DESIGN AND CONSTRUCTION OF A CUT-OFF WALL IN WARM PERMAFROST
Steven L. Anderson, P.E., Senior Geotechnical Engineer and Associate, Golder Associates Inc.
Brian W. Wilson, P.Eng., Regional Leader – Western Canada, GAIA Contractors
Matthew Tonner, E.I.T., Engineer, GAIA Contractors
A cut-off wall has been designed and is currently under construction along the Back Dam of the
existing tailings impoundment at Red Dog Mine, Alaska. The cut-off wall, which will be in the
order of 5,000 ft long and up to 150 ft deep, when complete, is being excavated using a 40 metric
ton Bauer trench cutter head mounted on a Liebherr crawler crane, in combination with a closed
loop slurry pumping and de-sanding system.
The tailings impoundment is situated in an area of warm permafrost and high potential seismic
activity. Design incorporated an assessment of rock jointing to define the final depth of the wall
along the alignment requiring the use of plastic concrete backfill to address the concerns
associated with potential cracking of the wall due to settlement and during seismic activity.
The results of the geotechnical investigation program, particularly with respect to variation in rock
hardness along the wall alignment are discussed, as are the design criteria for the plastic
concrete backfill, the challenges faced during construction, including scheduling of the work with
the on-going dam raise activities of the mine, and the lessons learned with respect to potential
productivities and selection of cutter wheels and teeth. Also described are the requirements for
and advantages of utilizing a slurry boosting system to meet pumping requirements, re-design of
the plastic concrete mix to consider workability and optimization of cutter teeth configurations to
reduce wear through highly varied rock hardness.
Keywords: Cut-off wall; dam design; geotechnical investigation; trench cutter.
Introduction
In 2002 Teck Alaska Inc. (TAK) commissioned Golder
Associates Inc. (Golder) to design a Back Dam to
minimize the potential for seepage at the southern end
of the tailings impoundment at Red Dog Mine, Alaska.
The Back Dam was to be designed for an elevation of
960 ft with a planned closure elevation of 986 ft and
was to include provisions to reduce long-term seepage
through the native materials and weathered bedrock.
Figure 1: Back Dam Location
The initial design concept included construction of a
cofferdam with installation of a geomembrane liner on
the upstream face of the Overburden Stockpile that
would be keyed into silty native soil of relatively low
permeability. A geotechnical investigation completed
by Golder in 2004 found ice-rich and organic materials
as well as highly fractured bedrock near the liner keyin elevations, effectively rendering this concept
unacceptable.
During revision of the Back Dam design to account for
the findings of the 2004 investigation, and to address
the concerns of regulatory bodies, TAK requested that
Golder revise the design and incorporate a cut-off
through the proposed Back Dam embankment,
extending to low permeability frozen bedrock to reduce
the risk of long-term seepage through the native
materials and weathered bedrock. Several cut-off wall
options were evaluated including sheet pile
installation, vibrated beams and slurry trench
installation, extension of the geomembrane liner into a
trench, and a cut-off wall constructed using a deep soil
mixing process; specifically with the Bauer Cutter Soil
Mixer (CSM). Following discussions with Golder
Associates Innovative Applications (GAIA) Inc., and
review of the CSM operation at a site in British
Columbia, Canada, the option of constructing the
cut-off wall using the Bauer CSM through an
embankment situated between the Overburden
Stockpile and the cofferdams was selected.
Subsequent discussions with the regulators indicated
that completion of a cut-off to the frozen bedrock alone
would not be adequate and that the design should
consider the effects of some thawing of the permafrost
with possible seepage through the thawed bedrock. In
2005 and 2006, Golder completed additional
geotechnical investigations along the southern
cofferdam to determine the depth to unweathered, low
permeability bedrock, assuming thawed conditions.
Ice-rich materials and organics were encountered in
the overburden soils overlying weathered and highly
fractured bedrock. Low permeability bedrock was
encountered and recorded at variable depths along
the alignment, with the deepest section at about 170 ft
below the proposed dam crest elevation of 960 ft.
Following this investigation it was obvious that use of
the Bauer CSM would no longer be practical due to
the extent of rock excavation required, hence this
method was withdrawn and replaced by a deeper
cut-off constructed using a Bauer Trench Cutter.
Design proceeded on this basis with TAK planning to
construct the embankment dam using their own
forces, whilst retaining GAIA in 2007 in the role of
general contractor for construction of the cut-off
through the Back Dam.
that surface water and shallow groundwater greatly
influenced the thermal stability of the permafrost soils.
Between 1995 and 1997, seven thermistor strings
were installed within the Overburden Stockpile.
Temperature readings from these thermistor strings
indicate the permafrost had aggraded into portions of
the stockpile and the depth to the top of permafrost
varied between about 15 ft to 70 ft below the ground
surface (Water Management Consultants 1999).
Thermistor readings were also taken during the
2005/2006 geotechnical investigation that indicated an
average thaw depth of 18 ft below the native soil
surface and average permafrost temperatures of about
31°F (-0.6°C). The deepest thaw depths were located
near historic drainages.
Subsurface Conditions
Subsurface conditions have been characterized near
the proposed cut-off wall location from the
geotechnical investigations completed in 2004 through
2006 that included 15 boreholes near the north face of
the Overburden Stockpile and 26 boreholes along the
southern cofferdam and near the abutments. The
locations of these boreholes are shown in Figure 2
with subsurface profiles shown in Figures 3 and 4.
These investigations also included test pits, thermistor
string readings, packer tests, and laboratory testing.

Site Characteristics
The Red Dog Mine is located approximately 90 miles
north of Kotzebue, Alaska at about 68 degrees
latitude. The proposed Back Dam is located at the
north end of the Overburden Stockpile, as shown in
Figures 1 and 2.
Climate
The climate at the Red Dog Mine is characterized by
long, cold winters and moderately warm, windy and
somewhat rainy summers.
The average annual
temperature is 24°F (-4°C) with an air freezing index
(AFI) of 4731°F-days and an air thawing index (ATI) of
1899°F-days. Total precipitation is generally between
10 to 13 inches per year and the average annual
snowfall is 48 inches.
Thermal Conditions
In the early 1980’s the Red Dog Mine site was the
subject of various feasibility and design studies.
These early investigations (including the installation of
about 50 thermistor strings) indicated that relatively
warm permafrost was present at the mine site, and


Fill materials were encountered on the haul roads,
cofferdam, and Overburden Stockpile. The waste
rock fill materials at the haul road and cofferdam
generally had a composition of silty gravelly soil
with occasional cobbles and boulders.
Fill
materials at the Overburden Stockpile mainly
consist of Kivalina shale that had been stripped
from the mining area.
The native soils are colluvial and alluvial materials
comprised of organic soil, ice and ice-rich soil, silty
clayey soil and sandy gravelly soil.
The shale bedrock underlying the colluvial and
alluvial materials is typically highly to completely
weathered near the contact surface becoming
fresher with depth. The rock is generally fractured
with a rock quality designation (RQD) of less than
20%. Rock strengths vary from weak (R2) to very
strong (R5) with estimated uniaxial compressive
strengths (UCS) up to 22,010 psi. The majority of
bedrock encountered during the investigations,
about 75%, had a UCS that was less than 7,500
psi.
Design Considerations
Cut-off Wall Alignment
The embankment dam and associated cut-off wall
alignment was selected to limit the amount of
excavation into the Overburden Stockpile and
minimize disturbance of the cofferdam. Using an
excavation surface derived from the borehole and
ground topography data, an alignment for the
embankment and cut-off wall was developed based on
criteria that included:
Figure 2: Borehole Locations
Figure 3: Section A through Cofferdam


Maintaining the cut-off wall within the center of the
constructed embankment to protect the cut-off wall
from slope instability issues.
Using 1H:2V (horizontal to vertical) slopes for the
excavation prism starting a distance of 1.5 ft out
from the alignment centerline at an elevation of
986 ft. The offset distance was based on a 3 ft
cut-off wall width and the slope angle was based
on the 2 to 1 stress distribution method, which is
an empirical approach based on the assumption
that the area over which the load acts increases in
a systematic way with depth.
Figure 4: Section B through Overburden Stockpile
Cut slopes were developed based on natural slope
angles with flatter cut slopes expected on the
northern side of the alignment within the
impoundment area. A limited amount of excavation
into the cofferdam was considered tolerable
depending on the seepage flows encountered after
dewatering.
Cut-off Wall Embankment Section
The cut-off wall embankment was designed to
consist of a central section composed of select fill
produced from screened crushed rock and
compacted in controlled lifts to 95% maximum dry
density (modified proctor) to optimize strength and
limit impacts from settlement. The select fill width
was arbitrarily set at 20 ft for crest elevations of
986 ft and higher with 1H:3V side slopes. This
provides a base width that is equivalent to the
estimated deepest fill height (a criterion similar to
what is used for impervious core dams) and keeps
the select fill within the base excavation limits. The
20 ft width also allows for future raises, if required.
Rockfill, placed in controlled lifts and compacted with
a minimum number of passes with a roller, was used
as fill outside of the select fill prism to provide
additional stability with 3H:1V side slopes. The
rockfill materials were produced from waste rock
from the mining operations or borrowed from other
rock quarry areas.
Some maintenance and
additional fill placement is expected as the
underlying ice-rich materials slowly thaw and
consolidate, possibly including as much as 9 ft of
differential settlement in areas where the ice thaws
quickly. The integrity of the cut-off wall, which is
supported by the select fill section, would not be
affected by these deformations since they would
likely be restricted to the fill materials outside of the
select fill section.
The proposed trench cutter and base carrier require
a minimum work area of about 30 ft from the
centerline of the cut-off wall alignment; therefore, the
width of the embankment crest was designed to be
about 60 ft to keep the cut-off wall within the center
of the embankment. Due to the nature of the
equipment, cut-off wall construction requires a level
work surface for the trench cutter to sit on. Stepped
benches were therefore incorporated into the design
where the elevation increases at the abutments. In
addition, a stepped bench was required at the
deepest wall section due to the excavation limits of
the proposed trench cutter. The BC40 carrier was
equipped with a cable reel that could only extend the
cutter to 150 ft, necessitating a wall raise for the
deepest sections.
Cut-off Wall Panel Target Depths
The depth to low permeability bedrock, which was
defined as rock having a permeability of 1x10-6
cm/sec or lower, was established during the 2005/06
geotechnical investigation.
Since the rock was
frozen, conventional packer tests were impractical.
Permeability of the bedrock was therefore typically
determined using visual methods.
The visual
criterion for apparent low permeability was one or
less tight fractures per foot for a minimum length of
three feet. Based on this criterion, the depth to
apparent low permeability bedrock is shown in
Figure 3. The cut-off wall panels were designed to
be keyed 3 ft into the apparent low permeability
bedrock.
As is evident, the depth to the low permeability zone
varies significantly along the length of the wall. The
deepest panel areas are located near historic
drainages.
These deeper zones of higher
permeability bedrock are believed to be caused by
freeze/thaw cycles and other weathering affects.
The shallower panels are located outside of these
historic drainages and in areas where the bedrock
surface was encountered at a higher elevation.
Therefore, these higher areas have not been
exposed to the same degree of weathering and are
less fractured at depth. There appeared to be no
apparent correlation of rock hardness with depth,
hence productivity of the cutter could not be used as
an inferred guide to modify planned depths in the
field, and construction had to rely on the visual
interpretation of target depth from the investigation.
Thermal Considerations
The cut-off wall is expected to be susceptible to
degradation from freeze-thaw cycles; therefore, once
a section is complete, an insulated section is
installed over the top of the wall to prevent frost
penetration into the top of the cut-off wall.
Due to the warm permafrost temperatures, which are
near 31°F, the frozen ground conditions are not
expected to affect the cut-off wall installation, and in
particular the hydration of cement in the plastic
concrete mix.
After the cut-off wall has been constructed, we
expect the permafrost will aggrade into the fill
embankment as it did into the Overburden Stockpile.
Engineering Analyses
Engineering Analyses performed for the design
included seepage modeling, slope stability analyses
(for the constructed embankment and cofferdam),
hydrology and hydraulic analyses, thermal modeling,
settlement, trench stability, and mix design of the
cut-off wall materials.
Mix Design
The performance goals for the mix design were
developed based on the engineering analyses
described above with the requirement that the cut-off
maintain its function as a low permeability barrier
following settlement of the embankment as a result
of thaw of the underlying permafrost, or following
seismic activity. Since the analyses indicated that
settlements and stability are not expected to be
major design concerns, the minimum mix design
compressive strength and deformity was determined
qualitatively to be 100 psi and 5% strain,
respectively. A minimum permeability of 1x10-6
cm/sec was selected based on the seepage
analyses; however, a performance specification was
provided based on a flux value of 0.007 gpm/ft width
for a head loss of 23 ft and a minimum wall thickness
of 2.6 ft to possibly reduce material quantities and
save costs. This specification was to provide a
maximum flow rate of about 40 gpm across the
entire cut-off wall.
Mix design testing was carried out at Golder’s
laboratory in Burnaby, British Columbia, Canada
using the following available materials:




Potable water available at site (pH = 7)
Type I cement meeting ASTM C150
Bentonite meeting API 13A, Section 9
¾-inch minus crushed rock from site, which was
passed through an LA abrasion device to
simulate cuttings from the Trench Cutter
The mix design results are shown in Table 1. Based
on these results, Mix Design No. 4, which is 19%
water, 10% cement, 3% bentonite, and 68%
aggregate, met the performance criteria of 100-psi
minimum compressive strength and a maximum
permeability of 1x10-6 cm/sec at a strain of 5%. As is
discussed below however, this mix design was
modified in the field to better suit constructability
requirements.
Construction
Additional Investigation and Re-design
The original geotechnical investigation programs
were completed from the cofferdam or the
overburden stockpile and so were not on the actual
alignment of the proposed cut-off wall. The data
from these investigations had been used to formulate
the original design which had formed the basis of the
agency approvals to proceed with the construction.
Based on a review of this data, and in particular the
dip angle of the joints within the rock, it was
considered likely that additional investigations along
the final alignment may yield reduced excavation
depths as a result of encountering low permeability
rock at shallower depth. TAK agreed to complete
additional investigations and commissioned a drilling
program that was carried out in late 2007 along the
2008 alignment construction zone and late 2008
along the 2009 and 2010 alignment construction
zones. As suspected this investigation did indeed
indicate low permeability rock at shallower depths
directly on the cut-off alignment as shown in
Figure 5. These findings allowed development of a
revised configuration for the wall and removed the
need for a lower stepped bench along what was
previously the deepest portion of the cut-off wall.
The
revised
configuration
streamlined
the
embankment construction and effectively reduced
the cut-off wall surface area by approximately 70,000
ft2 with consequent savings in the order of $3M.
Figure 5: Revised Panel Depths (Cross Hatch Represents Area Removed from Original Panel Depths)
Excavation and Embankment Construction
Excavation and embankment construction is being
carried out by TAK in advance of the cut-off wall
construction, with construction quality control (CQC)
provided by Golder. All ice-rich and organic soils are
removed from within the embankment footprint to
expose suitable native sub-grade prior to placing the
select fill central section. The select fill material for
the embankment construction is crushed on site and
placed in one-foot lifts using a fleet of bottom-dump
haul trucks. The material is spread using a grader,
and compacted using an 8-ton vibratory roller. An
example of some of the ice-rich materials
encountered during the excavation is shown in
Figure 6.
Embankment construction started in May 2007 over
the western half of the alignment, but had only
advanced to an elevation of 960 ft prior to beginning
the cut-off wall construction in July/August of that
year. During the winter of 2007/2008 the design
crest elevation was raised to 970 ft due to discharge
limitations of the impoundment. This increased
design elevation required a 10 ft raise of the cut-off
wall sections constructed from the 960 ft elevation.
Due to the requirement for TAK resources on other
projects at the site further progress of embankment
construction was slow in 2008 and in the early part of
2009, increasing the necessary length of the 10 ft
wall raise.
These constraints were effectively
managed by TAK staff to keep the embankment
construction ahead of the cut-off wall construction.
Embankment construction continued throughout the
2009 season ensuring completion of the entire
alignment in advance of the 2010 cut-off wall
construction.
Figure 6: Massive Ice Encountered During Excavation
Table 1: Mix Design Results
Mix Type
Sample
Number
Age
Wet
Density
(days)
(cm/sec)
1A
46
83
3A
19
78
4B
42
129
Plastic
4A
80
129
Concrete
5A
19
131
Strain of Sample 5A limited by capacity of load cell
Plastic
Cement
Permeability
Before Strain
(cm/sec)
2.6E-07
6.3E-06
6.8E-08
6.0E-08
3.5E-08
Maximum
Deviator
Stress
(psi)
231
26
452
514
749
Tangent
Modulus
Maximum
Strain
Permeability
After Strain
(tsf)
2109
829
5127
3592
8135
(%)
8.3
8.1
7.7
17.7
1
(cm/sec)
2.8E-7
8.6E-6
8.4E-7
1.8E-6
6.9E-7
Cut-off Wall Construction
The cut-off wall is being constructed by GAIA
Contractors, the wholly owned specialized
contracting subsidiary of Golder’s Canadian
company.
The work is being completed with
specialist assistance from Bauer Maschinen and
subcontractor Alaska Interstate Construction (AIC)
who have provided much of the support equipment
and associated labor. About 4,000 ft2 of cut-off wall
was constructed in 2007 to evaluate the
performance of the cutter heads, the slurry
transport systems, and the mix design. Cut-off wall
construction continued in 2008 and 2009 operating
with two 12-hour shifts from May to October.
Approximately 148,000 ft2 of wall was completed in
2008 and 150,000 ft2 in 2009. In addition, 700
lineal feet of wall raise was completed in 2009. The
project is expected to be completed in late August
2010.
Methodology
The cut-off wall is constructed by excavating and
installing the wall in a series of primary and
secondary panels with 6” overlaps to ensure a
continuous cut-off wall. Construction begins by
installing reinforced concrete guide walls to help
support the near surface soils and provide
alignment and continuity for the cut-off wall. The
panel is then pre-excavated to sufficient depth and
slurry introduced into the shallow trench between
the guide walls to allow submergence of the slurry
mud pump situated on the trench cutter. Cutting of
the wall can then begin.
Proper installation of the guide walls is paramount
to maintain stability of the upper portion of the
trench and to control the alignment of the trench
cutter. The lateral load from the base carrier and
the low differential head provided by the bentonite
slurry near the surface render the guide walls
absolutely necessary to provide an active form of
support to the surrounding soils. The guide walls
also serve to properly support and align the cutter
guide frame, a vital piece of equipment used to
launch the cutter on a straight and aligned path. In
addition the guide walls act as a secure location for
marker pin installation on the outer limits of the
panels ensuring effective survey quality control. As
excavation proceeds the operator utilizes the
steering plates situated near the top and bottom of
the cutting frame to ensure that the alignment of the
cutter is plumb and the panel excavated vertically at
the desired location. The verticality of the panel is
monitored using inclinometers outfitted on the cutter
frame and recorded using real time software which
is downloaded daily for quality control purposes.
Excavation of each panel continues to the design
depths with additional slurry being added as
necessary to ensure that the hole remains full. The
deeper panels on the alignment take in the order of
14 hours to complete. During excavation, the
slurry, which contains the cuttings from the
overburden and rock, is pumped through a screen
and cyclone-based desander where the cuttings are
removed from the slurry down to a 0.060-mm grain
size. The re-conditioned bentonite is then recycled
and continually used for stabilizing the trench and
transporting the cuttings to the desander, or is sent
to the storage tanks when the system is overloaded
or shutdown. Once the panel has been cut to the
target depth, the working slurry in the excavated
column is tested for sand content. Slurry with a
sand content of less than 1% is adequate in
providing a clean displacement during the plastic
concrete backfilling process.
Alternatively, the
panel will be flushed with fresh bentonite to produce
the desired sand content before the plastic concrete
is backfilled using a tremmie pipe. An aerial view of
the on-going work is shown in Figure 7.
Wall Raises
As previously discussed, to accommodate the
delays in alignment fill placement, the trench cutting
test pilot work carried out in 2007 and much of the
work carried out in 2008 were completed from an
embankment crest elevation of 960 ft, ten feet
below the 970 ft design crest elevation. As a result
these sections of cut off wall (about 700 lineal feet)
required construction of a vertical extension to the
final grade.
Figure 7: Embankment Showing Bauer Trench
Cutter in Operation, On-Going Concrete Backfilling
Process and the Bentonite Treatment Plant
Originally it was envisioned that this work would be
completed
using
conventional
concreting
techniques by forming and pouring the wall,
effectively cutting off all alignment access for
remaining construction works. However, a method
was developed on site that allowed the
embankment to be filled to the final grade while
providing continual alignment access and no
subsequent delays in the trench cutting. The
method used as-built survey information to install
the guide walls at elevation 970 ft along the
alignment over the existing top of the cut-off
situated ten feet below grade at elevation 960 ft.
Similar to the trench cutting process, the installed
guide walls provided active support of the soil at
ground surface during a 30-foot long, 14-foot deep
trench excavation required to expose the top of the
existing wall and provide adequate key in. The wall
raise excavation was completed using a flat edge
clean up bucket that scraped the top of the existing
wall clean of any sloughed gravel and residual
hydrated bentonite. A cement bentonite grout was
poured and mixed on to the existing wall to provide
a bond between the existing cured plastic concrete
and the freshly backfilled plastic concrete bentonite.
Cutter Wheels and Cutter Teeth
Trench cutters are typically fitted with one of three
types of wheels and teeth. The configuration of
cutting wheel used in Red Dog is called the round
shank mount (RSM) wheels (Figure 8). These
wheels cut a variety of rock hardness ranging from
R1 to R5 and are ideally suited for the conditions
encountered along the Back Dam alignment. The
other two widely used wheel configurations include
the flat tooth wheels (the original wheel set used in
the 2007 pilot test) and bullet tooth wheels. Flat
tooth wheels are extremely efficient in soft rock and
clay like conditions as they are able to cut and
scoop large quantities of material, but are
unproductive for the R2 to R5 rock that makes up
60% of the required cutting through the Back Dam
alignment. Bullet tooth wheels are a configuration
of large conical shanks with multiple carbide tips
capable of grinding and cutting large columns of R4
to R6 rock, but were not economically feasible for
this particular application. For construction through
the 2008 season, ten types of RSM teeth were
purchased in an effort to optimize tooth wear and
cutting production. Supplied by Betek, Sandvik,
and Kenametal, all of these teeth proved capable
through various rock conditions; however, the small
surface area on the tips of the Betek and Sandvik
teeth made them more capable through the harder
rock zones.
The larger surface area of the
Kenametal teeth made them more useful in areas of
softer and more weathered rock. Due to varying
teeth lengths and thicknesses it is crucial that the
same types of teeth are used in unison on the
wheels for even wear. Since the teeth used were
adapted to match the anticipated rock hardness on
a day to day basis, the production variations among
the teeth types were negligible through the 2008
and 2009 season staying relatively consistent at
0.04 teeth/square foot. A new order of the less
expensive Sandvik teeth, capable of cutting in the
majority of the conditions found at Red Dog, and
some Betek teeth will finish out the 2010 seasons.
The smaller quantities of Betek teeth are required
as they prove most useful on the flap teeth, an
important tooth that cuts underneath the cutter gear
box, allowing the cutter to proceed down the hole
without getting hung up on a ridge of rock that
would otherwise be uncut.
Slurry Booster System
The slurry mud pump housed in the cutter is able to
efficiently pump cutting-contaminated slurry up to a
distance of 1500 linear feet back to the slurry
treatment plant. When the cutter is operating more
than 1500 linear feet from the treatment plant, the
cutting progress is held up by the inability to
transport the cuttings back to the plant at a rate that
can match the cutter progress. On initial project
set-up the slurry handling system was situated at
approximate station 16+00 along the 5,000 foot
long alignment. Under these circumstances it was
anticipated that the treatment plant including all of
the tanks, pumps, de-sanding equipment, piping,
electrical and water lines would be moved at least
once during the project to accommodate the
maximum pumping distance of the cutter mud
pump. However, scheduling issues meant that
relocation of the plant had the potential to seriously
impact the overall Back Dam construction schedule.
As a result a slurry booster system was added to
the existing system in 2009.
Figure 8: Bauer Cutter Heads Showing RSM Wheel
Setup
The slurry booster system included a 6” diesel
powered booster pump accompanied by an
increased pipe diameter in the slurry return line.
The ability to easily move the remote controlled
pump along the alignment as required maintains
consistent flows that, at a minimum, will keep up
with the faster cutting rates experienced in the top
30 to 50 feet of select fill. The booster system
increases production considerably in areas where
minimal cutting effort is required. The added cost of
the booster system equated to 2 weeks of crew site
time, insufficient time to move the treatment plant to
an alternate location, thus making the booster
system economically feasible and extremely
advantageous.
Plastic Concrete Mix Design and Placement
As previously discussed the review of preliminary
mix designs indicated that Mix No. 4 would provide
the appropriate properties for the design. However,
after initial use of this mix during the 2007 pilot tests
it was evident that the overall process and
productivity could be enhanced with a changed mix
design. The new mix design was developed around
two additional criteria to the strength and
permeability specifications. First, the new plastic
concrete mix incorporated lower density bentonite
slurry, as the slurry used in the original mix could
not properly circulate through the pumps in the
storage tanks. Second, the plastic concrete mix
was developed around a minimum allowable slump
of 7 inches to reduce the manual labor required in
the backfill tremmie process and eliminate the
bridging in the tremmie pipe that was inherent with
the low slumps in Mix No. 4 (slumps of
approximately 4 inches). Based on the desired
plastic concrete mix quantity the concrete trucks
load up with a volumetric amount of bentonite slurry
at the treatment plant and then drive to a concrete
batching plant to be loaded with aggregate and
cement. Mixing is then completed in the ready mix
truck prior to discharging into the panel excavations
with the use of tremmie pipes.
Instrumentation and Monitoring
The final element of construction will be the
installation of instrumentation and monitoring
equipment. Instrumentation, including thermistor
strings, piezometers, and survey monuments will be
important to monitor the performance of the
seepage reduction structure. Thermistor strings will
be installed to monitor degradation or aggradations
of the permafrost while standpipe piezometers will
be installed to monitor water levels and seepage
potential across the cut-off wall. Surficial survey
monuments will be used to monitor settlement.
Additional instruments installed within the cut-off
wall, such as inclinometers and extensiometers will
be used to monitor vertical and horizontal
movements, which are also vitally important to our
understanding of the performance of the cut-off
wall.
Conclusions
Experience on this project indicates that the trench
cutter is a very effective tool for the construction of
deep cut-off walls extending through both
overburden and rock. Careful selection of the type
of cutting wheel, as well as type of tooth, is
important to generate optimal production,
particularly when dealing with highly variable rock
strengths. Management of the slurry mixing and
delivery processes, as well as careful consideration
of the handling properties of the mix design for the
wall should also be given detailed consideration
during planning of the project.
References
HAMMER, T.A. et al. 1985. Ground temperature
observations. Thermal Design Considerations in
Frozen Ground Engineering, pp 8-52.
WATER MANAGEMENT CONSULTANTS. 1999.
The phase II hydrologic characterization of the
tailings impoundment.