Interface Shear-Strength Properties of Roughened HDPE

Abstract

The author should be congratulated on an excellent practice-oriented paper-it is well pre­ pared and thought provoking. The author has performed direct shear tests to determine the shear resistance of soil in contact with smooth and roughened HDPE geomembranes. The author concludes that roughened geomembranes can improve shear resistance significantly for fine­ grained soil. But for coarse-grained soil, the author's laboratory tests show no difference in frictional resistance for the smooth and roughened geomembranes. Three comments follow. TRIAXIAL TESTS ON REMOLDED SOIL The author tested two different soils: the first was a silt of high plasticity (MH), and the second was a poorly graded sand (SP) having no fines. For both soils, the author performed consolidated undrained triaxial tests with pore-pressure measurements on remolded samples. The results were ' = 28° and c' = 44 kPa (920 psf) for the silt, and ' = 30° and c' = 36 kPa (750 psf) for the sand. These are very high values of effective cohesion. They are probably due to the high effective confining pressures of 110-413 kPa (2,300-8,600 psf) used by the author. The effective cohesion values of 44 kPa (920 psf) for the silt and 36 kPa (750 psf) for the sand are most likely extrapolated values from the triaxial tests performed at high effective stresses. Studies have shown that at low effective stress, the shear-strength envelope for com­ pacted soil is curved and passes through or slightly above the origin [see figures 3 and 4 in Maksimovic (1989) and figure 7 in Day (1992)]. If the soil engineer should use these high effective cohesion values during design, the stability may be significantly overestimated for conditions of low effective stress, such as during or shortly after construction. TRIAXIAL VERSUS DIRECT SHEAR RESULTS FOR SILT The author performed direct shear tests to determine the interface friction values of the silt and geomembrane. The author obtained interface friction values for roughened HDPE of ' = 29-32°, which is higher than the effective friction angle of the silt material itself (' = 28°). The author concluded that this increase in effective friction angle was probably due to sample variation for the silt material and a conservative shear strength value from the triaxial test. It is not unusual for direct shear tests to give higher effective friction angles than triaxial tests. For example, Dounias and Potts (1993) state that direct shear tests overestimate the peak shear strength by as much as 7.5%. This translates into an overestimation of the effective friction angle of up to 2°, which is close to the difference between the author's direct shear and triaxial tests.