Experimental Investigation of the Interaction Mechanism at the EPS Geofoam-Sand Interface by Direct Shear Testing

The results of laboratory direct shear tests conducted at the expanded polystyrene (EPS) geofoam-sand interface are presented in this paper. The sands used in the tests were composed of rounded to subangular particles with values of mean particle size ranging from 0.28 to 2.17 mm and void ratios ranging from 0.51 to 0.72. The EPS geofoam was of two different densities: 10 and 20 kg/m³. The test results are expressed in the form of shear stress versus normal stress in direct shear failure envelopes, which were found to be nonlinear. The curved interface failure envelopes were approximated by piecewise linear envelopes composed of three linear segments corresponding to three types of phenomenological interaction mechanism, which develop successively for increasing values of normal interface stress: purely frictional, frictional-adhesional, and purely adhesional. A simple conceptual framework is proposed that qualitatively explains the observed interface behavior in terms of the normal interface stress, EPS geofoam hardness (or density), interface relative roughness (which incorporates the mean particle size), shape of sand particles, and void ratio of sand. The experimental results are compared to similar results reported in the literature for HDPE geomembrane-sand interfaces. Apparent values of interface friction angle, δ, and adhesion, c_a, are proposed in the paper for the tested sands and for specific ranges of normal interface stresses and the two EPS geofoam densities.