing results to a numerical model. Smith (2000) developed a coupled

contaminant transport model for quasi-steady-state conditions that considers the movement of

contaminants in the dissolved phase via pore fluid flow and in the adsorbed phase by transport

associated with solid particle movement. Smith stated that if contaminant transport due to solid

movement is not accounted for, the approach may lead to underestimating contaminant

breakthrough times.

Moo-Young et al. (2001) developed a centrifuge-based model to investigate the contaminant

transport through capped sediments. The centrifuge tests were performed using a 6.5 m radius

device with 10 to 350g acceleration range. The sediment specimen (initial water content, wi =

41%) was 42 cm in diameter, 6.5 cm high, and capped with a 1.5 cm thick clean sand layer.

Peters and Smith (2002) derived a solution for solute transport through a deforming porous

media for both material and spatial coordinate systems, and compared their model with some of

the existing contaminant transport models for transient and steady-state flow conditions. It was

observed that the effect of consolidation becomes more pronounced with decreasing cv without

sorption of contaminants. Peters and Smith (2003) examined the effective of coupled chemical

transport (by considering the effect of pore fluid chemistry on the diffuse double layer) and

mechanical consolidation of solute transport through a deformable soil layer. It is interesting to

note that their results indicate that applied osmotic flow can actually cause a negative Darcy

velocity (i.e., velocities counter to the applied gradient) and retard the contaminant transport

within the clay liner for the consolidation-induced advection case.

2.4.3.3 Piecewise-linear Approach to Coupled Contaminant Transport

45

Fox and Berles (1997) introduced the piecewise-linear approach that he and his co-workers had

applied to large strain consolidation into the coupled contaminant transport problem. The

versatility and flexibility of the piecewise-linear approach have enabled the introduction of some

very specific parameters such as non-linear and non-equilibrium sorption.

Fox (2003) developed a piecewise-linear model (CCT1) for contaminant transport in

consolidating soft soils that combines the large strain consolidation model CS2 (Fox and Berles,

1997) and the movement of contaminants in their liquid and solid phases. The consolidation part

of the model uses an Eulerian (spatial) coordinate system, and the contaminant transport part

uses a Lagrangian (material) coordinate system. Fox (2007a) improved the CCT1 model and

developed a two-dimensional solute transport model (CST1) with linear, equilibrium sorption

assumption coupled by one-dimensional large strain consolidation. A saturated soil layer is

modeled with a series of thin layers, and the movement of the solid contaminant phase and pore

fluid (with dissolved contaminants) are treated separately, as shown in Figure 2.15.

Consolidation and breakthrough results from CST1 showed great similarity to those from Peters

and Smith (2002, Fig. 2.16), and also provided a good match with the analytically obtained

results for different combinations of contaminant transport (Figure 2.17). In the CST2 model,

Fox (2008) added a non-linear, non-equilibrium sorption capability to CST1. Lee and Fox (2008)

performed experimental investigation of consolidation-induced contaminant transport. They

investigated contaminant transport from the kaolin specimen (initial water content wi = 104%) in

a consolidation test apparatus that applies dead weight loading, and uses a peristaltic pump to

provide additional advective submarine flow with potassium ion (K+) as reactive tracer and

bromide ion (Br-) as non-reactive tracer. Results have shown that CST2 is fairly good at

46

estimating non-reactive and reactive coupled contaminant transport observed in experimental

tests.

2.4.4 Summary and Need for Further Work

The contaminant transport problem through reactive/non-reactive cap layers and various

deformable porous medium have been investigated by numerous authors, ranging from

advective/diffusive flow within non-deforming media to non-linear, non-equilibrium coupled

transport in large strain applications. Most of the basic questions about the consolidation coupled

contaminant transport have been addressed for single compound or independent multiple

reactive/non-reactive compounds. Further research on coupled contaminant transport should

focus on competitive sorption controlled coupled transport of multiple contaminants for more

accurate and deeper investigation of the phenomenon.

47

Table 2.1 Summary of theoretical large strain consolidation studies Reference

(type of solution approach)

Significance General Equation

Terzaghi (1925) (analytical)

• First approach to mathematically define the one-dimensional infinitesimal strain

• Despite the well-known limitations still valid and most commonly used consolidation theory

McNabb (1960) (analytical)

• Material coordinate first defined rather than particle approach

• Linearized solution for Terzaghi’s 1-D consolidation theory obtained

Mikasa (1965) (analytical)

• Eulerian strain is governing factor. • Permeability and compressibility can be

variable. • Self-weight effect can be taking into account for

thick layers.

Gibson et al. (1967) (analytical)

• Darcy’s law related to velocity of the soil skeleton and pore fluid to the excess pore pressure gradient.

• Non-homogeneity, time-intrinsic effects to soil skeleton and compressible pore fluid and solids are allowed in the theory.

• Widely used as foundation for deriving theories (Fox and Berles, 1997).

Poskitt (1969) (analytical)

• Solved the large strain consolidation equation by perturbation method using a power series.

Berry and Poskitt (1972)

(analytical)

• Developed large strain consolidation theory which also considers the effect of secondary compression.

• Considered amorphous granular peat material of interest.

48

Monte and Krizek (1976)

(analytical, semi-empirical)

• Used semi-empirical power relations and finite-element approach to solve large strain consolidation problem.

Schiffman (1980) (analytical)

• Developed a new approach which considers the variation throughout the soil depth by using Gibson et al.(1967) as base model.

Huerta et al. (1988) (analytical)

• Developed a one-dimensional mathematical model with large strain theory to solve seepage-induced consolidation phenomena.

Cargill (1984) (graphical solution

charts)

• Developed solution charts based on linearized and normalized Gibson et al.(1967) theory.

Not Applicable

Mikasa and Takada (1986)

(graphical solution charts)

• 3 new graphical methods to modify the experimental Cv value for site estimation.

McVay et al. (1986) (semi-empirical,

analytical)

• Compared the experimental and theoretical prediction of consolidation of soft soil.

• Revealed nonlinear approach is more realistic than linear attempts.

Kiousis et al. (1988) (analytical)

• Developed a computational model based o advanced elasto-plastic large strain deformation and solved using the finite element method.

Fox and Berles (1997) (numerical)

• Development of a piecewise-linear model which is numerical attempt based on constitutive relationship of the soil.

Fox (1999) (graphical solution

charts)

• Solution charts based on CS2 piecewise linear method for hand application.

Morris (2002) (analytical)

• New analytical method allows for the settlement estimation for unconsolidated soil based on Gibson et al.(1967) and Cargill(1984), linear and normalized equations.

Fox and Qiu (2004) (numerical)

• New numerical model which accounts for compressible pore fluid in addition CS2 results.

49

Figure 2.1 Slurry consolidometer used to prepare samples of illitic slurry (Sheeran and Krizek, 1971)

50

Figure 2.2 Large strain consolidation device to test sedimented Bridgewater clay followed by incremental loading consolidation (Lee, 1979)

51

Figure 2.3 Slurry consolidation cell to test dredged sludge material (Carrier and Keshian, 1979)

52

Figure 2.4 Slurry consolidation device to prepare reproducible clay samples for cone penetration tests (Kurup, 1993)

53

Figure 2.5 Slurry consolidometer to obtain consolidation characteristics of Madison Metropolitan wastewater sludge (Aydilek et al., 1999)

54

Figure 2.6 Seepage induced consolidation device to test phosphatic waste clay consolidation characteristics (Abu-Hejleh et al., 1996)

55

Figure 2.7 Self-weight consolidation column (2 m high) used to test Combwich Estuarine mud (Been and Sills, 1981)

56

Figure 2.8 Self-weight consolidation column to obtain the mechanical behavior of oil sand slurry (Scott et al., 1986)

57

Figure 2.9 Comparison between large strain consolidation models and linear/nonlinear small deformation models (McVay et al., 1986)

58

Figure 2.10 Comparison of settlement versus time from large strain consolidation theory and field results (McVay et al., 1986)

59

Figure 2.11 Comparison of experimental average degree of consolidation versus time data from slurry consolidometer and conventional consolidometer, and CS2 model estimations

(Aydilek et al., 1999)

60

Figure 2.12 Comparison of settlement versus time in Madison Metropolitan Sewage sludge field test data and CS2 model estimation (Aydilek et al., 1999)

61

Figure 2.13 Schematic of capped sediment section used as base model in finite element analyses (after Potter et al., 1994 and Loroy et al., 1996)

Overlying Water

Waste Layer

Sand Cap

Silt Stratum

Aquifer

62

Figure 2.14a Model of subaqueous sediment section used for contaminant transport modeling (Alshawabkeh et al., 2005)

Figure 2.14b Breakthrough curve of contaminant in 25cm thick sand cap without consolidation, with retardation factor =10 (Alshawabkeh et al., 2005)

Figure 2.14c Breakthrough curve of contaminant within 25cm thick sand cap with consolidation, with retardation factor = 10 (Alshawabkeh et al., 2005)

63

Figure 2.15 Saturated soil layer configuration a) before loading b) after loading, for piecewise-linear approach to coupled contaminant transport (Fox, 2007a)

64

Figure 2.16 Comparative settlement versus time curves (left) and contaminant breakthrough concentrations with time (right) curves for CST1 Model and Peters and

Smith (2002) models (Fox 2007b)

65

Figure 2.17 Comparison of contaminant concentrations in a consolidating layer at different times using the CST1 model (Fox 2007b)

66

3. DESCRIPTION OF THE NEW DEVICE

3.1 Introduction

A new testing device was developed to provide valid data about the efficiency of the reactive

core mat for possible use in the remediation of sub-aqueous environments in Superfund projects.

The new device, known as the Integrated Contaminated Sediment Testing Apparatus Column

(ICSTAC), has a multifaceted purpose. It is a large, or finite, strain consolidation device with a

mechanism for incremental consolidation loadings as well as seepage tests for very soft

subaqueous sediments, with the goal of measuring their resulting mechanical behavior. It is also

used as a contaminant transport device to assess the capability of the reactive mat to remediate

and/or sequester contaminants in these sediments by creating consolidation-induced advective

and dispersive transport of pore fluid contaminants up through the reactive mat, where they are

hypothesized to be either adsorbed or rendered inert. The third function of the ICSTAC is as a

bioavailability test device. A clean sand layer is placed over the reactive mat and consolidation

loading piston, and after the column test, this material is used for bioaccumulation exposure tests

on polycheate worms. These three functions represent the complexity of the field problem, and

were integrated into the design of the ICSTAC. In addition, attempts were made to design and

fabricate the ICSTAC for optimum usability and within a reasonable budget.

3.2 Design and Fabrication of the New Device

3.2.1 Mechanical Design and Fabrication

Figure 3.1 shows a schematic of the ICSTAC at conceptual design phase. With the contaminated

sediment, reactive core mat (RCM), overlying clean sediment (“biogeneration zone”) and water

column, the device is intended to be an accurate physical model of the subaqueous sediment

67

“stack” with the RCM and biogeneration zone in place. An overview of test procedures and

device usage is presented. First, contaminated sediment with a known initial concentration is

placed in the acrylic test cylinder and a section of RCM placed on it. The loading platen is then

put in contact with the RCM and locked off to ensure that no load is applied. The clean sand

(mixed with an organic material, 3% Omega One® “trout chow” by mass to promote bio-

adsorption of any contaminants that break through the RCM and piston) is placed on the loading

platen, and the rest of the column filled with deionized water. The water column can be

pressurized for any depth up to an equivalent pressure of 25 m (82ft) of water.

The primary component of the ICSTAC device is the acrylic column, 20.3 cm (8 in) diameter,

40.6 cm (16 in) high, which serves as both the vertical process column for the testing, and also

serves as a guide and sealing cylinder for the loading piston to travel through. Two independent

pressurized water cylinders, actuated by deadweight hangers, provide flow and pressure through

the top inlet to the overlying water column, and through the bottom inlet to the base of the

sediment specimen (one pressure cylinder for each inlet). This arrangement allows either equal

backpressure to be applied across the specimen for system saturation, or differential pressures for

constant gradient fluid flow (versus those induced by consolidation stresses). Sampling ports in

each fluid line allow chemical samples to be taken during the test. The loading piston is actuated

by deadweight loads applied via an innovative pulley system. Details of the design of each

component are discussed in depth later in this chapter.

3.2.1.1 Computer Aided Design of the Components

Traditionally, two-dimensional computer aided design (CAD) tools such as AutoCAD™

(Autodesk Corp. 2010) are used to generate drawings for fabrication in Civil Engineering. The

use of static drawings that result from this software often leads to issues in visualization and

68

assembly. As a result, parts often need to be sent back to be modified or adjusted. In order to

avoid this, a new type of design software, SolidWorks™ (by Dassault Systémes SolidWorks

Corp.) was used during the design of ICSTAC device.

The foundation elements of the SolidWorks™ environment are so-called “sketches” and

“features.” Sketches are small drawings, meant to formulate the geometry of the model. Features

utilize the defined geometry to create a three-dimensional solid. Sketches and features can be

linked to other sketches and features in a parent-child relationship, or can be defined absolutely.

The advantage of a sketch- and feature-based environment is that changes in design can be

rapidly implemented and visualized. Additionally, many alternatives can be generated and

compared by activating or suppressing features. SolidWorks™ models, once created, can be

assembled virtu

a new coupled consolidation and contaminant transport ...825/fulltext.pdfa new coupled consolidation...

Documents