Geosynthetics August/September 2022 - 20

Construction considerations for wick drain ground improvements
The foundation conditions have been
The construction
was comprised
of earthworks to
prepare a graded
foundation ground
surface, placement
of a geogrid, with
crushed granular
material, followed by
the installation of
the wick drains.
characterized based on geotechnical
investigations, including boreholes, cone
penetration tests (CPTs), bedrock probe
holes and electrical resistivity imaging
(ERI) geophysics survey. At selected locations,
sonic drilling was carried out to
obtain continuous cores to log the overburden
units, collect samples, determine
the thicknesses of the deposits, and check
for the existence of pre-sheared (slickensided)
layers. At other locations, mud
rotary drilling was carried out to collect
high-quality samples using a thin-wall
piston tube sampler and to carry out field
vane shear tests. An ERI geophysics survey
was carried out to profile the depth
to bedrock. The undrained shear strength
was measured using an electrical shear
vane. Index testing and a program of
advanced laboratory testing were carried
out to measure the foundation clay
deposits index parameters and the total
and effective stress shear strengths, and to
determine the consolidation parameters.
The advanced laboratory tests carTABLE
1 Geotechnical
design parameters
Material
Overburden Waste
Waste Rock
UGU, Silt and Clay
UGU, Silty Clay
Clay Till
LGU
Sand Till
Bedrock
Unit Weight
kN/m3
18
21.5
19.5
19.5
18.5
18
20
n/a
Friction Angle ˚
n/a
LS
22 (FS)
12 (RS)
12 (RS)
15 (RS)
36
Cohesion kPa
n/a
Su/σ´v
0.24(a)
n/a
0.24
0.22
0.22
0.22
n/a
Impenetrable
Notes: (a) Applicable to the area of 3 m of clay fill and rockfill interval layers; UGU = upper glaciolacustrine unit; LGU = lower glaciolacustrine unit; FS = Fully softened (strain
weakened) strength condition; RS = Residual strength condition; LS = Leps Strength Function (Leps, 1970); n/a = not applicable
20
Geosynthetics | August September 2022
ried out consist of consolidated isotropic
undrained (CIU) triaxial compression,
consolidated anisotropic
undrained (CAU) triaxial compression,
consolidated constant volume direct
simple shear (DSS), oedometer, constant
rate strain (CRS), direct shear and
ring shear tests.
Effective Strength Parameters
Fully Softened
Sumin kPa
40
n/a
40
30
30
30
n/a
Su/σ´v
n/a
n/a
0.24
0.22
0.22
0.22
n/a
The three cohesive soil units (i.e., the
upper and lower glaciolacustrine units
and the clay till) control foundation geotechnical
stability of the stockpile.
Wick drain design
Geotechnical design parameters were
selected based on a review of the results
from field investigation and laboratory
testing, and test fill observations. Selected
design parameters are shown in Table 1.
The overconsolidation ratio is generally
more than 10 at the ground surface and
reduces to 2 at 49 to 66 feet (15 to 20 m)
below the ground surface.
The clay layers normally consolidated
vertical coefficient of consolidation is 23
square feet/year (2 m2/year) and horizontal
coefficient of consolidation is 108
square feet/year (10 m2/year). A triangular
wick drain installation pattern was
used in the design. Both radial drainage
and natural (vertical) drainage were considered
in the analyses. The spacing of
wick drains was varied until the required
rate of consolidation for the duration of
stockpile construction was obtained. The
required rate of consolidation at year
six of the stockpile construction (end
of construction) was 97%, resulting in
7-feet (2-m) wick drains spacing. A test
fill was constructed to assess the performance
of wick drains (Etezad et al. 2019).
Undrained Shear Strength Parameters
Peak
Sumin kPa
n/a
n/a
65
60
60
60
n/a
Geosynthetics August/September 2022

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