The Long-term Performance of Cement-Bentonite Slurry ... yr projects 2007... · in close contact...

i

The Long-term Performance of

Cement-Bentonite Slurry Trench Cut-Off Walls

in Contaminated Ground

by

J.P.K. Muriuki (DOW)

Fourth-year Undergraduate Project in Group D,

2006/2007

“I hereby declare that, excepts where specifically indicated,

the work submitted herein is my own original work.”

J.P.K. Muriuki 30/05/2007

TECHNICAL ABSTRACT

ii

Cement-bentonite slurry trench cut-off walls were first used in the UK at a landfill site in

1986. They are a type of low permeability in-ground barrier that remediate contaminated land

by interrupting the pollution pathway, and hence isolating the contaminant source from a

vulnerable receptor.

Cut-off walls are deemed a cost-effective method for of remediation, and hence over 100 such

walls exist in the UK today. A single-phase method of construction is generally used, in

which a continuous trench is excavated under the support of self-hardening cement bentonite

slurry, which then sets to form the required low permeability barrier.

A legislated ICE National Specification exists to provide guidance on the design,

construction; of cement-bentonite cut-off walls, to ensure the final product meets the required

minimum performance specifications. Although there has been no known failure of a cut-off

wall in the UK, there is increasing concern within the construction industry over the fact that

the National Specification does little to do address the long-term durability of slurry walls in

aggressive, contaminated ground.

As a result a, a test-site was commissioned at a former gasworks site in April 1996. A total

length of 120m of cement-bentonite slurry trench cut-off walls was constructed in the

chemically aggressive ground. Over the past 11 years, research has been conducted by the

Building Research Establishment in collaboration with the Cambridge University Engineering

Department, to monitor any changes in the hydraulic and mechanical properties of the wall.

Between 1997 and 2005, parts of the wall have been excavated throughout their entire depths,

to monitor changes in the wall structure. During all of these excavations the walls were found

to be intact, however, parts of walls constructed adjacent to contaminant hot spots were found

to be soft to touch, which pointed to a possible loss in mechanical strength. Block samples

were exhumed from the excavated walls at different depths and stored underwater for further

laboratory testing.

The block samples obtained provide a valuable resource for further research as they are a

direct representation of the in-ground structure that in some cases had been exposed to

aggressive contaminants for over 8 years. It was therefore decided to conduct a research

TECHNICAL ABSTRACT

iii

project focussed on assessing the effects of aggressive contaminants on the permeability,

mechanical strength and chemical composition of the cement-bentonite wall structures.

The first tests examined the properties of 11-year-old samples that had been exposed to

varying levels of contaminants on the field. The results indicated that there was a noted

change in the chemical composition of the samples that had been exposed to contaminants,

which was characteristic of sulphate attack. Permeability testing of these samples revealed

that this noted change had little or no effect on the hydraulic properties of the material, which

researchers say is dominated by the presence of inclusions within the material.

Further tests were carried out by immersing various cement-bentonite samples in an

aggressive magnesium sulphate solution, aimed at simulating a contaminant hot spot. The

samples were carefully selected to test the effect of the contaminant on samples of varying

age, block samples obtained from varying wall depth and samples previously exposed to

varying levels of field contamination.

A significant loss in mechanical strength was noted in all the samples and was characterised

by the formation of gypsum on the sample surfaces. The loss in mechanical strength was

noted to decrease towards the core of the samples. An interesting observation was that some

sample cores exhibited little or no change in mechanical strength, which was characterised by

the formation of ettringite as opposed to gypsum. This was mainly noted in 11-year-old

samples that had little or no previous contaminant exposure and block samples exhumed from

shallow depths of the wall. Such intact cores were found to indicate a slow moving reaction

front.

From these tests, it was also noted that younger samples were more susceptible to sulphate

attack as they exhibited more surface cracking, and their core interiors were softer than their

elder counterparts were. These results suggest that it is unadvisable to construct a slurry wall

in close contact with a contaminant plume.

It would be useful to conduct further research to quantify the loss in mechanical strength

associated with the inclusion of varying levels of contamination within the cement-bentonite

mix before hardening. This would help to further assess the degree of wall vulnerability at a

young age.

CONTENTS

1

CONTENTS PAGE

1.0 INTRODUCTION

1.1 UK Contaminated Land Management 1

1.2 Cement-Bentonite Slurry- Trench Cut-Off Walls 2

1.3 Design & Construction 2

1.4 Permeability Testing 5

1.5 Unconfined Compression Testing 7

1.6 The Need for Further Research 7

1.7 Report Outline 8

2.0 BRE TEST SITE & PROJECT MOTIVATION

2.1 Introduction to the BRE test site 9

2.2 Opportunity for further research 11

2.3 Research Objectives 13

3.0 EXPERIMENTAL METHODS

3.1 Risk & Harzard Assessment 14

3.2 Permeability Testing 14

3.3 Evaluating Chemical Durability 20

3.4 Evaluating Chemical Compostion 24

4.0 THE EFFECTS OF PREVIOUS CONTAMINANT EXPOSURE


4.2 XRD Results 29

4.3 Permeability Results 31

5.0 THE EFFECTS OF IMMERSION TESTING ON 11 YEAR OLD

SAMPLES


5.2 Observational Results 36

5.3 XRD Results 39

6.0 THE EFFECTS OF IMMERSION TESTING ON SAMPLES

OF VARYING AGE 6.1 Research Objectives 43

6.2 Observational Results 43

6.3 XRD Results 45

7.0 FINAL REMARKS

7.1 Conclusions 47

7.2 Opportunities For Further Research 48

8.0 REFERENCES 49

9.0 APPENDIX 50

1 INTRODUCTION

1

1.0 INTRODUCTION

1.1 UK Contaminated Land Management

A risk-based contaminated land management method has been adopted in the UK for some

years. It is legislated by the Part IIA Environment Protection Act (EPA) 1990 and 2000,

which is aimed at controlling specific threats to human health and the environment. Risk

assessment is carried out to ensure that land is fit for its current use or, in the case of

redevelopment, its intended use.

Land is considered to be contaminated when it appears to the Local Authority, in whose area

the land is within, by reason of substance in, or under the land, that;

(i) Significant harm is being caused or there is a significant possibility of such harm

being caused, or

(ii) Significant pollution of controlled water is being, or is likely to be caused. [1]

The widely recognised source-pathway-receptor pollutant linkage model illustrated in Fig.1 is

used for assessing the risks from contaminated land. Under Part IIA of the EPA 2000, all

three elements of linkage must be present for a risk to exist. If any of the elements of a

pollutant linkage is absent, then there can be no risk and the land is not contaminated.

Fig. 1.1: The pollution linkage model

Contaminated land remediation aims to control, modify or destroy pollutant linkages. It is

achieved by one of the following:

(i) Isolate or remove the source

(ii) Interrupt or manage the pathway

(iii) Protect the receptor of modify its exposure [2]

SOURCE RECEPTOR PATHWAY

1 INTRODUCTION

2

1.2 Cement-Bentonite Slurry- Trench Cut-Off Walls

As defined by the Institution of Civil Engineers:

“Cement-bentonite slurry trench cut-off walls are a common type of low permeability, in-

ground barrier for control of the migration of groundwater, leachates, chemical contaminants

and gases” [3].

The first use of a slurry trench cut-off wall was to control the flow of leachate at a landfill site

in 1983. Since then, over 100 slurry walls have been constructed in the UK, with some several

kilometres long.

Cut-off walls provide a cost-effective method for remediating contaminated land. It is a

contaminant containment technology that performs its remedial action by interrupting or

removing the pollution pathway, therefore eradicating any risk posed to the receptor as

defined by the EPA. It is the most common form of in-ground vertical barrier used for

controlling the lateral migration of pollution in the UK [4].

Slurry walls can be used in conjunction with pump and treat technologies, which facilitate the

in-situ treatment of contaminant plumes whereby polluted groundwater is extracted from the

ground and sent to a treatment plant. Once treated, the clean water may be re-introduced in to

the ground beyond the slurry wall where further contamination cannot occur.

1.3 Design & Construction

There is considerable experience in the design and construction of cement-bentonite slurry

trench cut-off walls (CBSTCW) in the UK. The Specification for the construction of slurry

trench cut-off walls was issued by the ICE in 1999, and is more commonly known as the

Specification. The documents aim to provide a standard consistent approach to the design,

construction, testing and monitoring of CBSTCW and guidance on the appropriateness of this

technique [5].

1 INTRODUCTION

3

Fig. 4 is a typical cross section through

a CBSTCW. Although the diagram has

been dimensioned, these will vary from

site to site depending on the geological

conditions and the design parameter for

that particular site. However, the

minimum width of the wall is generally

accepted to be 0.5m.

Fig. 1.2: Typical CBSTCW cross section

The key features that one must pay particular attention to are:

(i) Clay cap: A compact, low permeability clay liner is placed after the wall has hardened

and its main purpose is to prevent drying that would cause cracking, and provide some

mechanical protection to what can be a brittle material.

(ii) Key: The Key (-in) or otherwise know as the toe, locks the wall in to the underlying

aquiclude1, thus ensuring no contaminants seep underneath the wall.

(iii) Geomembrane: A thin sheet (~2mm) HDPE with a typical permeability of 10-14

– 10-13

(m/s) is often placed at the centre of the wall during construction, as can be viewed in

Fig. 1.2 [6]. The geomembrane acts as a second line of defence against aggressive

leachate/and or gas transmission, and is more common in modern cut-off walls.

In the UK, the single phase method of construction is generally used, in which a continuous

trench is excavated under the support of a self-hardening cement bentonite slurry, which then

sets to form the required low permeability barrier as illustrated in Figures 1.3 and 1.4. Slurry

is continuously pumped during excavation and when the desired depth is reached, pumping

continues to compensate for slurry loss to the surrounding ground, which contributes to the

formation of a filter-cake 2.

1 Any geological formation that absorbs and holds water but does not transmit it at a sufficient rate to supply

springs, wells, etc. 2 An impermeable layer formed by colloidal fractions at the soil-slurry interface.

1 INTRODUCTION

4

Fig. 1.3: Slurry wall construction [7] Fig. 1.4: Construction schematic [8]

Once crucial aspect in the design and construction of CBSTCWs is producing a slurry mix

that will manifest the required mechanical and hydraulic properties on hardening, which must

be in accordance with ICE specifications.

Table 1: Typical design mix [9]

The contents of a typical design mix are

listed in Table 1. What can be noted is

that water accounts for approximately

84% of the mix by mass.

Ground blast furnace slag (GBFS) can be substituted with pulverise fuel ash (PFA), which is

said to be more resistant to chemical attack [10]. However, much more cement and PFA are

required to produce a mix that will set to give properties required by the Specification [4].

Mixing of the slurry is generally a two-stage process where the sodium-activated bentonite is

first pre-mixed with water in a high-shear mixture and the slurry allowed to hydrate for a

minimum of 8 hours, before the cement is added. Once all the materials are added, the slurry

mix is homogenised before being pumped in to the trench. The mixing process is illustrated in

Fig. 1.5.

Material

Mass/1000kg

water

Sodium activated bentonite 40kg

Ordinary Portland Cement (OPC) 30kg

Ground Blast Furnace Slag (GBFS) 120kg

1 INTRODUCTION

5

Throughout the whole process, the mix

is closely monitored to ensure a

desirable uniform mix is achieved.

Density, viscosity and bleed are all

measured as control tests on the fluid

slurry in accordance with the

Specification.

Fig. 1.5: Slurry batching plant schematic [11]

Although every effort can be made to produce a homogenous slurry mix that will give the

required properties once set, the method of construction employed will inevitably involve

some of the ground being incorporated in to the slurry. The inclusion of these heterogeneities,

mainly in the form of soil and rocks, increases with depth and in some cases can significantly

affect the properties of the set slurry.

Therefore, to ensure the ‘heterogeneous’ mix adheres to specification, the following tests are

conducted using the slurry sampled from the trench:

(i) Permeability test

(ii) Unconfined Compression test

1.4 Permeability Testing

The Specification clearly states that the permeability of the set cement-bentonite material is

the fundamental parameter by which to assess performance. The slurry wall must be able to

resist permeation by groundwater, gases and leachates. There are various ways through which

the chemical contaminants (in various phases) can migrate across the wall, and these are

outlined below:

(i) Hydrostatic pressure: A pressure-head difference across the wall provides are driving

force for the permeation of liquid or gaseous phases through the wall. This is regarded

to be the most dominant driving force at levels of permeability in accordance with the

specification.

1 INTRODUCTION

6

(ii) Osmosis: Varying water potentials across the wall may provide a driving force for

osmosis to occur.

(iii) Chemical potentials: A difference of chemical concentrations will provide a driving

force for the diffusion liquid chemicals and gases through the wall.

(iv) Electric potential: Varying electric potentials may provide a driving force that may

cause contaminant migration through electrolysis, electro-osmosis or electrophoresis.

(v) Temperature differentials: This may drive flow by influencing microbiological

processes.

[12]

The Specification provides the following guidelines on permeability:

“A target permeability of less than 1x10-9

m/s is required. However, due to inherent

variability of trench mixes, sampling and testing, at least 80% of results shall be less than

1x10-9

m/s and at least 95% of the results shall be less than 1x10-8

m/s, with no individual

result in excess of 5x10-8

m/s, when measured……. at an age of 90 days.” Pg. S5

Samples for compliance testing for both permeability

testing and UCS testing are taken from the mixer, and

towards the top and bottom of the trench. It is

important to obtain the different samples to realise

how inclusions affect the final properties of the set

slurry. Part of this process can be viewed in Fig. 1.6.

The fluid samples are then cast in 100mm diameter

plastic mould and transferred to the laboratory for

testing.

Fig. 1.6: Slurry Sampling [7]

Permeability testing is conducted after 90 days because it has long been established that the

permeability of cement-bentonite increases exponentially before reaching a stable

equilibrium. Young samples are also weak and become increasingly brittle with age. Once

hardened, the material is very susceptible to drying shrinkage, which causes irreversible

cracking and damage. Therefore, it is of utmost importance that the samples obtained are

stored in water and handled appropriately until the time of testing, to obtain representative

results.

1 INTRODUCTION

7

1.5 Unconfined Compression Testing

Unconfined compression testing is conducted to ensure the CBSTCW has the required

strength (called UCS) to limit the formation of cracks and fissures that would provide

preferential flow paths for contaminants within the material, which may significantly impair

the walls’ permeability. The National Specification stipulates that:

“The minimum unconfined compressive strength at an age of 28 days shall be 100 kPa”

Pg. S5

A strain at failure of greater than 5% was issued in previous issues of the Specification,

however this was later made obsolete as it became evident that this was almost impossible to

achieve within the new specified limits of permeability.

1.6 The Need For Further Research

Fig. 1.7 indicates various mechanisms by

which the wall is degraded over time.

Although there is considerable experience in

the design, specification, construction, and

validation of CBSTCWs, there is growing

consensus within the industry that little is

known about their performance, especially in

chemically aggressive ground. The potential

health, environmental and cost implications if

failure of such a wall were to occur, would be

significant.

Fig. 1.7: Slurry wall degradation mechanisms

Therefore, since 1996 the Cambridge University Engineering Department (CUED) in

collaboration with the Building Research Establishment (BRE), have been testing samples

1 INTRODUCTION

8

obtained during the construction and later excavation of a CBSTCW at a field test site. The

research conducted addresses the following issues:

(i) Does the set slurry in the trench have the required permeability?

(ii) Are the laboratory measurements representative of in-situ behaviour?

(iii) Will the wall crack due to either drying shrinkage or movement?

(iv) Is the wall deep enough and continuous?

(v) Is the wall material durable in chemically aggressive ground in the long term

considering both change of permeability and cracking because of chemical

interaction?

(vi) What is the confidence in the design life?

[4]

1.7 Report Outline

This report documents the findings of laboratory work conducted from October 2006 to April

2007. The samples used in the experimental work were obtained from or are specific to the

BRE test-site commissioned in 1996. The following is an outline of the information contained

in this report:

Chapter 2 gives and introduction to the BRE test-site, which includes a brief description of the

slurry wall configuration found on the site and the chemical properties of the aggressive

ground. The chapter closes with a brief proposal of the project research objectives.

Chapter 3 documents the experimental methods and techniques employed in conducting the

experimental work and is followed by a discussion of the results obtained in chapters 4, 5 and

6.

The conclusions of the research conducted are documented in chapter 7, which also closes

with a useful insight of the practical implications of the results obtained.

2 BRE TEST SITE & PROJECT MOTIVATION

9

2.0 BRE TEST SITE & PROJECT MOTIVATION

2.1 Introduction to the BRE Test Site

Although over 100 CBSTCWs have been constructed in the UK

since 1983, concern over the long-term performance and

durability of slurry walls led to the commissioning of a test site

by BRE and SecondSite on a disused gasworks in 1996. The site

was formerly owned by British Gas and was operational between

1890 and 1975.

Fig. 2.1: Test site geology

As indicated by Fig 2.1, the underlying geology at the site consists of 3m of made ground,

with the water table 2m below the surface and 1m above the clay aquiclude [4].

This site was specially selected for trials as the previous manufacture of coal-gas had heavily

polluted and contaminated the ground with spent oxide3, coal residues, carbon black and foul

lime, some of which can be view in Figs. 2.2 to 2.4. Trial pits and boreholes at the site

together with rigorous chemical analyses were used to assess the ground conditions, and

narrow down the type and level of contamination found on the site.

Fig. 2.2: Spent Oxide [7] Fig. 2.3: Foul Lime [7] Fig. 2.4: Carbon Black [7]

Table 2.1 provides a brief summary of the contaminants identified within the spent oxide and

others arising from previous site operations. A detailed table containing results from

groundwater analysis of samples taken from wells at the site can be found in the appendix on

3 Spent oxide is the residue from iron oxide used in the purification of coal gas to remove hydrogen sulphide and

hydrogen cyanide. Whe the sulphur content reached 50-60%, the material was termed spent and was either

dumped or used for the the production of chemicals such as sulphuric acid. The removal of hydrogen cyanides

during purification resulted in the formation of complex cyanides with total cyanide contents up to 6%. [4]


10

page 50. The table lists the concentrations of various chemical compounds found on the site

as measured by the BRE and British Gas between 1995 and 1996.

Table 2.1: Contaminants present at test site4 [4]

Chemical Comment Effect on Human Health

*Sulphate Within spent-oxides respiratory toxicant

*Sulphide Within spent-oxides respiratory toxicant

*Total Cyanide - blood/developmental/kidney/neuro -toxicant

*Sulphur 50-60% of spent-

oxides by weight respiratory toxicant

*Arsenic toxic carcinogen

*PAH’s5

Volatile organic

compounds toxic carcinogen

Foul Lime -

Carbon Black Due to coal storage toxic carcinogen

*Exceeds UK Trigger Threshold Concentration6

A total of 120 metres of cut-off wall, 0.6m wide by 5m deep was constructed, comprising 2

test cells (also referred to as boxes), 10m square in plan, and four independent lengths of wall

as shown in Fig. 2.5.

Fig. 2.5: CBSTCW layout at BRE test site.

4 A detailed table of all the specific contaminant compounds found on the site can be found in the appendix.

5 Poly Aromatic Hydrocarbons

6 Trigger threshold concentrations are values set by UK Soil Guidline Values for contaminants potentially

harmful to human health, and plant life, the set values indicate the concentration of a contaminant in soil below

which no action is required [2].


11

The main purpose of installing the walls was to assess and aid the development of improved

insitu permeability monitoring and measurement methods [4]. There are currently no accepted

methods for measuring insitu permeability within cut-off walls. However, any method of

measuring insitu permeability in a cut-off wall must satisfy the requirement that it must not

damage or impair the performance of the wall.

The results of insitu permeability measurements using piezometers, BRE packers, piezocones,

and self-boring permeators differed significantly from accepted laboratory measurements and

were thus deemed inaccurate methods of assessing wall permeability.

Since the wall was constructed, research has been conducted by students at CUED in

collaboration with the BRE. The following section outlines some key research findings to date

and serves as the basis of motivation for the experimental work conducted in this project.

2.2 Opportunity for further research

Test cell 1, constructed in heavily contaminated ground where significant quantities of spent

oxide were mixed in to the slurry during construction was excavated in 1999, while wall 1

(trench 1), constructed in uncontaminated ground was exhumed in 2004. Block samples

throughout the depths of both walls were collected and stored underwater for further testing as

shown in Figs. 2.6 – 2.8.

Fig. 2.6: Exhumed wall Fig. 2.7: Sample carving Fig. 2.8: Sample storage


12

The objectives of the exhumations were to:

(i) examine the continuity of the wall

(ii) compare the insitu condition of a wall in relatively uncontaminated ground with one in

very contaminated ground

(iii) examine any obvious signs of deterioration due to the mixing of the contaminated

ground

(iv) obtain high quality samples for laboratory examination and testing

The wall contained in the uncontaminated ground showed no obvious signs of deterioration or

leakage, and was thus deemed satisfactory. However, a close examination of the wall

exhumed in test cell 1 proved otherwise. The samples obtained had a pungent sulphurous

smell and were soft to touch compared to the unreacted samples [4].

Table 2.2: Sample UCS strength [4]

The noted loss in strength was similar

to observations made during wall

construction in 1996 as shown in Table

2.2.

Table 2.2 clearly indicates that the strength of contaminated samples falls considerably

despite the fact that one would expect the strength of young samples to increase as the

material sets and hardens. Although the mean strength of the 28-day-old samples exceeds the

100 kPa figure stipulated by the Specification, one must note from the range that some

samples failed to satisfy the minimum strength criterion, which is a cause for concern

Age Mean Range Mean Range

28 days 360 263 - 415 260 54 - 282

90 days 890 790 - 968 583 447 - 700

ContaminatedUncontaminated

UCS (kPa)


13

2.3 Research Objectives

Although there has been no noted failure of CBSTCWs in the UK, the observations made

during wall construction and excavation resonated concern within the industry regarding the

long-term durability of CBSTCWs in aggressive ground conditions. To help ease these

growing concerns it was decided to conduct research aimed at investigating the following:

(i) The effect of aggressive contaminants on the hydraulic properties of cement-bentonite

(ii) The effect of aggressive contaminants on the mechanical properties and chemical

composition of cement-bentonite samples of:

a) varying age

b) varying wall depth

The above research topics were selected as they would help to assess the effects of field

contaminants on the cement-bentonite material, while establishing how these effets vary along

the depth of the wall and the relative vulnerability of the material at different ages.

It is inevitable that any reactions that do occur between the slurry wall and the contaminants

will alter the chemical properties of the material, however, what this research aims to

investigate is how these changes will affect the mechanical and hydraulic properties of the

structure.

3 EXPERIMENTAL METHODS

14

3.0 EXPERIMENTAL METHODS

3.1 Risk & Hazard Assessment

Table 2.1 in chapter 2 clearly indicates that some samples obtained from the BRE test site

may be contaminated with toxic substances such as arsenic, sulphur, cyanide, that pose

significant risk to human health . A comprehensive literature review and risk assessment

concluded that the level of contaminant concentrations found on or in the samples would only

pose a significant risk if directly ingested through the mouth.

However, it was decided to take adequate precautionary measures by wearing lab coats, latex

gloves, eye goggles and facial masks during all practical procedures to mitigate any potential

risks.

3.2 Permeability Testing

(i) Permeability defined

Permeability is the fundamental parameter by which to assess the performance of CBSTCWs

[4], and can be defined as “the rate of discharge of water under laminar flow conditions

through a unit cross-sectional area of a porous medium under a unit hydraulic gradient and

standard temperature conditions” [13].

Permeability is described by Darcy’s law, stated below.

v = water flow velocity in m/s

k = permeability7 in m/s

i = hydraulic gradient8

7 Can often be referred to as the hydraulic conductivity

8 A dimensionless quantity found by dividing the head difference by the length of the flow path

v = k i


15

(ii) Types of permeability tests

There are three types of permeability tests, namely:

a) Constant head test: The head difference and hydraulic gradient are kept constant

across the sample, the corresponding flow rate through the sample is measured and

from this, the permeability can be inferred.

b) Falling head test: The flow rate through the sample can be computed from the fall in

head across the sample over time. Once known, the permeability can be calculated.

c) Constant flow test: The flow rate through the sample is kept constant; the subsequent

loss in head can then be used to calculate the permeability.

The constant flow test otherwise known as the triaxial permeability test is recommended by

the Specification for testing low permeability materials, such as cement-bentonite. This is

because high pressures can be used to force the flow of water through the sample to obtain

practical permeation times, which would otherwise be in years as opposed to days.

(iii) Triaxial permeability experimental apparatus

The National Specification recommends that all

permeability tests for cement-bentonite materials

be carried out in accordance with BS

1377:19909, Part 6, Clause 6 using a flexible-

wall permeator, which can be viewed in Fig. 3.1

[15].

Fig 3.1: Flexible Wall Permeator [16]

9 BS 1377:1990. British Standard Methods of Test for Soils for Civil Engineering Purposes


16

The standard requires that samples are tested under a backpressure and that a specified

minimum saturation is achieved before permeation is started. Due to the inherent stiff nature

of the material, high confining pressures may be necessary to demonstrate the required

saturation. Flexible permeators are therefore recommended as the high confining pressure

maintains a tight contact between the sample and the membrane to prevent any leakage during

the experiment [15].

The full experimental setup can be viewed in Fig. 3.2. All the experiments are carried out in

triplicates to provide multiple sets of data for averaging and to hedge against any risks of

experimental failure that may render some trials obsolete.

Key:

[1] Air-water pressure interface chamber

[2] Constant flow pump

[3] Pore pressure transducer

[4] PC with data logging software

[5] Triaxial permeator

Fig. 3.2: Experimental setup

(iv) Experimental method

Figure 3.3 is a plot of pressure (bars) against time, and is a real time representation of the

experiment as it progresses through the different phases.

Fig. 3.3: Experimental timeline


17

Key:

[1] Start of consolidation

[2] End of consolidation – Start of saturation

[3] End of saturation – Start of injection

[4] End of injection phase I – Start of injection phase II

Green: Cell pressure (Pc)

Blue: Back pressure (Pb)

Pink: Injection pressure cell 1

Navy: Injection pressure cell 2 (Pi)

Red: Injection pressure cell 3

a) Operating pressures

It was necessary for the effective stresses (σv’ = Pc - Pi ) experienced by the sample obeyed the

following expressions:

σv’BOTTOM > 0: Else, the membrane and the test specimen will separate, hence causing

experimental failure [13]

σv’TOP = 1 bar (100 kPa): Effective stress as recommended by the specification

σv’TOP > σv’BOTTOM : To facilitate the flow of water during injection

The cell and backpressures are supplied by air-water pressure interface chambers as shown in

Fig. 3.2, while the injection pressure is supplied by a constant flow pump, also shown in the

same figure. All the pressures are measured using pore pressure transducers that are

connected to the data logger that then creates the real time plot in Fig. 3.3.

For low permeability materials such as cement-bentonite, with permeabilities less than 1x10-

7, it is recommended that a hydraulic gradient of 30 be used [13]. However, it is best practice

to use a hydraulic gradient close to that found on the test site.


18

b) Consolidation

As shown in Fig. 3.3, consolidation is the first stage of the experiment whereby, the specimen

is slowly consolidated by increasing the effective stress from zero to 1 bar or 100 kPa as

recommended by the Specification. This process is usually completed within 48 hours.

c) Saturation

This is the second phase of the experiment

during which, all gaseous phases are removed

from the sample and the experimental setup.

The presence of compressible air-bubbles

within the setup leads to inaccuracies which

can be overcome by attaining 100% saturation,

where possible.

Fig. 3.4: Suggested back pressure [13]

Air-bubbles are eliminated in two ways. The first method is to use de-aired water for

permeation (injection) and to provide the cell pressure. Secondly, backpressure can be applied

to the sample to dissolve any air-bubbles that may be contained within the sample .

As depicted in Fig. 3.3, saturation is completed in numerous stages. Recommended values of

backpressure to achieve varying levels of saturation can be viewed in Fig. 3.4. Saturation may

be verified by measuring the B coefficient 10

. The specimen is considered fully saturated

when B = 1. [13]

During this phase of the experiment, it is important that the back, injection and cell pressures

varied according to the following expression:

Pc > Pb > Pi

10

The B coefficient is the change in pore water pressure in the porous material divided by the change in confinig

(cell) pressure [13].


19

d) Injection

Before injection commences, it is important to ensure that all prerequisite pressure conditions

are reached. Once this is verified, de-aired water is injected through the sample from top to

bottom at a constant flow rate, within the specified limits of the hydraulic gradients. The

pressure caused by injection is allowed to stabilise before repeating the process twice at

different flow rates.

e) Calculating permeability

( )

( ( )

i b

i b

vk

i

P P g Li

L

qv

A

q Lk

A P P g L

ρ

ρ

=

− +=

=

=− +

q = constant flow injection (m3/s)

A = cross-sectional area (m2)

ρ = density of water (kg/m3)

Fig. 3.5: Pressure system schematic

The cylindrical samples used in the permeability test were trimmed to the dimensions of

100mm x 100mm.

f) Mean average vs. Graphical average

In the absence of leaks, three permeability results should be obtained from each cell during

each experimental run. From this, a mean average and a graphical average can be calculated.

The graphical average is calculated from a pressure versus flow rate plot of the data obtained,

such as that in Fig. 3.6. The gradient of the line-of-best-fit provides the value of permeability.


20

Fig. 3.6: Block sample

cell 2 graphical

average

What is interesting to note is that sometimes, the mean and graphical averages may have large

discrepancies. This can be clearly viewed in Fig. 4.7 on page 36 whereby the mean average

obtained for the sample in cell 2 is 1.65x10-10 m/s compared to a graphical average of

6.00x10-11 m/s.

With large amounts of data, the graphical method would be the most preferred average

however, with only 3 data points, it can be difficult to obtain an accurate line-of-best-fit. The

R2 value ranging from 0 - 1 describes the accuracy of the line-of-best-fit, and a value close to

unity indicates a good fit with the data. The highest R2 value obtained was from these

experiment was 0.9973 from the cell 3 mixer sample, while the lowest value obtained was

0.3240 from the trench sample in cell 3.

3.3 Assessing Chemical durability

(i) Test selection

Three approaches can be used to assess the effects of contaminants on cement-bentonite:

a) Mixing test – the fluid slurry is mixed with a proportion of the contaminant

b) Immersion test – set slurry is placed in a solution of contaminants or site leachate

c) Permeation test – set slurry is permeated with solution of contaminants or site leachate

[17]

y = 6E-11x + 1E-11

R2 = 0.8988

1.5E-11

1.7E-11

1.9E-11

2.1E-11

2.3E-11

2.5E-11

2.7E-11

2.9E-11

3.1E-11

3.3E-11

0 0.1 0.2 0.3

Pressure -Ai (m2)

Flo

w rate

(m

3/s

ec)

Series1

Linear (Series1)


21

Hayes and Garvin (1999) report that the different test conditions all produce varying

information on the materials’ behaviour. In all three types of test, there are no generally

agreed methods or standards of testing and there is relatively little published data [4]. It was

therefore decided to use immersion testing as it provides a relatively simple means of

assessing the physical effects of chemical reactions between the set slurry and a particular

solution of contaminants or leachate.

In the tests, samples of hardened slurry are immersed in test solutions for a period and the

chemical-slurry interaction can be monitored by visual assessment, weighing, dimensional

measurements and strength changes. As there was limited time available to conduct the tests,

these attributes were deemed favourable.

(ii) Leachate selection

The most challenging aspect of experimental design was to select an appropriate leachate

solution for carrying out the immersion test. As it can be seen in Table 8.1 in the appendix, an

acidic cocktail of contaminants with variable concentrations have been identified across the

site. Garvin and Hayes (1999) document contaminants which are known and considered likely

to affect the integrity of cement-bentonite barriers, these include; organic and inorganic acids,

magnesium and ammonium salts and sulphates. Acidic solutions were found to have a limited

effect on the material relative to magnesium and ammonium salts and sulphates. [17]

Immersion tests conducted by Garvin and Tedd (year unknown) using a wide variety of

chemical contaminants indicates that; the degree of attack is linked to both the concentration

of the sulphate ion (attack increased with concentration of sulphate) and the counter ion. The

aggressivity or the rate of the reaction induced by the various sulphate solutions was found to

vary as:

Mg2+

= NH4+ > Na

+ > Ca

2+ [18]

MgSO4 solution was therefore selected, as it would represent a worst-case scenario, as it is

one of the most aggressive chemicals. Of the afore mentioned cations, magnesium ions

produce the fastest rate of reaction and this was seen as favourable due to the limited time

available to conduct the immersion tests.


22

The selection of a pure solution (single contaminant) also had the added bonus of simplifying

the future task of evaluating the chemical composition of the reacted cement-bentonite

samples, by limiting the chemical compounds that one would expect to find. A concentration

of 27.7 mg/l of hydrated11

MgSO4 solution was used to obtain a sulphate concentration of

11.5 mg/l, which is the average sulphate concentration found on the site.

(iii) Chemical reactions

The MgSO4 solution was expected to degrade the cement-bentonite in the following

reactions:

a) Ca(OH)2 + MgSO4 � CaSO4.2H2O + Mg(OH)2

b) C-S-H + MgSO4 � CaSO4.2H2O + M-S-H

c) C-S-H + Ca(OH)2 + 2CaCO3 + 2MgSO4 + 28H20 � 2CaCO3.CaSO4.CaSiO3.15H2O +

2Mg(OH)2 [19]

In reactions a) and b), MgSO4 reacts with the main components of Portland cement, which

are calcium silicate hydrates (C-S-H) and portlandite (Ca(OH)2). These reactions are reported

to cause a loss in strength as the magnesium silicate hydrates formed (M-S-H) have no

binding properties while C-S-H does [19]. The reaction product, gypsum (CaSO4.2H2O),

subsequently reacts with calcium aluminate hydrate yielding ettringite, which is an advanced

product of sulphate attack. The above reactions are associated with an increase in volume

leading to expansion and subsequently cracking of the material.

11

MgSO4. 7H2O


23

(iv) Apparatus

Fig. 3.7: Cement- bentonite Fig. 3.8: Empty mould Fig. 3.9: Confined sample

The simple apparatus used in this experiment can be

viewed in Figs. 3.7 – 3.9. Cylindrical cement-

bentonite samples of a height of 100 mm and

diameter of 50 mm (Fig. 3.7) were placed in

perforated HDPE moulds (Fig. 3.8), which were

finally secured using plastic tags, as shown in

Fig.3.9. This configuration was selected to simulate

the confined in-ground conditions that the wall

experiences on site.

Fig. 3.10: Immersed samples

The duplicate samples were then immersed in plastic containers holding 2 litres of MgSO4

solution between 12/12/2006 and 20/12/2006. Eight days were required to complete this

exercise as the process of trimming the cylindrical samples to size was incredibly time

consuming. The samples were extracted from the sulphate solution in the same sequence

between 02/04/2007 and 10/04/2007, to ensure that each sample was immersed in the solution

for exactly 110 days.


24

3.4 Evaluating Chemical Composition

(i) Process selection

Various methods of evaluating the chemical composition of the pre-immersed and immersed

cement- bentonite samples were identified, and some of which are listed below:

a) Fourier Transfer Infra-red Spectroscopy (FTIS)

b) Thermal Gravity-metric Analysis (TGA)

c) Nuclear Magnetic Resonance (NMR)

d) Scanning Electron Microscopy (SEM)

e) X-Ray Diffraction (XRD)

Of the five available methods, XRD was deemed the most appropriate because FTIS and

TGA would involve heating the sample and cement is known to be sensitive to thermal

change. SEM is an optical method that only analyses the surface rather than the bulk of the

material and one would have to identify the chemical composition from the microstructure

identified in the imaging, which would prove difficult for inexperienced researches. Although

NMR is a bulk analysis method, which would appear favourable, the interpretation of the

results is not straightforward and was hence eliminated [20].

(ii) XRD testing

XRD is a versatile, non-destructive, established analytical technique used for the

identification of various crystalline forms known as ‘phases’, of compounds present in

powdered and solid samples [21].

XRD is governed by Bragg’s Law:

nλ = 2dsinθ n = an integer

λ = the wavelength of x-rays

d = the spacing between the planes in the atomic lattice

θ = the angle between the incident ray and the scattering planes


25

Fig. 3.11: Phillips XRD machine Fig. 3.12: XRD operation schematic

An X-ray beam with a wavelength λ is incident on crystalline lattice a plane on the sample at

an angle θ. Diffraction occurs when the distance from successive planes differs by a complete

number of wavelengths n. By varying the angle θ, the Bragg’s law conditions are satisfied by

different d spacing in crystalline materials.

0

1000

2000

3000

4000

5000

6000

0 10 20 30 40 50

2 Theta

Inte

ns

ity

(a

.u.)

Fig. 3.13: XRD plot

Plotting the angular positions (2θ) and intensities of the resultant diffraction peaks produces a

pattern, which is characteristic of the sample, as exemplified in Fig. 3.13. Identification of the

different phases is achieved by comparing the XRD pattern – or ‘diffractogram’ obtained


26

from the unknown sample with an internationally recognised database containing reference

patterns for more than 70,000 patterns.

Fig. 3.13 shows the diffractogram of pure gypsum with its academic peak references overlaid.

What must be noted is that each crystalline material has more than one reference peak, and all

must be visible in the measured XRD pattern for accurate identification of the phase. Phase

concentrations are determined by peak intensities (heights), and therefore the tallest peaks are

always of the most interest. This especially true when there one phase present in the sample,

as the relative height of the tallest peaks of the different phases provides a useful indication of

the most abundant phase in the material.

(iii) Experimental procedure

One of the main advantages of using XRD is its great versatility that allows one to test

samples in either solid or powder form, which was a dilemma that was faced. To assist in

making this decision, preliminary tests were carried out on samples in powder form, solid

chunks, and paste. One major worry of testing samples in powder form was the fact that they

would not be representative of the hydrated samples that are found in the ground.

The results of the trials, which can be viewed in Fig. 3.14 indicated that all wet samples, be it

in chunk or paste form, produced a characteristic amorphous bump. The amorphous bump has

the effect of dampening some of the diffraction peaks, which are required for identification

purposes. This is clearly can be clearly seen in the difference between the 11 year

contaminated dry and paste samples. One observation that was made during the trials was the

fact that the wet sampled showed significant signs during the 33 minutes required to complete

each XRD measurement.

The pasty and chunky samples were also found to be disfavourable because it was difficult to

achieve an adequately flat sample surface that is crucial for obtaining accurate values of θ

during diffraction, while this was easily possible with sample is powder form. Powdered

samples also have the added benefit of freezing sample age. This is because the drying of

cement-bentonite is irreversible [4], thus the chemical composition of the material remains

constant, and the age of the sample is in effect ‘frozen in time’, which would not be the case if


27

the sample were immersed in water. It was therefore decided to use desiccator-air-dried

samples, which were then crushed and sieved to produce a fine powder.

0

500

1000

1500

2000

2500

3000

3500

0 10 20 30 40 50 60 70

2 Theta

Inte

ns

ity

(a.u

)

90 days dry

90 days dry 2

90days wet chunk

11 yr contaminated dry

11 yr contaminated dry 2

11 yr contaminated paste

11 yrs mixer dry

11 yrs mixer paste

11 yrs mixer chunk

Fig. 3.14: XRD trial results12

What was also established from the trials was the fact that majority of the diffraction peaks

contained in the uncontaminated and contaminated cement-bentonite samples, occur between

5 - 50 2θ-degrees. The Philips XRD equipment in Fig. 3.11 has an adjustable 2θ range of 0 –

90 degrees, which was adequately reduced to fit the required range, which had the added

benefit of compressing the measurement run time by the same amount.

12

In all the XRD results shown in this report, all but the first plot have an intesity off-set to enable the simple

comparison of results by amalgamating numerous plots on one axes. The intensity scale therefore only correlates

to the first plot.

4 THE EFFECTS OF PREVIOUS CONTAMINANT EXPOSURE

28

4.0 THE EFFECTS OF PREVIOUS CONTAMINANT EXPOSURE


There is increasing concern within the construction industry over whether cement-bentonite is

durable in chemically aggressive ground, and this raises question regarding the design life of

CBSTCWs [4]. Using permeability and XRD testing, this chapter aims to address the afore

mentioned issue by assessing the hydraulic properties and chemical composition of samples

that have experienced varying levels of contaminant exposure at the field test site. For this

purpose, the following samples were selected:

(i) Mixer: These samples were taken directly from the cement-mix on the site prior to

trench injection during construction in April 1996.

(ii) Trench bottom: These are samples taken from the bottom of the trench prior to

solidification during wall construction.

(iii) Block: These are samples exhumed from from wall 1 in 2004.

(iv) Heavily contaminated block: These are samples exhumed from test cell 113

(constructed in a contaminant hot spot), which was exhumed in 1999.

The level of contaminant exposure experienced by the samples varies as:

Heavily Contaminated Block > Block > Trench Bottom > Mixer

What is characteristic of these samples is the fact that they are all 11 years old and therefore a

direct comparison can be drawn between them.

Fig. 4.1: 11 years mixer Fig. 4.2: 11 years trench Fig. 4.3: 11 years block

13

Refer Fig. 2.5 Pg. 11


29

Figs. 4.1 to 4.3 are examples of some of the samples tested. What is immediately evident is

that the block sample has a completely different appearance to the other two. The block

sample is much darker than the rest. This change in colour could be attributed to chemical

reactions that have occurred in the ground or the heterogeneities that have been included in

the sample during its lifespan e.g. mud pockets, stone inclusions (one of which is clearly

visible in Fig. 4.3). A few, small inclusions, can be seen on the top surface of the trench

sample while as expected, there are none visible in the mixer sample

4.2 XRD Results

The following are the XRD results obtained from the three samples. While analysing XRD

data it is important to remember that the peak intensity (height) is representative of the

various phase concentrations within the samples. Please note that the characters on the graph

are labels for peaks to their immediate left, and have been placed level with the maximum

peak height where possible.

Fig 4.4: 11 year XRD results

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 10 20 30 40 50

2 Theta

Inte

ns

ity

(a

.u.)

11 years heavily

contaminated block

11 years mixer

11 years block

C

CC

C

C

E

E

E E

E

E

X

XX

X

X

X

Key:

C - Calcite

E - Ettringite

X - Unknonwn


30

Due to time restrictions, the 11-year trench sample was not tested, and therefore this result

was substituted with an 11-year block sample, which like the heavily contaminated sample,

was exhumed from the wall in 2004. However, this sample was not exposed to significant

amounts or concentrations of contaminants and was therefore deemed representative of a

trench sample.

As can be viewed in Fig. 4.4, the dominant crystalline mineral phases in the heavily

contaminated sample are calcite (CaCO3), which is characteristic of hydrated cement, and

ettringite, which is a product of advanced sulphate attack.

The composition of the 11-year, block samples is dominated by calcite with no traces of

ettringite visible in the diffractogram. As some of the 11 year block peaks, it was not possible

to identify all the mineral phases depicted in the diffractogram of the 11-year mixer sample.

However, the sample showed no traces of portlandite (Ca(OH)2), calcite or montmorrillonite

(Na-bentonite clay), all of which one would expect to find in this sample.

Despite this, what is clear from Fig. 4.4 is the fact there is a definite visible change in

chemical composition as the level of contaminant exposure to the sample increases. These

tests were followed with permeability test to see whether a similar trend in values of

permeability would be noticed.


31

4.4 Permeability Results

Fig. 4.8 provides a useful graphical representation of the all the results obtained. What is

immediately evident is the fact that the permeabilities of the three different types of samples

separate in to three bands, albeit with some overlap. The plot also provides a useful insight to

the fact the apparatus had no influence on the values of permeability measured, as there is no

visible relationship between permeability and cell number.

The unaffected mixer sample exhibits the lowest values of permeability. The lowest value

measured was 1.06x10-11

m/s (cell 1, Fig. 4.5), and the total results for all three mixer samples

have a mean average and graphical average of 4.74x10-11

m/s and 4.00x10-11

m/s (Fig. 4.5).

The trench samples occupy the second band with ‘median’ values of permeability. The lowest

value recorded from these samples was 6.01x10-11

m/s (cell 3, Fig. 4.6), which was over 5

times larger than the lowest value measured from the mixer sample. The mean average and

graphical average of the three samples are 1.17x10-10

m/s and 7.00x10-11

m/s (Fig. 1.6).

As can be viewed in Fig. 4.8, the block samples occupy the top band with the highest values

of permeability. The lowest value of permeability recorded from these samples was 4.97x10-11

m/s (cell 3, Fig. 4.7). Although this value was lower than the lowest value recorded from the

trench samples, the largest value recorded was 1.94x10-10

m/s compared to 1.73x10-10

and

7.97x10-11

m/s recorded from the trench and mixer samples. The three samples exhibit a mean

average and graphical average of 9.73x10 -11

m/s and 4.5x10-11

m/s (Fig. 4.7).

The results obtained from this permeability test were amalgamated with results from other

researchers who have conducted similar test on samples obtained from the same test site.

These results can be viewed in Fig. 4.8. A trend that can be noted in this plot is the fact that

the permeability of the 3 various samples appear to decrease with age. Although there is

variation in the values of permeability between the three types of sample, what is encouraging

is that all the values of permeability measured, are at least one order of magnitude lower than

that specified as a minimum in the Specification14

.

14

Refer Pg. 6


32

A holistic view of the trench and block sample data may imply that previous contaminant

exposure has limited effect on the hydraulic properties of the sample and hence the

CBSTCW, as the trench and block samples exhibit similar values of permeability. However,

there is a notable difference in the values of permeability of the mixer samples, which occupy

the lowest band of permeability and would thus suggest otherwise.

Experiments carried out by Sutherland (2004), indicate that

the hydraulic properties of cement-bentonite are influenced by

the degree of heterogeneities in the material, which create

preferential flows paths through cracks and fissures that are

caused by the alien inclusions. This effect is demonstrated in

Fig. 4.10, whereby a stream of dyed water can be seen

flowing through a fissure in the sample.

Fig. 4.10: Fissure flow [22]

This evidence would therefore suggest that the variance in the levels of contamination

exposed to the different samples does not have much effect on the hydraulic properties,

despite having a noted effect on the chemical composition of the various samples.

11 years Mixer Samples

0.00E+00

1.00E-11

2.00E-11

3.00E-11

4.00E-11

5.00E-11

6.00E-11

7.00E-11

8.00E-11

9.00E-11

0 1 2 3 4

Cell Number

Perm

eability (m

/s)

Mean Average

Graphical

Average

Fig. 4.5: 11 years mixer samples results


33

11 years Trench Samples

0.00E+00

2.00E-11

4.00E-11

6.00E-11

8.00E-11

1.00E-10

1.20E-10

1.40E-10

1.60E-10

1.80E-10

2.00E-10

0 1 2 3 4

Cell Number

Per

mea

bilit

y (

m/s

)

Mean Average

Graphical

Average

Fig. 4.6: 11 years trench samples results

Fig. 4.7: 11 years block samples15

results

15

Heavily contaminated samles

11 years Block Samples

0.00E+00

2.50E-11

5.00E-11

7.50E-11

1.00E-10

1.25E-10

1.50E-10

1.75E-10

2.00E-10

2.25E-10

0 1 2 3 4

Cell Number

Perm

eability (m

/s)

Mean Average

Graphical

Average


34

11 years All Samples

0.00E+00

5.00E-11

1.00E-10

1.50E-10

2.00E-10

2.50E-10

0 1 2 3 4

Cell Number

Per

mea

bil

lity

(m

/s)

Mixer

Trench

Block

Fig. 4.8: 11 year samples – All results

Cumulative Data

1.0E-12

1.0E-11

1.0E-10

1.0E-09

1.0E-08

1.0E-07

10 100 1000 10000Sample age, days

Perm

eabili

ty, m

/s

Mixer

Trench

Block

New

Data

Fig. 4.9: Cumulative BRE test site data to date [22]

5 THE EFFECTS OF IMMERSION TESTING ON 11 YEAR OLD SAMPLES

35

5.0 THE EFFECTS OF IMMERSION TESTING ON 11 YEAR OLD SAMPLES


The results from the previous chapter indicate that the level of previous field-contaminant

exposure has a limited effect on the permeability and hence hydraulic properties of the

cement-bentonite samples. Based on this result, it was decided to investigate the effect of

contaminant exposure on the long-term durability (mechanical properties) of cement-

bentonite, by immersion testing.

For this experiment, 8 different sample types were selected, all of which were 11 years old, to

investigate the following:

(i) The durability of samples previously exposed to varying levels of contaminants: mixer,

trench and heavily contaminated (block) samples selected for this purpose.

(ii) The change in durability of CBSTCWs with increasing wall depth: 5 blocks samples

obtained from 5 different depths during the wall exhumation were selected, and are

described in Table 5.1 below:

Table 5.1: Depth of block samples

No. BRE/CUED sample ID Depth below top of wall 9(m)

1 Block 2 0.3 – 0.5

2 Block 5 1.0 – 1.2

3 Block 8 1.9 – 2.1

4 Block 10 2.3 – 2.5

5 Block 15 3.3 – 3.5

In total 16 samples (2 of each type) were immersed in the MgS04 solution as described in

chapter 3.


36

5.2 Observational Results

The post immersion samples were soft to touch, as opposed to being hard and brittle. This

development pointed towards a significant loss in strength, which was confirmed by the fact

that no results were obtained from the UCS tests attempted on these samples, as they were too

soft and crumbled almost immediately. This observation supports the reactions postulated in

chapter 316

, which indicate that the presence of MgSO4 degrades the binding properties of

cement-bentonite by the conversion of C-S-H to M-S-H.

However, what is interesting is the fact that not the whole sample suffered a loss in strength.

Fig. 5.1 portrays a core found at the centre of a sample, which appeared to have maintained its

mechanical properties, as it was still hard and brittle. Cores of irregular shapes and sizes were

found in only 8 of the 16 samples immersed, or 4 of the 8 different sample types immersed.

Although distinct cores were not found in the other samples, the centre of these sample was

noted be harder than the surrounding cement-bentonite material.

Cores

05

101520253035

Mixer

Trench

HC Block 2

Block 5

Block 8

Block10

Block 15

Sample

Ma

ss

(g

)

Fig. 5.1: Domed core Fig. 5.2: Core mass

Fig 5.2 above is a plot of the average core mass found within the various samples. It is

thought that the size of the inner core could be an indication of the rate of penetration of the

reaction front within each sample, with a larger core inferring a slower rate of penetration.

Although there is not enough data to suggest any significant trend, there appears to be an

increase in penetration with an increase in wall depth.

16

Refer Pg. 23


37

Other notable characteristics of the immersed samples were the formation of surface cracks on

the unconfined surfaces of the samples, as is visible in Figs. 5.3 and 5.10 below. In some of

the samples, a crack along the whole longitudinal length of the sample was formed in line

with mould crevasse. An example of this can be seen in Fig. 5.4.

Fig. 5.3: Block 5

Before

Fig. 5.4: Block 5 After Fig. 5.5: Mixer

Before

Fig. 5.6: Mixer After

Fig. 5.7: Block 2

Before

Fig. 5.8: Block 2 After Fig. 5.9: HC17

Before

Fig. 5.10: HC After

Leaching of the sample surface colour was also observed in all the samples, but it occurred to

varying degrees as follows:

mixer > block > trench > heavily contaminated

As can be see in Fig. 5.6 above, the mixer sample turned completely white while only a

marginal change in colour was noted in the heavily contaminated samples as is visible in Fig.

5.10.

17

Heavily contaminated block samples


38

Surface crystals, white in colour, were found on all the samples and are clearly visible in Figs.

5.8 and 5.10, with some samples exhibiting more surface crystals than others do but in no

defined trend as that noted above. However, what was evident is that majority of the crystals

were formed on the unconfined circular surfaces as can shown in Fig. 5.8, or between the

mould and sample surface interface as shown in Fig. 5.10.

Change in Volume

0

0.00005

0.0001

0.00015

0.0002

0.00025

0 1 2 3 4 5 6 7 8 9

Sample

Vo

lum

e (

m3)

Before

After

Change in Density

0

300

600

900

1200

1500

0 1 2 3 4 5 6 7 8 9

Sample

De

ns

ity

(k

g/m

3)

Before

After

Fig. 5.11: Change in volume Fig. 5.12: Change in density

The formation of crystals was also associated with and average increase of 11.1% in sample

volume in all 8 samples as shown Fig. 5.11. This noted change was also reflected in a

marginal average increase in sample density of 2.0%. However, only 6 of the 8 different

samples tested recorded an increase in density, as shown in Fig. 5.12. The two samples that

were anomalies to this trend were block 2 and block 15, and this was because the two samples

recorder large increases in sample volume (31.4% and 15.2%).


39

5.3 XRD Results

Fig. 5.13: Surface chemical composition – effect of previous contaminant exposure

Fig. 5.14: Surface chemical composition – effect of depth

0

500

1000

1500

2000

2500

3000

0 10 20 30 40 50

2 The ta

Inte

ns

ity

(a

.u.)

Key:

G - Gypsum

Crystals

Mixer

HC

Trench

G GG

GGG

G

0

500

1000

1500

2000

2500

3000

0 10 20 30 40 50

2 Theta

Inte

ns

ity

(a

.u.)

G

GGG

GG

GG G

Crystals

Block 15

Key:

G - Gypsum

Block 8


40

Fig. 5.15: Core chemical composition – effect of previous contaminant exposure

Fig. 5.16: Core chemical composition – effect of depth

0

500

1000

1500

2000

0 10 20 30 40 50

2 The ta

Inte

ns

ity

(a

.u.)

Mixer

Trench

HC

Key:

G - Gypsum

E - Ettringite

Q - Quartz

X - Uknown

G

G

GG

E

EE EG

GG

E

Q

Q

G

X

X

X

0

500

1000

1500

2000

0 10 20 30 40 50

2 Theta

Inte

nsit

y (a.u

.)

Key:

G - Gypsum

C - Calcite

X - Unknown

Block 15G

G

GG

GG

G

G

G

G

C

C

CC

C

C

Block 8

X


41

Figs. 5.13 – 5.16 are the results obtained from conducting XRD tests on the crystals, samples

surfaces and sample cores of the immersed cement-bentonite samples. Due to time

restrictions, it was not possible to complete a full analysis of the 5 samples obtained from

different depths, and hence only 2 were selected namely; block 8 and block 15.

Figs. 5.13 and 5.14 incorporate XRD diffractograms of the crystals and cement-bentonite

surface samples of the immersed samples. What is immediately evident is that the fact that all

the plots are almost identical, and all characterise pure gypsum. The presence of gypsum

therefore explains the significant loss of ‘surface’ strength observed in all the samples, as it is

characterised by a loss in C-S-H due to the formation of M-S-H, which has no binding

properties.

One observation made in the previous section was the fact that ‘solid’ cores were found in

some of the samples, and served as an indication of a slow moving reaction front. Fig. 5.15

above analyses the effect that previous contaminant exposure has on post-immersion samples.

It is worth noting that a core was found in the mixer samples and not in the trench and heavily

contaminated samples.

The trench and heavily contaminated samples register very similar results that indicate that

the main phases found in the core are gypsum, calcite and quartz. The presence of quartz

indicates that there must have been rock or sand inclusions in the centre of the samples, which

was expected. A notable difference between the two results is that the heavily contaminated

sample has some unidentified phases.

The mixer sample, which possessed a solid core, exhibits the presence of gypsum, and most

notably ettringite, which is an advanced by-product of sulphate attack. This leads one to

question whether the presence of a core is an indication of a slow moving reaction front, or

whether the presence of ettringite as opposed to pure gypsum reduces the loss of mechanical

strength.

However, logic would suggest that ettringite would only be formed in samples with slow

moving reaction fronts, thus giving time for the sulphate attack to reach this advanced phase.

The presence of gypsum in the softer cores is most likely due to the fact that sulphates are


42

continually being supplied by faster diffusion paths, thus gypsum is perpetually created

without allowing time for the formation of ettringite.

If this were true, it would further imply that samples exposed to previous levels of

contamination are less resistant to chemical attack as no solid cores were found in the 11-

year-old trench and heavily contaminated samples.

Fig. 5.16 compares the effect of depth on the chemical composition of the core samples.

Although not all the samples were tested, the results obtained for block 8 and block 5 are very

similar, indicating that their cores are dominated by gypsum and calcite. It is also worth

noting that both these samples lacked a solid core and the chemical compositions obtained are

similar to those obtained for the trench and heavily contaminated samples, with the only

omission being quartz.

What would be of particular interest, and could possibly be an opportunity for further research

would be to identify the chemical composition of the cores of block 2 , 5 and 10, as they were

all solid. The presence of ettringite in these samples would confirm that the formation of

ettringite poses less risk to cement-bentonite strength in comparison to gypsum.

6 THE EFFECTS OF IMMERSION TESTING ON SAMPLES OF VARYING AGE

43

6.0 THE EFFECTS OF IMMERSION TESTING ON SAMPLES OF VARYING AGE


The results from the previous chapter indicate that there is a significant loss in mechanical

strength in samples exposed to aggressive contaminants for a prolonged period of time. As all

the samples tested in the previous section were 11 years old, it was thus decided to conduct

more immersion tests to investigate the effect of aggressive contaminats on samples of

varying age.

For this purpose, samples aged 4 days, 6 weeks, 8 weeks, 1 year and 2.5 years (on the date of

immersion) were selected.

6.2 Observational Results

The observations made were very to those made in the prevoius chapter. All the samples

suffered a loss in strength as before, however the cracks formed along the mould crevase were

much more pronoucenced in the younger samples as is shown in Fig. 6.2. The 1 year and 2.5

year old samples both possessed intact ‘solid’ cores as as previously shown in Fig. 5.11,

however the intererior of all the samples was stiffer to the touch in comparison to the

surfaces. This can be seen in Fig. 6.3 where the softer outer ‘mould’ has been separated from

the stiffer interior.

Fig. 6.1: 8 weeks before Fig. 6.2: 8 weeks after Fig. 6.3: 8 weeks after


44

All samples showed evidence of surface colour leaching as can be seen in Fig. 6.1 and 6.2.

however, the 4 day old sample exhibited leaching to a much larger depth than that witnessed

in other samples, and this can be seen in Fig. 6.4.

Crystals were seen to form on the unconfined, circular surfaces of the samples as shown in

Fig. 6.5 and this was again characterised with an average increase of 8.5% in sample volume

and an average increase of 4.5% in sample density as can be seen in Figs. 6.6 and 6.7.

Fig. 6.4: 4 days after Fig. 6.5: 2.5 years after crystals

Change in Volume

0

0.00005

0.0001

0.00015

0.0002

0.00025

0 2 4 6

Samples

Vo

lum

e (

m3)

Before

After

Change in Density

0

400

800

1200

1600

0 2 4 6

Samples

Den

sit

y (

kg

/m3)

Before

After

Fig. 6.5: Change in sample volume Fig. 6.6: Change in sample density


45

6.3 XRD Results

Fig. 6.7 indicates that the the surface of the samples contains pure gypsum as was found in the

previous chapter.

Fig. 6.7: Surface chemical composition – Effect of age

Fig. 6.7: Core chemical composition – Effect of age

0

200

400

600

800

1000

1200

1400

1600

1800

2000

2200

2400

2600

2800

3000

0 10 20 30 40 50

2 The ta

Inte

ns

ity

(a

.u.)

Key:

G - GypsumG G G G

G

G

G G

Crys tals

1 year

6 weeks

4 days

11 years

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 10 20 30 40 50

2 Theta

Inte

ns

ity

(a

.u.)

Key:

G - Gypsum

E - Ettringite

C - Calcite

X - Unknown

G

G

G

G

G

G

E

E

E

E

EE

E

E E

E

X

C

C

C

C

2.5 years

11 years11 years mixer


46

Due to time restrictions only 1 of the 5 sample cores was tested. The 2.5 year old sample was

one of the 2 that contained an intact, hard, solid core. The chemical composition of this core

was found to be remarkably similar to the result obtained for the 11 year mixer solid core in

chapter 5. Gypsum, ettringite and calcite were found within this core which again confirms

the hypothesis that the presence of ettringite as opposed to to pure gypsum is correlated with

lower loss in mechanical strength.

7 FINAL REMARKS

47

7.0 FINAL REMARKS

7.1 Conclusions

From the results in chapter four, it can be concluded that the contaminants on the BRE field

test site have a noticeable effect on the chemical composition of cement-bentonite, due to the

identification of ettringite in the heavily contaminated samples. However, this change in

chemical effect has limited effect on the long-term (11 years) permeability of the sample.

Despite the fact that the extremely aggressive conditions used in the immersion tests might be

considered unrealistic, as it is unlikely that a CBSTCW would be completely submerged in

MgSO4 solution. It can be concluded that sulphates have a significantly detrimental effect on

the mechanical strength of cement-bentonite. This loss in strength is associated with the

formation of gypsum (CaSO4.2H2O).

There was a notable degree of surface colour leaching and loss in mechanical strength in the

younger cement-bentonite samples. This therefore leads to the conclusion that young samples,

aged below 1 year, are more vulnerable to sulphate attack than their older counterparts are

The loss in mechanical strength was not witnessed to occur throughout the depth of each

sample, as the inner cores of the samples were found to be firmer than the surrounding

material. Ettringite was found in the two solid cores examined while only gypsum was found

in the four softer cores examined. On this basis, it can be concluded that the presence of

ettringite as opposed to gypsum causes less damage to the mechanical strength of cement-

bentonite.

However, this result does raise a few questions. Ettringite is an advanced product of sulphate

attack, but it was found at the furthest point from the chemical solution. This would imply that

the high concentrations of gypsum found on the surface are caused by the continual supply of

sulphate from the solution.

Solid cores were found in 11-year-old block samples obtained from shallower parts of the

CBSTCW, which would also imply that the occurrence of sulphate attack increases with wall

depth. On this basis, it may be concluded that young samples provide a faster route for

7 FINAL REMARKS

48

chemical diffusion as no cores were found in samples younger than 1 year, thus making them

more susceptible to sulphate attack.

These findings would suggest that CBSTCWs should be constructed away from contaminant

hot spots, to allow the material to age and harden before being exposed to any chemical

contaminants. This will improve the long-term chemical durability of the wall by helping to

maintain its mechanical strength.

7.2 Opportunities for further research

Due to time constraints, it was not possible to conduct a full XRD analysis of all the core

samples obtained. More data is required to ratify and substantiate the conclusions made

regarding the solid inner cores.

Samples of the post-immersion solutions from each experimental container were collected. It

would be interesting to analyse how much MgSO4 is left in each solution.

On page 21 of this report, three different methods are listed for assessing the chemical

durability of cement-bentonite. The mixing test could be used to complete an in depth study

on the effect of sulphate attack on young samples. This could be done by mixing differing

concentrations of MgSO4 in the regular cement-bentonite mix, and later testing the

mechanical strength of the cement-bentonite samples at different ages.

From these results, relationships between chemical concentration, sample age and mechanical

strength can be drawn to shed more light on the chemical durability of young samples. This is

a critical aspect of design due to the stringent requirements made by the National

Specification.

8 REFERENCES

49

8.0 REFERENCES

[1] Nathanail & Bardos, Wiley 2004 Pg.11;

Reclamation of Contaminated Land

[2] A. Tabba, Cambridge University 2007;

Module 4D14: Contaminated Land and Waste Containment

[3] BRE, ICE, CIRIA, DETR, 1999;

Specification for the construction of slurry trench cut off walls, Pg 1

[4] P. Tedd, 2005

BRE Client Report number 221- 476, Pg 1-2


Specification for the construction of slurry trench cut off walls, Foreward

[6] R. Murphy, E. Garwell, 1998;

Infiltration through landfill liners

[7] P. Tedd, 1996

Courtesy of

[8] LaGrega et al., 2001;

Hazardous waste management

[9] P.Tedd et al, 2003;

In-situ Assessment of a cement-bentonite containment system

[10] S. Garvin, P. Tedd, 1995;

Research on the performance of cement-bentonite containment barriers in the United

Kingdom

[11] E. Cairney et al, 1993;

Contaminated land, problems and solutions, Ch. 6, Pg. 123

[12] P. Williams, 2006

The long-term performance of cement-bentonite slurry trench cut-off walls in

contaminated ground

[13] American Standard for the Testing of Materails (ASTM), 1990;

Standard test mehtod for measurement of hydraulic conductivity of saturate porous

materials using a flexible wall permeator

[14] S. Jefferis, 2001;

Permeability a dynamic property of barrier materials


Specification for the construction of slurry trench cut off walls, Pg. N36

[16] D. Daniel, 1994;

State of the art: Laboratory hydraulic conductivity tests for saturated soils, Pg. 41

[17] S.L. Garvin, C.S. Hayes, 1999;

The chemical compatibility of cement-bentonite cut-off wall material

[18] S.L. Garvin, P.Tedd, (year uknown) ;

Research on the performance of cement-bentonite containement barriers in the United

Kingdom

[19] M J Shannag et al, 2003;

Sulphate resistance of high-performance concrete

[20] P.C. Hewlett (edited by), 1997;

LEA’s Chemistry of cement & concrete, 4th

edition, Pg. 202-203

[21] Phillips XRD manual

[22] K. Joshi, CUED 2007;

Courtesy of

9 APPENDIX

50

9.0 APPENDIX

Table X: Groundwater analysis of samples taken from wells before and after cut-off wall

construction in mg/l [4]

Prior to wall construction Dec 1995 Post wall construction Nov

1996

Test Lab BRE BRE BG BG BG BG BRE BRE BRE BRE

Location Well

1

Well

A

Well

B

Well

3

Well

1

Well

A

Well

A

Well

14

Box 2 Box

1

NH4 110 17 86 183 125 68 - - -

Phenol - - <0.05 <0.05 0.09 0.08 - - - -

Na 147 128 138 159 180 187 271 181 163 160

K 34 4 47 46 74 40 17.5 49.3 3.3 26.1

Ca 497 562 578 475 657 680 460 445 419 546

Mg 248 0.1 300 273 260 120 295 332 304 176

Total

SO4

10931 1440 3600 12000 14000 1570 21400 9800 35200 5000

As - - 0.012 0.014 0.200 0.019 - - - -

pH 2 7 3 2 2.5 6

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