Geosynthetics February/March 2021 - 44

GSI NEWS

By George R. Koerner

Extreme direct shear testing

GEOSYNTHETIC
INSTITUTE
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- institute.org

George R. Koerner is the
director of the Geosynthetic
Institute in Folsom, Pa.

he Geosynthetic Institute (GSI) is doing direct shear testing under extreme environmental conditions (i.e., specifically extreme temperatures). At the prompting of two member
companies, we have initiated an effort in this regard. This work looks at response curves
over a wide range of normal pressures, 0-43.5 psi (0-300 kPa). The work involves both hot
(185°F [85°C]) and cold (near 32°F [0°C]) temperatures. As you can see by Figures 1a and
1b, a modification to our ASTM D5321 or ISO 12957 direct shear box was necessary. It was
retrofitted with insulation and a water-circulation system, which controls the environmental
temperature of the entire shear box. The 11.8
× 11.8-inch (300 × 300-mm) shear box is typically only run at a testing temperature of 70°F
± 3.6°F (21°C ± 2°C).
The experimental design of this work is
straightforward and is outlined in the following steps:
1. Choose the most significant interface, needlepunch nonwoven (NPNW) geotextile
versus smooth and textured high-density
polyethylene (HDPE) geomembrane.
FIGURE 1a Photograph of circulation system,
2. Establish the existing uncertainty with the immersion heater and thermocouple
interfaces at room temperature.
3. Test the interface hot (185°F [85°C]) at a
range of normal pressures.
4. Test the interface cold (34°F [1°C]) at a
range of normal pressures.
5. Establish if there is a significant reduction
in shear strength at these extreme temperatures by analyzing the data and observing
the dismantled failure surface carefully to
FIGURE 1b Environmental controller with
see if there are new mechanisms at play insulated direct shear box
under these extreme conditions.
Postulations as to the direct shear behavior are offered for extreme conditions of landfill
liners experiencing subsurface exothermic events or landfill covers experiencing sudden
freeze/thaw cycling. Giroud et al. (1995) and Koerner et al. (2019) shed some light on the
subject. However, the situation seems to need further research, particularly in light of climate
change and extreme weather events.
It should be clearly stated that the geomembranes used for this testing were from cast lines
utilizing patterned calendars and not from the nitrogen-expulsion technique of a blown-film
line. This choice was made because there is less variability of friction sheet produced from
cast lines. The geomembranes used for this testing were manufactured under strict quality
control with top-quality HDPE resin, carbon black, antioxidants and thermal stabilizers.
The smooth and textured HDPE used for this experiment are shown in Figures 2a and 2b,
respectively. The NPNW geotextile is a staple filament polypropylene, shown in Figure 2c,
which is commonly used as filter or puncture protection.

Geosynthetics February/March 2021

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