Geosynthetics July/August 2024 - 14

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A
GMA TECHLINE
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George R. Koerner
GCL SHEAR STRENGTH CONCERNS
Q: We are worried about the midplane
shear strength of our GCL in a seismically
active zone of our landfill. The GCL is a
single component of a double composite
liner system. It is backing up the primary
geomembrane in the upper composite liner
system. Should we be concerned?
A: GCL shear strength is a primary design
consideration for facilities involving slopes
due to the low strength of hydrated bentonite.
One needs to account for both interface and
midplane failures in a comprehensive slope
stability design. However, I have never seen a
midplane needlepunch GCL in my career. For
this reason, the GRI GCL-3 specification value
is rather low at 2.1 lb./in (360 N/m). There
has been much research on this topic. The
following references may help you:
Fox, P.J., Nye, C.J., Morrison, T.C., Hunter, J.G., and Olsta,
J.T. 2006. " Large Dynamic Direct Shear Machine for
Geosynthetic Clay Liners. " Geotechnical Testing Journal,
29(5), 392-400.
Fox, P.J., and Stark, T.D. 2004. " State-of-the-Art
Report: GCL Shear Strength and Its Measurement. "
Geosynthetics International, 11(3), 141-175.
Koerner, R. M., Soong, T.Y. and Gontar, A. 1998.
" Selected Aspects of GCL Shear Strength Testing. "
Proc. GeoBento '98, BRGM Foundation, Paris, France,
pp. 97-110.
McCartney, J.S., Zornberg, J.G. and Swan, R.H. 2004.
" Effect of Specimen Conditioning on Geosynthetic Clay
Liner Shear Strength. " Proc. GeoAsia 2004, Seoul, Korea,
pp. 635-643.
Von Maubeuge, K. P. and H. Ehrenberg. 2000.
" Comparison of Peel Bond and Shear Tensile Test
14
Geosynthetics | Summer 2024
Methods for Needlepunched Geosynthetic Clay Liners. "
Geotextiles and Geomembranes, 18(2-4), 203-214.
Please note, I would be much more concerned
with an interface failure in your complex cross
section and the landfill filling sequence. Please
understand that I am not minimizing your
concern; it is real. However, there may be more
pressing items to worry about.
DIRECT SHEAR MACHINE TESTING
Q: We just bought a direct shear machine
and want to start ASTM D5321 testing. The
pneumatic piston applying the vertical loads for
normal pressure has a range from a low of 1,124
pounds to a high of 20,000 pounds. Can we use
this direct shear box for landfill cover testing?
A: Unfortunately, your low load of 1,124
pounds equals 7.8 psi, assuming your box
has the appropriate 1-square-foot area. This
load is far too great to capture the normal
pressure range for most landfill covers,
which is usually in the range of 0.5 to 3 psi.
Remember that the site-specific normal
pressure generally is the middle normal
pressure of the three-point test.
Since this box appears to be built for the
testing of a landfill liner system instead
of a cover system, you need to ensure the
capacity of the " shear " load cell is operating
in the 10%-90% load range. This is why most
labs doing accredited direct shear testing use
two boxes, with each one configured very
differently: one for light loads (cover system)
and one for heavy loads (liner systems).
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Geosynthetics July/August 2024

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