1 INTRODUCTION EPS geofoam is characterized by very low density (nearly 100 times lesser than soil) with potentially high compressibility, good flexural strength and high rupture strength in shear. The weakest plane in the failure of EPS geofoam under fill load was found to be at the interface between EPS geofoam blocks and between EPS geofoam block and interfacing material in the contact. For any construction material, basic engineering properties of EPS geofoam are required to perform analysis and design. The friction factor or interface friction coefficient is the ratio of the resisting shear stress to the applied normal stress which is an important parameter for assessing the stability of the structure containing EPS geofoam. Interface friction coefficient can be derived from the results obtained from the laboratory testing. The evaluation of the interface strength parameter should consider different materials and field applications. The test should be conducted at representative normal stresses. Wagner (1986) investigated interface strength behavior of geofoam of density 22 kg/m 3 by using two test methods tilt test and direct shear test. He reported the interface friction coefficient from tilt test whereas the peak and residual interface friction coefficient from direct shear. Kuroda et al. (1996) conducted shaking table test to calculate interface friction coefficient value of geofoam to geofoam of density 20 kg/m 3 under both static and dynamic loading conditions. They investigated interface strength using geofoam of density 20 kg/m 3 . Miki (1996) conducted direct shear test to determine interface friction coefficient between geofoam to geofoam and geofoam to sand using density of geofoam 20 kg/m 3 . The investigation of geofoam to sand show decrease in the value of interface friction coefficient due to increase in the thickness of sand layer below geofoam. Negussey (1997) reported that the interface friction between geofoam and sand is comparable to the angle of internal friction of sand alone. Sheeley and Negussey (2004) investigated interface strength behavior of geofoam by conducting large scale direct shear test by using density of geofoam 18 and 29 kg/m 3 . The investigation consist of interface strength between geofoam to geofoam in both dry and wet condition, geofoam to cast in situ concrete and geofoam to geomembrane (both HDPE and PVC). They have reported geofoam to geofoam interface strength was found to be equal in both dry and wet condition. They reported Direct Shear Test on Expanded Polystyrene (EPS) Geofoam A. H. Padade & J. N. Mandal Indian Institute of Technology Bombay, Powai Mumbai – 400 076, India ABSTRACT: Expanded Polystyrene (EPS) geofoam is an interesting area in the field of geotechnical engineering. In the present study an attempt has been made to understand the behavior of shear strength parameters of EPS geofoam through direct shear tests. Direct shear test has been carried out on EPS geofoam of four different densities i.e. 0.15, 0.2 0.22 and 0.30 kN/m 3 under four different normal stress values of 15, 30, 45 and 60 kN/m 2 . The dimensions of the specimen used in the investigation are having length 100 mm, width 100 mm and height 50 mm. The test results show that with the increase in density of EPS geofoam, cohesion value increases whereas marginal increase in angle of internal friction of material is observed. Shear factor ‘z’ has introduced to know the effect of normal stresses and relationship of cohesion and density of EPS geofoam has also developed.