Vertical stress distribution in soil – bentonite slurry walls Rahul V Mukherjee MDH Engineered Solutions, Member of SNC Lavalin Group, Saskatoon, Saskatchewan, Canada. Moir D Haug Department of Civil Engineering, University of Saskatchewan, Saskatoon, Saskatchewan, Canada. ABSTRACT Backfill arching or wall “hang-up” of backfilled slurry wall trenches has the potential to significantly reduce the effectiveness of these walls. Soil-bentonite (SB) slurry walls are one of the most popular techniques to minimize the horizontal migration of contaminants. This technique involves the excavation of a narrow trench in the presence of bentonitic slurry down through permeable strata and backfilling with a low permeability material. This paper presents the results of a laboratory testing program and modelling study to investigate the vertical stress distribution in slurry walls. The program was conducted to supplement the design and installation of an 11,000 m long slurry wall. This slurry wall is being installed through low hydraulic conductivity glacial till containing intermediate permeable zones. The depth of this 1 m wide slurry wall is approximately 50 m. Backfill hang-up in these deep walls is a particular concern. In order to calculate “hang-up” it is necessary to determine friction along the wall of the trench. The testing program involved large strain consolidation testing of selected field backfill samples. A Marriott bottle system was also employed to continuously measure hydraulic conductivity and lateral stress during testing. Additional backfill samples were tested to provide data on sensitivity of horizontal and vertical stress to variations in backfill fines content. An analytical model and a coupled seepage stress-strain finite element model (FEM) were used to predict vertical stress changes with time and depth for the different backfill materials. The results of the research study found that trench width was the most important variable in determining wall hang-up. It also showed that the upper portions of the backfilled trench may take years to consolidate and thus provide the desired level of impermeability. RÉSUMÉ La voûte ou la paroi moulée de tranchées remblayées a le potentiel de réduire significativement l’efficacité de ces murs. L’utilisation de parois moulées constituées d’un mélange de sol -bentonite (SB) est l’une des techniques les plus populaires pour limiter la migration horizontale des contaminants. Cette technique implique l’excavation d’une tranchée étroite, en présence d’une suspension de bentonite, à travers des strates perméables et le remblayage avec un matériau à faible perméabilité. Cet article présente les résultats d’un programme d’essais en laboratoire ainsi que d’une étude de modélisation de la distribution des contraintes verticales dans les parois moulées. Le programme a été réalisé dans le but de compléter la conception et l’installation d’une paroi moulée de 11 000 m de long. Cette paroi est en cours d’installation dans un till à faible conductivité hydraulique contenant des zones perméables intermédiaires. La profondeur de cette paroi moulée de 1 m de large est d’environ 50 m. Les conditions d’accrochage du remblai l e long de ces parois profondes est une importante préoccupation. Afin de calculer l’accrochage, il est nécessaire de déterminer le niveau de friction le long de la paroi de la tranchée. Le programme d’essais comprend des essais de consolidation à grande déformation d’échantillons sélectionnés de remblai. Un système de bouteille Marriott a également été employé pour mesurer en continu la conductivité hydraulique et la contrainte latérale durant les essais. Des échantillons de remblai supplémentaires ont été testés afin de fournir des données sur la sensibilité des contraintes horizontales et verticales en fonction de la fraction de particules fines contenues dans le matériel de remblai. Un modèle analytique et un modèle par éléments finis (FEM) ont été utilisés pour prédire les changements de contraintes verticales en en fonction du temps et de la profondeur de chacun des différents matériaux de remblai. Les résultats de l’étude ont montré que la largeur de la tranchée était la variable la plus importante po ur déterminer l’accrochage aux parois. Il a également été démontré que les parties supérieures de la tranchée peuvent prendre des années avant d’être consolidées et donc fournir le niveau d’imperméabilité désiré. 1. INTRODUCTION The potential for slurry wall backfill arching and hang-up was examined in a laboratory testing program. The permeability of clay till slurry backfill is highly dependent on the load placed on the backfill. As a result permeability usually decreases with depth as the backfill consolidates slowly under increasingly high loads. If the width of the backfill trench is narrow and the depth great, there is a potential for arching and backfill hang-up. Laboratory testing was carried out to characterize changes in hydraulic conductivity and the coefficient of lateral earth pressure with increase in effective stress. Soil-bentonite (SB) slurry walls are one of the most popular techniques to minimize the
1. Vertical stress distribution in soil bentonite slurry walls
Rahul V Mukherjee MDH Engineered Solutions, Member of SNC Lavalin
Group, Saskatoon, Saskatchewan, Canada. Moir D Haug Department of
Civil Engineering, University of Saskatchewan, Saskatoon,
Saskatchewan, Canada. ABSTRACT Backfill arching or wall hang-up of
backfilled slurry wall trenches has the potential to significantly
reduce the effectiveness of these walls. Soil-bentonite (SB) slurry
walls are one of the most popular techniques to minimize the
horizontal migration of contaminants. This technique involves the
excavation of a narrow trench in the presence of bentonitic slurry
down through permeable strata and backfilling with a low
permeability material. This paper presents the results of a
laboratory testing program and modelling study to investigate the
vertical stress distribution in slurry walls. The program was
conducted to supplement the design and installation of an 11,000 m
long slurry wall. This slurry wall is being installed through low
hydraulic conductivity glacial till containing intermediate
permeable zones. The depth of this 1 m wide slurry wall is
approximately 50 m. Backfill hang-up in these deep walls is a
particular concern. In order to calculate hang-up it is necessary
to determine friction along the wall of the trench. The testing
program involved large strain consolidation testing of selected
field backfill samples. A Marriott bottle system was also employed
to continuously measure hydraulic conductivity and lateral stress
during testing. Additional backfill samples were tested to provide
data on sensitivity of horizontal and vertical stress to variations
in backfill fines content. An analytical model and a coupled
seepage stress-strain finite element model (FEM) were used to
predict vertical stress changes with time and depth for the
different backfill materials. The results of the research study
found that trench width was the most important variable in
determining wall hang-up. It also showed that the upper portions of
the backfilled trench may take years to consolidate and thus
provide the desired level of impermeability. RSUM La vote ou la
paroi moule de tranches remblayes a le potentiel de rduire
significativement lefficacit de ces murs. Lutilisation de parois
moules constitues dun mlange de sol-bentonite (SB) est lune des
techniques les plus populaires pour limiter la migration
horizontale des contaminants. Cette technique implique lexcavation
dune tranche troite, en prsence dune suspension de bentonite,
travers des strates permables et le remblayage avec un matriau
faible permabilit. Cet article prsente les rsultats dun programme
dessais en laboratoire ainsi que dune tude de modlisation de la
distribution des contraintes verticales dans les parois moules. Le
programme a t ralis dans le but de complter la conception et
linstallation dune paroi moule de 11 000 m de long. Cette paroi est
en cours dinstallation dans un till faible conductivit hydraulique
contenant des zones permables intermdiaires. La profondeur de cette
paroi moule de 1 m de large est denviron 50 m. Les conditions
daccrochage du remblai le long de ces parois profondes est une
importante proccupation. Afin de calculer laccrochage, il est
ncessaire de dterminer le niveau de friction le long de la paroi de
la tranche. Le programme dessais comprend des essais de
consolidation grande dformation dchantillons slectionns de remblai.
Un systme de bouteille Marriott a galement t employ pour mesurer en
continu la conductivit hydraulique et la contrainte latrale durant
les essais. Des chantillons de remblai supplmentaires ont t tests
afin de fournir des donnes sur la sensibilit des contraintes
horizontales et verticales en fonction de la fraction de particules
fines contenues dans le matriel de remblai. Un modle analytique et
un modle par lments finis (FEM) ont t utiliss pour prdire les
changements de contraintes verticales en en fonction du temps et de
la profondeur de chacun des diffrents matriaux de remblai. Les
rsultats de ltude ont montr que la largeur de la tranche tait la
variable la plus importante pour dterminer laccrochage aux parois.
Il a galement t dmontr que les parties suprieures de la tranche
peuvent prendre des annes avant dtre consolides et donc fournir le
niveau dimpermabilit dsir. 1. INTRODUCTION The potential for slurry
wall backfill arching and hang-up was examined in a laboratory
testing program. The permeability of clay till slurry backfill is
highly dependent on the load placed on the backfill. As a result
permeability usually decreases with depth as the backfill
consolidates slowly under increasingly high loads. If the width of
the backfill trench is narrow and the depth great, there is a
potential for arching and backfill hang-up. Laboratory testing was
carried out to characterize changes in hydraulic conductivity and
the coefficient of lateral earth pressure with increase in
effective stress. Soil-bentonite (SB) slurry walls are one of the
most popular techniques to minimize the
2. horizontal migration of contaminants. Backfill arching or
wall hang-up of the backfilled trenches has the potential to
significantly reduce the effectiveness of slurry walls. A 45 m plus
deep, 11,000 m long slurry wall is currently being constructed at a
Potash mine in eastern Saskatchewan. This wall is being constructed
through high clay content drift. The slurry wall is being keyed
into low permeability upper Cretaceous clay-shale. The purpose of
this deep slurry wall is to cutoff seepage of brine through stacked
aquifers which exists between the glacial till under the site.
These aquifers represent pathways for the migration of chloride
impacted fluids. 2. BACKGROUND Soil bentonite (SB) slurry walls are
vertical barriers constructed by excavating a narrow trench through
high hydraulic conductivity material or zones and backfilling with
a mixture of soil, bentonite and water to achieve a low hydraulic
conductivity barrier (Xanthakos 1979, Haug 1983, Evans 1995). The
backfill soil either comes from the excavated trench or from a
source nearby, if the soil is deemed to be unsuitable for use. The
slurry in the SB wall is a mixture of water and dry bentonite (4% -
7%) by weight (Evans 1994). Before placement in the trench the
backfill is mixed with desired amount of slurry from the trench to
provide a desired consistency. Since SB walls are mainly used as
low hydraulic conductivity barriers and constructed as a part of
polluted site remediation process, hydraulic conductivity value
plays an important role. In general, they range from 10 -7 m/s to
10 -11 m/s (DAppolonia 1981, Haug 1987, Evans 1995, Yeo et al
2005). The hydraulic conductivity for the backfill mixture depends
upon many factors such as (i) the amount of dry bentonite added to
the mix (Ryan 1987, Evan 1994, Yeo et al 2005), (ii) the amount of
fines in the mixture (Xanthakos 1979, DAppolonia 1981, Yeo et al
2005), (iii) resistance of the mix against the contaminant being
contained and (iv) the state of stress in the slurry wall (Evans
1995, Filz 1999). The performance assessment of these walls in
terms of hydraulic behaviour is generally done through laboratory
testing of the backfill mixture. Laboratory models and field
studies on the slurry walls have shown that the stress in the wall
does not increase linearly with depth as assumed earlier
(McCandless and Bodocsi 1987, Evans 1995).The friction at the
trench/backfill interface governs the consolidation behaviour of
the backfill- slurry mix, which is fairly compressible. This
phenomenon can be explained by arching. 3. FOCUS OF THE STUDY
Backfill hang-up and arching is of particular concern for deep
slurry walls. This potential is a function of the width of the
trench, the depth of the trench, shear strength of the backfill and
potentially the sequence and spacing of permeable zones cut-off by
the slurry wall. In 2009, construction was started on an 11,000 m
long, 45 m plus deep slurry wall through glacial till at a
Saskatchewan potash mine. This slurry wall is intended to control
the migration of chloride impacted water. The hydraulic
conductivity and the potential for osmotic consolidation is a
function of the stress environment of the trench backfill. As a
result, the potential for arching or wall hang-up is a concern.
Further complicating the situation is the presence of stacked
aquifers which will result in more rapid backfill consolidation at
various depths in the trench. Figure 1 shows the general situation.
Figure 1 Schematic diagram of complex site geology 4. EXPERIMENTAL
PROGRAM The experimental program involves slump cone tests, direct
shear test, large strain consolidation testing along with hydraulic
conductivity measurement of the backfill mixes. A Marriott bottle
system was also used to enable continuous monitoring of hydraulic
conductivity during loading. Miniature load cells were installed on
the walls of the mold at different depths to measure the stress
profile. Figure 2 shows a sketch of the large strain modified
consolidation mold used in the present study. 4.1 Test Materials A
glacial till brought from the site was used as a host soil for
preparing the backfill mixes. The natural water content of till was
about 6% 8%. Air dried and pulverized till passing through 4.75 mm
was used for preparing the trial backfill mixes. The hydraulic
conductivity of the till was about 1 x10 -8 m/s. The
preconsolidation pressure of the host soil ranged from 1200 1500
kPa. A total of four different backfill mixes were examined in the
testing program. Three backfill trial mixes (TM1, TM2 and TM3) were
prepared in the laboratory by varying the fines content (particle
size < 75 m) and the fourth mix was collected in the field (FB)
from the site of the slurry wall construction. The percentage of
fines in the backfill mix made in the laboratory (TM1, TM2 &
TM3) was varied as 10%, 25% and 50% respectively. Table 1 shows the
description of the backfill mixes used in this study. Bentonite
slurry was prepared in the laboratory in the same proportion as
that in the field to simulate the field conditions. The bentonite
was mixed with tap water using a high-speed colloidal shear mixer.
Then the bentonite-water
3. slurry was allowed to hydrate for 48 hrs 72 hrs in order to
properly hydrate the bentonite powder. The density of the slurry
was 0.86 g/cc and the Marsh cone viscosity was 39 s (API 1990). The
pH of the slurry was about 9.20. Table 1 Description of backfill
mixes Mix Percent of fines without bentonite Percent of fines with
bentonite Amount of bentonite in the mix Trial Mix1 (TM1) 10 12.9
2.9 Trial Mix2 (TM2) 25 28.9 3.9 Trial Mix3 (TM3) 50 52.8 2.8 Field
Backfill (FB) N.A 28 N.A 4.2 Experimental Setup A test apparatus
for measuring the co-efficient of lateral earth pressure (K) during
large strain consolidation was developed. This apparatus was based
on original design by Gan et al; 2011 (Nov 2011). The Proctor-type
stainless steel mold shown in Figure 2 (150 mm diameter) was
modified to measure the co-efficient of lateral earth pressure and
consolidation characteristics of the backfill mixtures under
increasingly large strains. Hydraulic conductivity (k) of the test
mixes were also measured simultaneously. Button type load cells
were installed in the mold to measure the lateral pressure exerted
on the wall of the mold by the backfill mixes during consolidation.
The mold was modified to contain ports for measuring hydraulic
heads at 8 different depths. The bottom outlet of the mold was
connected to a Marriott bottle setup to continuously run constant
head hydraulic conductivity test along with consolidation. The
modified mold (Figure 2) was loaded by a Conbel up to 900 kPa. Two
load cells and the linear potentiometer were connected to a
computer by a data logging system. Lab view software was used to
communicate with the data logging system to record the raw data
automatically on to the computer. A Marriott bottle was connected
to the bottom outlet of the mold to run the constant head hydraulic
conductivity (k) test simultaneously along with consolidation. All
the eight ports in the mold were connected to a manometer to enable
reading heads at different heights in the sample. 4.3 Test
Procedure Preliminary tests such as determination of Atterberg
limit were performed on all the soils used in the test program,
namely the glacial till (host soil) obtained from the field, the
field backfill (FB), TM1 (having 10 % fraction passing 75 m), TM2
(having 25 % of fraction passing 75 m) and TM3 (having 50% of
fraction passing 75 m). Slump cone tests were conducted on the
samples. This helped in determining the consistency of the SB mix.
This is an indirect way of measuring the flowability of the mix. In
the present work, each backfill mix was mixed with 5%
bentonite-water slurry in various proportions to evaluate the
relationship between the gravimetric water content of the resulting
backfill-slurry mixture and the resulting slump. The slump was
measured according to ASTM C 143, and the slump tests performed for
each specimen at a given water content were repeated two to three
times to account for variability in the measured values of slump.
Figure 2 Modified mold Consolidated drained (CD) shear box tests
were conducted to study the shear strength parameters of the
backfill mixes. All the samples were sheared under increasing
normal pressures of 54.7 kPa, 163.2 kPa and 316 kPa. In total 12
samples were tested. Samples from different mixes were allowed to
consolidate in large consolidation molds at different specified
normal loads. After, consolidation the samples were extruded and
trimmed to fit into a 150 mm x 150 mm diameter direct shear box.
The samples were left for 1 - 3 days depending upon the normal
load. The shear box test was then carried out according to ASTM
D-3080-04. The saturated samples in the molds were compacted at a
water content corresponding to a slump of 120 mm and at a constant
dry density. The depths in the field were simulated corresponding
to the load applied onto the sample. The samples once compacted
were left to consolidate under the weight of the loading plate,
till the load cell and potentiometer reading stabilized. The whole
setup was then loaded using Karol-Warner Conbel. Load cell and
deflection readings were recorded and monitored continuously. A
constant head hydraulic conductivity test was run simultaneously,
with use of the Marriott bottle arrangement (Figure 2). Once the
equilibrium was attained, the next incremental load was applied to
the sample. 5. DISCUSSION OF LABORATORY TEST RESULTS Figure 3
shows, the failure envelopes of TM1, TM2, TM3 and FB obtained from
the shear box test data. The effective angles of internal friction
() measured from the failure envelope were 32 , 29 , 26 o and 23 o
for TM1, FB, TM2 and TM3, respectively. This was because the sand
contents in TM1, TM2 and TM3 were 87%, 71% and 47.2% respectively.
The amount of fines present in the mix also justifies the values.
These results were used as an input, for modelling consolidation of
the SB wall. These values are
4. considerably lower than 35 degrees previously reported in
the literature. Figure 3 Failure envelope for different types of
backfill mixes Figure 4 shows the e - log p curves for all the test
materials tested for compressibility in a large proctor mold, the
diameter of which was 150 mm. The sample thickness was 160 mm.
Consolidation test was performed by applying air pressure on the
sample and by monitoring settlement of the sample. The equilibrium
void ratio corresponding to the applied pressure was plotted
against log of applied pressure as shown in Figure 4. TM3 showed
highest compressibility because it had highest fines content and
the other mixes, namely TM2, FB and TM1 which had lower fines
content resulted in reduced compressibility. Figure 5 shows the
variation of measured hydraulic conductivity, k (m/s) with the
applied vertical stress. These tests were run simultaneously with
consolidation. This was done using Marriott bottle arrangement as
described in previous section. All the tests were run 3 times to
account for variability in results. The data shows that hydraulic
conductivity (k) decreased with increasing in vertical stress (i.e.
decrease in equilibrium void ratio). The data also show that the
hydraulic conductivity value of TM3 which had highest fines content
of all the materials was lower than other materials. The applied
pressure hydraulic conductivity relation for FB and TM2 was close
because the fines content (silt and clay) in these materials was
nearly equal. TM1, which had the least fines content and highest
coarse content resulted in the highest value of hydraulic
conductivity corresponding to the equilibrium void ratio. Figure 4
e log p curves from large strain consolidation test Figure 5
Hydraulic conductivity (k) plots for various backfill mixes The
hydraulic conductivity for TM1 decreased from 1x10 - 4 m/s to
2.5x10 -7 m/s with the increase in stressfrom 9 kPa to 900 kPa.
Whereas the k for TM3 decreased from 8x10 -9 m/s to 4x10 -10 m/s
with the same increase in stress. Hence, the amount of fines
present in the mix plays an important role in controlling k.
Similar results have been reported in literature by Yeo et al, 2005
where hydraulic conductivity for their test mixes also decreased
with the increase in fines content. 6. MODELLING A numerical method
based on principles of statics (discrete model) and secondly the
finite element method was used for this study. This method does not
consider time dependent behaviour such as secondary consolidation,
creep etc. The results of modeling using both methods have been
discussed and comparisons between both have been drawn. 6.1
Discrete model Discrete model is based on the principle of statics.
A body is said to be in a state of static equilibrium, if and only
if all the forces acting on it are balanced. Assumptions in statics
model include; 1. The co-efficient of lateral earth pressure, K = h
/v where h and v are horizontal and vertical effective stresses
(Figure 6). These are not principal stresses. 2. For a given trench
width, at any depth,v is uniform across the trench. 3. Co-efficient
of lateral earth pressure (K) is constant all throughout the wall.
4. Friction is fully mobilized along the two vertical interfaces of
the slurry wall. Note that this solution does not take into account
stress- strain relations of the material.
5. Figure 6(a) shows a trench of B unit wide which has been
discritised into small elements of thickness, h. Consider an
element of thickness h, at a depth of h from the ground surface
(Figure 6a). vn is the stress acting on the top of the element
which is the cumulative stress being transferred by the elements
above it. Figure 6(c) shows the free body diagram for the shaded
element under consideration. Pn (force acting downwards) = Force
being transferred due to the normal stress to the top of the
element under consideration by the elements above it. w is the self
weight of the element. F/2 is the frictional force acting upwards
on each side of the block. Pn+1 is the net force acting upward on
the segment to keep it in static equilibrium. From equilibrium,
Figure 6(c) Po + W P1- 2*F/2 = 0 P1 = Po + W 2*F/2 Writing the
forces in terms of stresses, vn+1 * B * 1= (vn * B * 1) + ( * B *1
* h) 2*F/2 Where, F/2 = (tan ) h h= (tan ) Kv h vn+1 = (vn) + ( *
h) 2*(tan Kvn/B) h [1] Hence, from the above equation, vertical
stress on any element is the function of the co-efficient of
lateral earth pressure K, width of the wall B and angle of internal
friction, 6.1 Parametric study using discrete model The variation
of vertical effective stress in the wall, with depth, for different
width of the wall and for K = 0.35 and = 29 is shown in Figure 7.
The width (w) of the wall is varied as 0.5, 1, 2, and 4 m. The
dashed line (w = 1m) represents the stress distribution for the SB
wall. The vertical stress at any depth moves towards the geostatic
stress (h) distribution with an increase in the width of the wall.
Thus it can be concluded that arching reduces as the width of the
wall increases. Figure 7 shows that for all trench widths the
stress up to 2 m depth is approximately equal to geostatic stress.
Below that depth, the vertical stress deviates significantly from
geostatic stress. Figure 8 shows the variation of vertical
effective stress (v) with depth of the wall for different angles of
internal friction (for K = 0.35 and w =1 mBased on Rankine active
and passive earth pressure theory, K should lie between 0.35 and
3.25. This is the lowest and highest values of K calculated using
the value shown in the figure. The angle of internal friction, 'for
the SB mixes varied from 2 to 2 . he dashed line in Figure 8
represents the properties of backfill from the slurry wall
construction site. The width of this wall was 1m. Figure 6.
Discrete Model Figure 7 Variation of vertical effective stress (v)
with depth for different widths of the wall Figure 8 Variation of v
with depth for different angle of internal friction () of the SB
mix
6. Stress distribution in the wall was found to move further
away from the geostatic stress with increase in '. For all the
values of ', v in top 3 m, is approximately equal to geostatic
stress and thereafter starts to move away (less than geostatic
stress). As ' increases, the vertical effective stress decreases.
This is because friction along the side walls acts upward and is
function of '. Hence, with increase in ', the backfill has
increased resistance to settlement. It also resists consolidation
and strength gain. Figure 9 Variation of v with depth for different
values of K The vertical effective stress in top 2 m for all values
of K is approximately same. The stress at D = 8 m, 20 m and 50 m
decreases as K increases from 0.25 to 0.75. Based on Rankine earth
pressure theory for 29; K should lie between 0.34 and 2.88. The
percentage difference between the geostatic stress and v at 50 m
depth for K = 0.35 and 0.75 is 95.4% and 98%. This difference is
not overly significant. Hence, K has little impact on arching in
the SB walls. Figure 10 Vertical stress distribution for different
SB mix used in the present study It can be concluded that arching
in slurry wall can be reduced by selectively choosing the mix
parameters. The width of the wall plays a significant role in the
performance of SB walls. If possible, the wall should be wider if
the trench is deep (Figure 7). The stress distribution using trial
mix 3 (TM3) would result in the least arching (Figure 10). The
vertical effective stress at 50 m depth is 22 kPa, 25 kPa, 29 kPa
and 36 kPa for TM1, FB, TM2 and TM3 respectively. Stresses are
about 90% of the geostatic stress (h) for a given depth. 6.2 Finite
Element Model A finite element model (FEM) of Soil Bentonite cutoff
wall was developed for this research program. The model simulates
the consolidation of the backfill material. Finite element software
Geo-studio 2007 was used for analysis. Seep and Sigma packages in
Geostudio were coupled for this work to simulate fully coupled
behaviour. The coupled analysis simultaneously solves two groups of
nodal equations, equilibrium (stress-deformation) and continuity
(flow) equations, across the finite element mesh (GEOSLOPE 2007).
The basic material-parameters required for such analysis on fully
saturated systems are: Youngs Modulus, E; Poissons ratio, ; Unit
weight of the material, ; and saturated hydraulic conductivity, k.
However, the software does not take in to account secondary
consolidation and the deformation due to creep. The model-domain
was delineated as a 1 m wide soil bentonite column from the top of
the ground surface and keyed at a depth of 50 m into a stiff
material such as heavily consolidated clay or bedrock (which is
shale in this case). Figure 11 shows the simplified representation
of the complex site geology. Only fully saturated SB wall was
modelled, with water table at the top. The hydraulic and stress
boundary conditions used in the model are laid out in the Table 2.
Figure 11 Cross-section of aquifer used for the present study 4.3
Test Procedure Preliminary tests such as determination of Atterberg
limit were performed on all the soils used in the test program,
namely the glacial till (host soil) obtained from the field,
the
7. field backfill (FB), TM1 (having 10 % fraction passing 75
m), TM2 (having 25 % of fraction passing 75 m) and TM3 (having 50%
of fraction passing 75 m). Slump cone tests were conducted on the
samples. This helped in determining the consistency of the SB mix.
This is an indirect way of measuring the flowability of the mix. In
the present work, each backfill mix was mixed with 5%
bentonite-water slurry in various proportions to evaluate the
relationship between the gravimetric water content of the resulting
backfill-slurry mixture and the resulting slump. The slump was
measured according to ASTM C 143, and the slump tests performed for
each specimen at a given water content were repeated two to three
times to account for variability in the measured values of slump.
Table 2 Boundary conditions used in FEM analysis Type of analysis
Flow boundary Displacement boundary In-situ Water table at top
Sides - x direction fixed (x = 0) Bottom x & y direction fixed
(x and y 0) Transient Water table at top (Inherited from In- situ
analysis) Zero pressure at top Flow into the side walls (total head
= 50 m throughout the depth) Sides and bottom fixed (x and y 0)
6.2.1 Discussion of results from finite element model It was
important to wait for excess pore pressure to dissipate completely
as it reflects the end of consolidation process (primary
consolidation). In the present case (Figure 12), it takes 1.75
years to dissipate more than 95 % of the excess pore pressure. With
the increase in fines content of the mix , the time taken for more
than 95% of excess pore pressure to dissipate increases. This is
because the amount of void in the soil matrix decreases with the
increase in fines content of the mix. Construction of soil
bentonite (SB) slurry wall is a site remediation technique to
contain waste at the contaminated site. In the present study, 11 km
long SB wall is being constructed around a potash mine tailing
management area (TMA) to retard and prevent the flow of brine into
the nearby aquifers and water channels. Since the top few meters of
the wall is prone to attack by brine which can cause osmotic
consolidation, it is important to understand the vertical effective
stress distribution in SB walls with time and depth of wall. The
variation of v with time and depth of the wall for FB is shown in
Figure 13. Similar curves for other trial mixes used in this study
were also plotted. The time taken for effective stress to build up
at any given time and depth, increases with the increase in fines
content in the mix. For example, the stress at time, T = 0.5 years
at 20 m depth for TM1 and TM3 are 6 kPa and 2 kPa. This is about
65% and 95% less than the effective stress at the same depth at the
end of consolidation. The stress distribution shown in Figure 14
compares the Final v at the end of consolidation i.e. when all the
excess pore pressures have dissipated. In the case of above mix, it
took 1.75 years (Figure 12) for the excess pore pressure to
dissipate completely and reach the hydrostatic state (steady state
in this case).The dashed line in Figure 14 shows the vertical
stress distribution as predicted by the discrete model. It shows
that after a depth of about 5 m from top, the stress becomes
constant with depth. The solid line representing FEM prediction
shows an increasing trend of stress from top to bottom of the wall
with the increase in depth. Figure 12 Dissipation of excess pore
pressure with time Figure 13 Variation of vertical effective stress
(v) with time The FEM and discrete model prediction seems to match
over the entire depth of the wall. The stress at a depth of 5 m
from the ground surface is 65% and 62% less than the geostatic
stress whereas the stress at 50 m depth (at the bottom of the
trench) is 96.5% and 95.5% less than geostatic stress as predicted
by FEM and discrete model respectively. Hence, it can be concluded
that the vertical effective stress in SB walls never reaches
geostatic stress condition.
8. Figure 14 Comparison of final vertical effective stress, v
with depth of the wall as predicted by the two models 7.
CONCLUSIONS The results of this study show that there is a
significant loss of effective vertical stress in initial slurry
wall backfill with depth, and that this loss increases for narrower
a trench. The following conclusions can be drawn from the
laboratory test results. i. Grain size distribution has significant
impact on but minimal impact on K. The value for the SB mix ranged
from 2 for M (maximum amount of fines) to 2 for M1 (minimum amount
of fines) ii. Hydraulic conductivity (k) of the SB mix was a
function of amount of fines. At a given consolidating pressure, k
decreased with increase in fines content in the backfill mix.
Similar results have been reported by Baxter, 2001 and Yeo et al,
2005. iii. Hydraulic conductivity for TM3 and TM1 at consolidating
pressure of 900 kPa was 2.80 x 10 -10 m/s and 5.28 x 10 -8 m/s
respectively. TM2 and FB which had nearly the same fines content
have similar k at any given consolidating pressure. iv.
Compressibility of the mix was proportional to the amount of fines.
TM3 which had the maximum amount of fines (50%) was the most
compressible, while TM1 with the least amount of fines (10%) was
the least compressible. Vertical stress distribution in SB walls
was predicted using analytical and finite element methods. Both
models show arching in the SB wall. Arching is defined as a
phenomenon of stress transfer from a yielding mass to an adjacent
stationary solid body. In the present study, arching is observed by
stress distribution pattern in long narrow SB slurry walls. Arching
in SB walls decreases with the increase in the width of the wall
i.e. the stresses move more towards geostatic stress condition.
This was due to decrease in frictional contribution which opposes
the consolidation of the SB mixes. i. Co-efficient of effective
angle of internal friction (') of the material influences the
consolidation of materials in narrow trenches. The greater the
amount of fines in the mix, the smaller is the arching. Hence,
arching in deep narrow walls is largely proportional to '. ii.
Arching in the SB walls decreases with an increase in trench width.
iii. Although the final stress in the wall may be reached in a few
years, it never reaches the geostatic stress condition. 8.
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