Geosynthetics October/November 2021 - 10

RESIDUAL STRENGTH OF A GCL
Q: I was wondering if I could get some input
about the residual strength of a geosynthetic
clay liner (GCL). We have lab results where it is
" flatlining " for the residual case (Figures 1 and
2, and Tables 2-4). The GCL is proposed for
use in a landfill cell with ultimately a maximum
of 98 feet (30 m) of waste on top. We have
done global stability modeling, and the GCL
internal shear strength as presented fails our
specification for the residual case.
FIGURE 1 GCL midplane
shear stress vs. displacement
Regarding the mechanics of the test, it makes
sense to me that if it is gripped, squashed
and sheared aggressively, that it will pull the
needlepunching apart and result in a low
residual shear strength. However, I don't think
this is a likely failure mechanism within a
landfill, as the displacements are unlikely to
be that high on the cell floor (i.e., if the peak
strength is enough, the residual shouldn't be
a concern).
On review of GRI Report 30, you appear to be
able to measure a residual strength at high
confining stresses. I was wondering if there is a
different method used for this test?
FIGURE 2 MohrCoulomb
plot
Friction Angle (degrees):
Y-Intercept or Adhesion (kPa):
TABLE 2
Test Conditions
Upper Box: GCL, carrier layer facing down
Lower Box: Subbase compacted at 95% of 1,660 kg m3 @ 20.5% moisture content
Box Dimensions: 305 mm X 305 mm X 102 mm
Interface Conditioning: GCL soaked under 10 kPa for 48 hours, step loading applied for 24 hours prior to shear.
Test Condition: Wet
Shearing Rate: 0.1 mm/minute
TABLE 3
Test Data
Specimen No.
Bearing Slide Resistance (kPa)
Normal Stress (kPa)
Corrected Peak Shear Stress (kPa)
Corrected Large Displacement Shear Stress (kPa)
Peak Secant Angle (degrees)
Large Displacement Secant Angle (degrees)
TABLE 4
10
Geosynthetics | October November 2021
1
100
50
31
26.4
17.1
2
300
131
50
23.5
9.4
3
5
500
171
31
18.8
3.6
Test Results
Peak
16.8
26
Would appreciate any thoughts you have on
the matter.
Large Displacement (@75 mm)
0.1
37
Internal shear of the GCL occurred at 100, 300 and 500 kPa
A: The third test looks strange at NP=72.5 psi
(500 kPa). It has a hiccup in the beginning of
the test and then a post-peak dramatic drop to
residual. Check with the lab to see if all three
modes of failure are the same and explainable.
Gripping, clamping and friction plate
compatibility are important at high normal
pressures. The lab might have photographs of
the failure surface after the test.
In short, I think you are comparing apples
to oranges if modes of failure are different.
Boundary conditions and box configuration
should be changed to reconcile this situation.
Clamping on the front of the box, gripping
along the sides of the box and a high friction
bottom surface with a free back (trailing end) is
typical for high normal pressure situations. The
Geosynthetic Institute (GSI) does have a guide,
GRI-GCL-4, which gives recommendations
for securing GCLs for midplane shear. We also
recommend looking for skid marks posttest
and not a wide-width tensile test of the
geosynthetic being tested. G
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Geosynthetics October/November 2021

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