Construction of the Dewatering Dikes at

CONSTRUCTION OF THE
DEWATERING DIKES
MEADOWBANK
GOLD MINE, NUNAVUT
Fiona Esford, M.A.Sc., P. Eng., Associate, Golder Associates Ltd., Vancouver, BC, Canada
Grant R. Bonin, P. Eng., Associate, Golder Associates Ltd., Vancouver, BC, Canada
Michel R. Julien, Ph.D., P.Eng, Corporate Director Environment, Agnico Eagle Mines Ltd.,
Toronto, ON, Canada (formerly Principal, Golder Associates Ltd.)
12
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AERIAL IMAGES COURTESY OF AGNICO EAGLE MINES LIMITED, MEADOWBANK DIVISION
ABSTRACT:
Between 2008 and 2010, a series of dikes were constructed within Second and Third Portage Lakes at
Agnico Eagle Mines Limited’s Meadowbank gold mine,
located approximately 80 km north of the hamlet of
Baker Lake, Nunavut. The dikes are key infrastructures
for the mine that permit dewatering of portions of the
lakes to expose areas for the development and extraction
of ore from open pits. This paper, the first in a series
of three papers, previously presented and published
as part of the CDA 2013 Annual Conference, describes
the techniques used to construct approximately 3 km
of dikes within the lakes over a three-year period. The
dikes consist of broad rockfill shells, granular filters,
and a central low permeability element. The low permeability element is comprised of a combination of soilbentonite and/or cement-soil-bentonite cut-off wall that
was excavated using slurry trench technology through
the granular filters to bedrock or into a dense foundation
soil. Depth of excavation was up to 15 m below the lake
elevation. In deeper portions, where the cut-off wall did
not reach bedrock, jet grout columns were constructed
between the base of the cut-off wall and the bedrock
surface, up to a depth of 22 m. Grouting of the shallow
bedrock and the interface between the bedrock and the
base of the cut-off wall and/or jet grout columns was
then carried out.
The earthworks component of the construction, the
subject of this paper, occurred over the short, open
water season of each summer approximately 12 weeks
in length, between mid-July and early October. The
dikes were constructed within the lakes, prior to any
dewatering. The construction techniques employed are
now considered as viable methods for dike construction
within the Canadian Arctic environment. The key lessons
learnt as well as the main challenges that were faced
while constructing the dikes in the Arctic environment
and the logistical constraints are also presented.
RÉSUMÉ:
Entre les années 2008 et 2010, des digues ont été
construites dans les lacs Second Portage et Third
Portage à la mine d’or Meadowbank de Mines Agnico
Eagle Limitée, située à environ 80 km au nord du village de Baker Lake, dans le territoire du Nunavut. Ces
digues sont des infrastructures essentielles permettant
le dénoyage des zones à exposer pour l’extraction du
minerai par fosses à ciel ouvert. Cet article, le premier
d’une série de trois, décrit les techniques utilisées dans
le cadre de la construction d’environ 3 km de digues
à l’intérieur des lacs, sur une période de trois ans. Les
digues sont formées d’une enveloppe en enrochements,
de matériaux granulaires filtrants et d’un élément central
de faible conductivité hydraulique. L’élément central
consiste en un mur parafouille constitué d’un mélange
de sols et de bentonite, avec ou sans ajout de ciment,
creusé en utilisant la technologie de la tranchée de boue
à travers les matériaux granulaires jusqu’au roc ou à
l’atteinte d’une couche dense de sols de fondation. La
profondeur du mur parafouille atteint jusqu’à 15 m sous
le niveau des lacs. Dans les sections plus profondes, là
où le mur parafouille n’a pas atteint le socle rocheux,
des colonnes constituées de coulis de ciment injectées
sous haute pression ont été mises en place entre la base
du mur parafouille et la surface du rocher jusqu’à une
profondeur de 22 m. La partie superficielle du socle
rocheux et l’interface entre la surface du rocher et la
base du mur parafouille ou des colonnes de ciment ont
ensuite été injectées par l’injection de perméation de
coulis de ciment (cementitious permeation grouting).
Les travaux de terrassement ont été réalisés à l’intérieur de la courte saison estivale d’eau libre des lacs
entre les mois de juillet et d’octobre, soit environ 12
semaines. Les digues ont été construites à l’intérieur des
lacs avant leur dénoyage. Les techniques de construction
employées sont maintenant considérées comme des
méthodes viables pour la construction de digues dans
l’environnement de l’Arctique canadien. Les principaux
défis rencontrés, les contraintes logistiques ainsi que les
leçons tirées de la construction de ces ouvrages dans
l’environnement arctique sont également présentés.
1 INTRODUCTION
Agnico Eagle Mines Limited (AEM) operates the
8,500 tonne-per-day Meadowbank gold mine, located
approximately 80 km north of the hamlet of Baker Lake,
Nunavut (Figure 1). The mine is accessed year round by
air and seasonally, between the end of July and midOctober, by barge to Baker Lake. Materials barged to
Baker Lake are then transported to Meadowbank over
an approximately 110 km All-Weather gravel road which
is maintained throughout the year by AEM. The mean
monthly temperature at the mine varies from a low of
-32°C in January to a high of 12°C in July. The mean
annual temperature is -11.4°C with the mean monthly
temperatures below zero Celsius for eight months of the
year, October through May. Average annual precipitation is 295 mm with approximately half of it falling as
snow. Meadowbank is located within the zone of continuous permafrost with a mean average temperature
ranging between -6 to -8°C, and estimated depth of
450 to 550 m. The thickness of the active layer varies
and taliks exist beneath lakes where water depths are
greater than 2 to 3 m.
Mine construction began in 2008. Production commenced in the first quarter of 2010. The mine consists
Canadian Dam Association • Winter 201513
Figure 1:
Meadowbank
Gold Mine
location
Figure 2: Meadowbank site
of a series of open pits, with conventional processing
and slurried tailings deposition within a tailings storage facility.
Broad rockfill dikes with a low permeability element
were constructed within Second and Third Portage
Lakes and are key infrastructures for the mine to permit
dewatering of portions of the lakes to expose areas for
the development and extraction of ore from open pits
(Figure 2).
This paper, the first in a series of three papers, previously presented and published as part of the CDA 2013
Annual Conference, describes the techniques used to
construct approximately 3 km of dikes within the lakes
over a three-year period. The earthworks component
of the construction, the subject of this paper, occurred
over the short, open water season of each summer,
approximately 12 weeks in length, between mid-July
and early October. The dikes were constructed within
the lakes, prior to any dewatering.
The first dike, East Dike, was constructed in 2008, followed by the north and south portions of the Bay-Goose
Dike in 2009 and 2010, respectively. The depth of water
through which each dike was constructed, along with
the depth to bedrock, successively increased each year
of construction. As such, the complexity of the design,
construction, and risk associated with each structure
also increased. Designs for the dikes were prepared
progressively, with adaptations made to incorporate
lessons learned from the previous year of construction
into the design for the subsequent construction season.
The construction techniques employed are now considered viable methods for dike construction within the
Canadian Arctic environment. The key lessons learnt
as well as the main challenges that were faced while
constructing the dikes in this challenging environment
and the logistical constraints are presented in this paper.
2 DIKES
Both the East Dike and Bay-Goose Dike were constructed within lakes, prior to any dewatering. The
earthworks component of the East Dike was constructed,
during the open water season of 2008, within Second
14
Portage Lake. The East Dike serves to isolate a portion of the lake to permit development of the Portage
Open Pit and concurrently provide an area for tailings
deposition. The East Dike is approximately 800 m in
length and was constructed through a maximum water
depth of 5.5 m and maximum depth to bedrock of 8 m.
The earthworks component of the Bay-Goose Dike
was constructed over two open water seasons. This dike
was constructed within Third Portage Lake to isolate and
permit dewatering of a portion of the lake for development of the Goose Pit. The north portion, approximately
900 m in length, was constructed in 2009 through a
maximum water depth of 8 m and maximum depth
to bedrock of 12 m. The south portion, approximately
1.3 km in length, was constructed in 2010 through a
maximum water depth of 8.5 m and maximum depth
to bedrock of 22 m.
The East Dike is a permanent structure. After mine
closure, flooding of the Portage Pit will occur and eventually only a 1 m head will exist across the structure. The
Bay-Goose Dike is considered temporary and at closure
will be breached to allow water from Third Portage Lake
to enter and flood both the Goose and Portage Pits.
2.1 Investigations
Bathymetric surveys were conducted during the open
water season, in the year preceding construction to
profile the lakebed surface.
Geotechnical investigations were conducted during
the winter, with drilling occurring through the ice.
Investigations were performed to obtain information
on the depth to bedrock and thickness of lakebed soils.
Information obtained from the geotechnical and bathymetric surveys was then utilized to assist in the selection
of the dike alignment, prepare the design, construction
drawings, and quantity estimates. Three drilling methods
were used: rotary-percussive drilling, diamond coring
and vibrosonic. Rotary-percussive drilling was effective
in providing estimates of the depth to bedrock where
water depths were less than 4 m, and provided chip
samples of the lakebed soils in relatively shallow areas
where the ice was grounded on the lakebed surface.
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However, samples could not be obtained in deeper areas,
nor was it possible to drill in deeper water due to the
lack of support for the drill rods. Diamond coring was
successfully used to provide estimates of the depth to
bedrock and in recovering core samples of the bedrock.
However, no soil samples could be recovered. Having
closely spaced information points about the depth to
bedrock, beneath the proposed cut-off alignment, was
found to positively aid in the design, construction planning, and execution of the works.
For the south portion of the Bay-Goose Dike, the
thickness of lakebed soil beneath where the alignment
crossed three underwater channels was considered to
be more substantial (up to 14 m). As such, during the
winter of 2010 a sonic drill rig was mobilized to site to
investigate the lakebed soils in these areas. The sonic rig
was successful in retrieving a continuous profile of the
lakebed soils, through the water column and beneath
the ice. This permitted visual classification, sampling,
and testing to characterize the materials. The lakebed
soils were found to be variable with a dense, low permeability glacial till inter-layered with lenses of silt,
sand, and gravel, and mixtures thereof. Figure 3 shows
the distribution of geotechnical holes drilled beneath
the south portion of the Bay-Goose Dike and Figure 4
shows a sample of the stratigraphic profile developed
utilizing the information obtained from the various
investigation programs.
Thermistor data has shown that permafrost exists on
land and extends into the lakes with a transition into
the talik, unfrozen area. The depth of the active layer
varies as does the quantity of ice within the frozen soil.
Therefore, frozen ground conditions were anticipated
to be present on and near the abutments of each dike
and near the islands.
2.2 Design
The design intent for each dike was to isolate portions of the lakes, permit dewatering and open pit
development, and to minimize seepage from the lakes
entering the pits during operation. Using Canadian Dam
Association (CDA, 2007) classification system, both the
East Dike and Bay-Goose Dike are rated as having a high
consequence due to the potential risk to workers within
Figure 4: Sample of stratigraphic profile developed for Bay-Goose Dike
from investigation information
the open pits and the potential for economic loss for the
mine. The dikes were required to have an adequate factor of safety: 1.5 long term static; and 1.0 pseudo-static.
In addition, the dikes were to be situated in such a
manner that the potential for pit wall instability to affect
the performance of the dike would be low.
The dikes consist of broad rockfill shells, granular
filters and central low permeability elements. The selection of materials for construction of the low permeability
element varied based on the depth to bedrock. The East
Dike consists of a soil-bentonite cut-off wall that was
excavated using slurry trench technology through the
granular filters to bedrock. For the Bay-Goose Dike,
the low permeability element was comprised of a combination of a soil-bentonite and/or cement-soil-bentonite
cut-off wall, which was similarly excavated using slurry
trench technology through the granular filters to bedrock
or into a dense foundation soil. The maximum depth
excavated for cut-off wall construction was 15 m below
the lake level. In deeper portions, where the cut-off
wall did not reach bedrock, jet grout columns were
constructed between the base of the cut-off wall and
the bedrock surface, up to a depth of 22 m. Grouting
of the shallow bedrock and the interface between the
bedrock and the base of the cut-off wall and/or jet grout
columns was then carried out.
Typical cross sections of the Bay-Goose Dike are
shown in Figure 5, with the cut-off wall to bedrock and
with jet grouting. Material placement angles shown in
Figure 5 are approximate. Figure 6 shows the profile
view of the Bay-Goose Dike low permeability elements.
Based on bedrock depth, variations in the typical design
cross section were specified. These included: width of
rockfill platform, width of the central trench excavation,
material used for construction of the low permeability
element, and depth of grouting.
For the deeper portions of the Bay-Goose Dike, a
partial cut-off system was considered. However, once the
results from the sonic drilling program were obtained,
it was decided that due to the presence of potentially
erodible silts and uncertainty in the continuity of lenses
Figure 3: Bay-Goose Dike south portion boreholes
Canadian Dam Association • Winter 201515
Figure 5: Bay-Goose Dike typical sections
of soil with higher conductivity, that the low permeability
element would be extended to bedrock.
3 CONSTRUCTION
Construction occurred during the limited open water
season of each year. At Meadowbank, ice typically melts
from the surface of the lakes by mid-July and begins
to re-form by mid-October. The general sequence of
construction activities consisted of:
installation of silt curtains for turbidity control;
construction of the rockfill platform;
excavation of the central trench to bedrock or target
surface;
backfilling of the trench with granular material (fine
and coarse filters);
vibro-densification (deep areas);
raising the height of the rockfill platform;
dynamic compaction of fine filter;
excavation of the slurry trench; and
backfilling of the slurry trench with soil-bentonite
and/or cement-soil-bentonite.
Throughout the construction period, multiple activities were carried out concurrently along the length of
the dike. The following subsections provide details of
each of the construction activities.
Following the open water season, the following
construction activities occurred, and are described in
Part 2 (Bonin et al., 2013) and Part 3 (Esford and Julien,
2013)of this series of papers:
jet grouting of soil, where the cut-off wall was not
extended to bedrock (carried out during the winter
months);
grouting of bedrock and the interface with the base
of the cut-off wall (carried out during the winter
months); and
installation of instrumentation for dike performance
monitoring.
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to reduce the migration
of suspended particles
(turbidity) beyond the
active work area. Control
of turbidity was found to
be challenging and each
year modifications to
the method of turbidity
control were employed
to improve conditions for
the subsequent year of
construction.
During construction
of the East Dike, a single
row of silt curtains was
installed on each side of the proposed dike alignment,
approximately 25 m from the dike crest. The barriers were
found to be too close to the active work front, as construction activities were found to be affecting the volume
of water between the barriers which caused pressure, in
the form of wave action, to be applied to the curtains.
Based on the East Dike silt curtain performance, a
double line of silt curtains was installed during construction of the north portion of the Bay-Goose Dike.
Upstream from the dike, the inner curtain was installed
approximately 50 m from the dike and the outer curtain
approximately 100 m. Downstream from the dike, the
inner curtain was installed approximately 150 m from
the dike and the outer curtain approximately 200 m.
The intent of the outer curtain was to provide secondary containment in the event that turbidity migration
occurred through the inner curtain. The locations of the
inner and outer curtains were selected based on water
depths and information on currents and water movement
within Third Portage Lake. Significant improvement in
the control of turbidity was noted with the double curtain system. However, in early September, 2009 a large
storm with heavy winds lifted portions of the curtains
out of the water and strong waves and currents destroyed
some segments.
Prior to construction of the south portion of the
Bay-Goose Dike, turbidity control measures were again
re-evaluated. The benefit of the double curtain system
was recognized. However, to improve the security of the
inner curtain, protection from wind and currents was
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3.1 Turbidity Control Methods
Each year, prior to commencement of construction
activities, silt curtains were installed within the lakes
Figure 6: Bay-Goose Dike profile of the low permeability elements
Canadian Dam Association • Winter 201517
Figure 7: Bay-Goose Dike; aerial view of turbidity curtain configuration
(deployment in progress) and rockfill platform construction
required. Ideally, the inner curtain would be constructed
in smaller cells such that if one portion of the curtain
failed, the entire control system would not be compromised. To accomplish this, the upstream portion of the
rockfill platform was constructed during the winter by
breaking and removing ice, and dumping rockfill into
the lake at a slow rate. The completed upstream portion of the platform provided the required protection
and anchoring system for the inner cells of the turbidity
curtain. Upstream from the dike, the inner curtains were
installed approximately 20 m from the dike. The distance
between the outer curtain and the dike varied with a
minimum separation of 100 m. Downstream from the
dike, a single curtain was installed with the minimum
separation of 150 m. This sequence of construction and
method of turbidity curtain installation was successful in
maintaining adequate control. Figure 7 shows an aerial
view of this turbidity curtain configuration (in progress).
Additionally, to reduce the potential pressure applied
to the curtains due to water movement caused by the
construction, submersible pumps were used to remove
water from between the curtains during rockfill placement, central trench excavation, and filter placement.
3.2 Rockfill Platform
The broad rockfill platform of each dike provided
structural support for each dike and a working surface
for the remainder of the construction activities. The
platforms were constructed using rockfill sourced from
either a designated quarry (East Dike) or from open pit
development activities (Bay-Goose Dike). Rockfill was
transported to the leading edge of the platform using
mine haulage equipment, dumped on the surface and
then pushed into the water by dozers. Figure 7 shows
an aerial view of the upstream portion of the rockfill
platform that was constructed during the winter of 2010
and the partially completed expansion of the platform
during the summer of 2010. Figure 8 shows the expansion of the platform and excavation of the central trench.
3.3 Central Trench Excavation
Behind the advancing front of the rockfill platform
construction, the central trench was excavated. The
Figure 8: Bay-Goose Dike: rockfill platform construction and central trench excavation
excavation was conducted to remove lakebed soils down
to bedrock or target surface. In deep areas, where bedrock was located beyond the reach of the available equipment, the excavation was terminated on a target surface,
defined as competent (dense to compact) well graded
soil (i.e. till not silt). For the majority of the alignment,
the central trench was excavated to bedrock, thereby
removing lakebed soil, including boulders or potentially
erodible materials. The excavation was backfilled with
granular material, and thereby provided a downstream
filter and improved the ability to excavate the cut-off
trench. Excavation of the central trench from the single
rockfill platform was found to be faster and eased construction rather than if the excavation was done from
two parallel platforms.
The dimensions of the trench varied based on the
depth to bedrock. For the East Dike the base of the
trench was approximately 5 m wide, while for the BayGoose Dike it ranged between 8 and 12 m, with the
width increasing as the depth to bedrock increased.
As portions of the excavation were completed, a
detailed bathymetric survey of the excavated trench
was conducted with either sonar and/or dip tape and
rod sounding measurements. Verification of excavation depths using a dip tape and rod soundings were
important and also used to detect silt accumulation.
Cross-sections through the trench at 5 m intervals and
profiles along the centreline of the trench were created
using the survey data by the Contractor as part of the
foundation approval process. Foundation approval, by
the Engineer, for the base excavation depth, width,
and termination point (bedrock or target surface) was
required before backfilling of the trench with filter
material could commence. Figure 9 shows the central
trench and long reach excavator.
3.4 Filter Placement with Central Trench
Upon approval of the foundation, the trench was then
backfilled with crushed granular materials. Fine filter
was placed in the middle, coarse filter on the downstream side, and either coarse filter or fine rockfill on the
upstream side. The grain size distribution envelopes for
the fine filter and coarse filter are shown in Figure 10.
Canadian Dam Association • Winter 201519
Figure 9: Bay-Goose Dike central trench excavation with long reach excavator
Placement of the materials occurred simultaneously, with
the fine filter being advanced slightly ahead of the coarser
materials. An excavator was used to push the fine filter
material into the excavation from the water surface to
minimize the potential for segregation. The coarser materials on the upstream and downstream sides were either
placed by the excavator or dozer using the same method.
Figure 11 shows the placement of backfill within the central trench excavation, with the fine filter in the middle of
the excavation, fine rockfill on the right (upstream) and
coarse filter on the left (downstream). Samples of the fine
filter were collected above and below the waterline to
check gradations. Results indicated that gradations met
the specification and that significant segregation was not
apparent. Periodic measurements of the slope of the fine
filter and coarse filter were taken longitudinally along
the placement front as a general check on the geometry
of the backfill. They indicated that the zone of fine filter
placement was sufficient for excavation of the cutoff wall.
In some locations, based on the geometry of the section,
fine filter was placed on the downstream slope of the
excavation, so that this material was in contact with the
lakebed soils, to provide filter compatibility.
3.5 Vibro-Densification (Deeper Areas)
Along portions of the alignment where the depth to
bedrock was 8 to 10 m or greater below the initial platform elevation, or where the excavation did not extend to
bedrock, vibro-densification was required to compact the
fine filter materials sufficiently to permit excavation of the
slurry trench. Vibro-densification was used to compact
the lower portion of the fine filter material, beyond the
depth where dynamic compaction is effective to achieve
the required compaction. Vibro-densification was performed from the initial rockfill platform elevation, with
probe holes either side of the centerline, approximately
3 m centre to centre. Subsequently, dynamic compaction
was used to compact the upper portion of the fine filter
material and is described in Section 3.7.
3.6 Raising Height of the Initial
Rockfill Platform
Above the central trench, the elevation of the rockfill
platform was increased by 2 m to provide a working
Figure 10: Fine filter and coarse filter particle size distribution envelopes
Figure 11: Bay-Goose Dike backfilling of central trench excavation with fine rockfill,
fine filter, and coarse filter [from upstream (right) to downstream (left) of the
photograph]
surface for the dynamic compaction and slurry trench
excavation, for construction of the cut-off wall. The
raised platform consisted of zones of fine filter, coarse
filter, fine rockfill, and rockfill.
3.7 Dynamic Compaction of Fine Filter
Dynamic compaction was conducted to increase the
density of the fine filter material to permit the excavation of the cut-off wall. A crane was used to drop the
15 tonne weight from a height of 18 m for the dynamic
compaction. Figure 12 shows the craters created by
dropping of the weight from the crane, used for the
dynamic compaction. Multiple passes were made with
craters being measured and backfilled, prior to the next
round of densification. The pattern for each phase of
compaction varied based on conditions.
Canadian Dam Association • Winter 201521
3.8 Cut-off Wall Construction
(Excavation and Backfilling)
Following compaction of the fine filter material,
an approximately 1 m wide (i.e., width of excavator
bucket) cut-off wall was excavated using the slurry
trench method, along the designated alignment. A
5% bentonite slurry was pumped into the trench as
excavation occurred and the slurry level was continually maintained in order to provide wall support. The
working platform for the cut-off trench excavation
was 2 m above the lake elevation which provided a
positive hydraulic head between the trench and lake
for further wall support. The trench was excavated
to bedrock or target elevation. The cut-off wall was
excavated to bedrock along the entire East Dike
alignment and majority of the Bay-Goose alignment.
In deeper areas of the Bay-Goose Dike, where bedrock was located beyond the reach of the excavation
equipment, the cut-off trench was extended until competent soil was reached, approximately 1 m beyond
the depth where the central trench excavation was
terminated. Soundings of the base of the trench were
taken at 2 m intervals and compared to the design
depth and surveyed base elevation of the central
trench excavation. Figure 13 shows the excavation
of the slurry trench.
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12/02/13 5:29 PM
Figure 12: Bay-Goose Dike dynamic compaction of fine filter
Figure 13: Dike cut-off trench excavation using slurry trench method and dozer backfilling
trench with soil-bentonite
The requirement for construction was that the excavaTo prepare the cement-soil-bentonite mixture, a voltion and backfilling of the cut-off trench was to be comume of till with the dry bentonite was placed within a
pleted prior to the onset of freezing conditions, which
container, along with approximately 6% dry cement (by
occurs approximately during the first week of October.
weight of till), and the bentonite slurry. An excavator was
Soil-bentonite material was used to construct the cutthen used to mix the materials. Once a uniform mix was
off wall for the East Dike. For the Bay-Goose Dike, in
achieved, the density and slump were measured. Upon
shallower conditions (i.e., less than approximately 6 m
approval, the excavator would then remove the cementto bedrock) where the hydraulic gradient acting on the
soil-bentonite mix and place it at the crest of the leading
structure was low, soil-bentonite material was also used
front. At the beginning of each shift, measurements of
to construct the cut-off wall. Where the hydraulic gradithe slope of the leading front of the soil-bentonite or
ent was higher, in deeper water, the cut-off wall was
cement-soil-bentonite mixture within the trench were
constructed using cement-soil-bentonite to reduce the
recorded to track progress.
potential for erosion. During construction of the southFigure 6 shows the longitudinal profile of the cut-off
ern portion of the Bay-Goose Dike, plugs of cement-soilwall constructed for the Bay-Goose Dike, the depth of
bentonite were constructed in select locations along the
cut-off trench excavation, and backfill material placecut-off wall alignment. The plugs served to separate work
ment: soil-bentonite (SB), or cement-soil-bentonite
fronts for the cut-off wall construction and facilitated
(CSB) are depicted, along with zones where jet groutchanges in backfill material.
ing was conducted and the extend of bedrock groutPrior to construction, samples of till were collected
ing. Jet grouting and grouting works are described in
from site and mix design testing was performed by the
greater detail in Part 2 of this series of papers (Bonin
slurry trench contractor to determine relative ratios of
et al, 2013).
materials to be used. To prepare either the soil-bentonite
Permeability testing was carried out on representative
or cement-soil-bentonite mixtures for backfill, till was
samples of the soil-bentonite and cement-soil bentonite
farmed either from on-land borrow areas for the East
materials. Samples were collected during placement
Dike, or for the Bay-Goose Dike from the dewatered
in cylindrical molds or as a bulk sample. Testing was
lakebed, downstream of the East Dike. Oversized cobconducted in rigid wall or flexible wall permeameters.
bles and boulders were removed by the excavator and
A summary of the results from the quality assurance
approximately 1.5 to 2% dry bentonite powder (by weight
and quality control testing are presented in Table 1.
of till) was mixed into the till. Once
prepared, the till-bentonite mixture Table 1: Summary of permeability results from quality assurance (QA) and quality control (QC) testing
Structure
Material
QA/QC Min. Permeability Max. Permeability Avg. Permeability No. of
was then hauled to the crest of the
m/s
m/s
m/s
Samples
soil-bentonite
QA/QC 3.4x10-10
2.7x10-10
4.6x10-9
32
dike and stockpiled along one side East Dike
QA
1.4x10-10
2.5x10-8
4.4x10-9
8
of the cut-off trench alignment. To Bay-Goose Dike soil-bentonite
soil-bentonite
QC
1.1x10-10
5.9x10-10
2.4x10-10
11
prepare the soil-bentonite mixture, Bay-Goose Dike cement-soil-bentonite QA
9.8x10-10
2.5x10-8
8.6x10-9
6
cement-soil-bentonite QC
2.2x10-9
4.5x10-8
1.4x10-8
6
5% bentonite slurry was then added to
the dry till-bentonite, and the tracks
and blade of a dozer were used to mix the slurry and dry
3.9 Subsequent Construction Activities
mixture until a uniform semi-fluid mix was achieved.
Following completion of the cut-off wall construcThe slump and density of the mix was tested, and upon
tion, an additional 1 m high working platform was
approval, the excavator would gradually push the soilconstructed over the cut-off wall centreline from which
bentonite mixture into the trench at the crest of the
subsequent construction activities were carried out. In
leading front. Figure 13 shows mixing and placement
deeper portions, where the cut-off wall did not reach
of the soil-bentonite mix into the trench.
bedrock, jet grout columns were constructed between the
Canadian Dam Association • Winter 201523
Figure 14: Bay-Goose Dike following dewatering and
Goose Pit development (Spring 2012)
base of the cut-off wall and the bedrock surface. Grouting of the shallow
bedrock and the interface between
the bedrock and the base of the cutoff wall and/or jet grout columns was
also carried out. Further details of
the grouting and jet grouting works
are provided in Part 2 of this series
of papers (Bonin et al. 2013).
Instrumentation to monitor the
performance of the dikes was also
installed from this platform. Part 3
of this series of papers describes
the instrumentation and presents
results from the monitoring (Esford
and Julien, 2013). Figure 14 shows
the Bay-Goose Dike following
dewatering along with development
of the Goose Pit.
4 SUMMARY AND
LESSONS LEARNED
Dikes were successfully constructed over three open water seasons, on the shoulders of extreme
Arctic conditions (freezing temperatures over most of the year, ice,
high winds, frozen ground, blizzards). Isolation and limited access
necessitated a high level of logistical
support, planning and sequencing.
Challenges for the construction
included:
A limited construction season and
short period free of ice (mid-July
to mid-October);
Continuous permafrost (frozen
ground, active zone, ice-rich soils);
Thawing of the active zone – high
water content, potential for differential settlement and/or lateral
spreading; and
Limited availability of soil and
alluvial materials suitable for dike
construction.
The key lessons learnt during the
construction included:
Pre-construction information on
the depth to bedrock along the
longitudinal profile of the cut-off
wall aids in design, construction
planning and execution.
Geometry of the central trench
(base width and side slopes)
should minimize the occurrence
of rocks from side slopes falling
down on to the base, and in particular entering the cut-off wall
alignment.
During the final verification stage
for each segment of the central
trench excavation, the excavator
bucket should make multiple
passes across the surface and the
quality assurance/control team
should witness this activity (visual
movement and sound of bucket
moving) for approval.
Compare bathymetric survey data
recorded within central trench
excavation with estimated depth
to bedrock from investigation
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data and with manual soundings
(tape and rod) to verify depths
and determine if sediment/silt
has accumulated at the base of
the trench.
If silt is present, quantify the elevation of sediment and remove
as much as possible, prior to
backfilling.
Be cognisant of potential silt during densification and cut-off trench
excavation.
Be aware of areas of potential silt
accumulation and if removal is
not possible, adjust the grouting
program and treat the zone by jet
grouting.
Carry out cut-off wall construction using cement-soil-bentonite in
deeper portions to reduce erosion
potential.
Use plugs constructed of cementsoil-bentonite for transitions
between materials being utilized
for backfilling the cut-off wall or
for joining multiple work fronts.
Dynamic compaction is suitable for
densification up to 8 to 10 m below
lake elevation, where the central
trench was excavated to bedrock.
Vibro-densification is required to
densify fine filter materials beyond
depths where dynamic compaction is effective, or when the central trench was not excavated to
bedrock.
Track and cross-check elevations
of the bedrock and/or target surface through each stage of the
construction to avoid potential
windows of untreated material
within the low permeability element of the dike.
Construction pre-planning, and
sequencing of the works are essential to make use of the short construction season.
Identify potential contingency
measure where simplified dike
design components are to be
utilized.
Incorporate experience gained
through the construction process
(by owner, engineer, contractor)
and carry forward the adaptations in subsequent construction
seasons.
E
mploy very experienced (i.e.
senior) construction supervision
staff on site to make key design
decisions.
Document and rigorously checktrench, cut-off wall, bedrock and
platform elevations throughout
works for comparison and verification purposes.
I nstall instrumentation (thermistors and piezometers) for
real-time monitoring during
dewatering.
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The effectiveness and high efficiency of Smith-Root
electric barriers successfully prevented fish from
entering the tailrace of the hydroelectric facility.
GREN Biologie Appliquée Sàrl
REFERENCES
Canadian Dam Association, 2007. Dam Safety
Guidelines.
Bonin, G., Rombough, V. and Julien, M., 2013.
“Part 2: Grouting Techniques Employed for Dike
Construction at the Meadowbank Gold Mine,
Nunavut” CDA 2013 Montreal Conference.
Esford, F. and Julien, M., 2013. “Part 3: Performance
Monitoring of Dikes During Dewatering and
Operation at the Meadowbank Gold Mine,
Nunavut” CDA 2013 Montreal Conference.
The authors would like to thank Agnico Eagle Mines
Limited for its support and leadership during the
design and construction of these dikes. The successful completion of these works would not have been
possible without collaboration between the owner,
the contractor and the designer.
VESSY HYDROELECTRIC
Arve River, Switzerland
WE PROVIDE EXPERTISE AND TECHNOLOGY FOR:
• Fish entrainment and impingement prevention
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Smith-Root has fifty years of
expertise in fish guidance solutions
WWW.SMITH-ROOT.COM
[email protected] • (360) 573-0202 • Vancouver, Washington, USA
Canadian Dam Association • Winter 201525
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