Soil-geotextile interface friction by direct shear tests

This paper describes large-size direct shear tests on soil-geotextile interfaces. Medium-grained, uniform sand and three varieties of woven and nonwoven geotextiles manufactured with different techniques are utilized to investigate the soil-geotextile interface friction coefficient (f*). Tests were carried out using an apparatus specifically designed for interface testing, and results were compared with those obtained from the conventional direct shear equipment. The results obtained from this study indicated that the determination of peak interface behaviour was not a trivial matter, as the results were significantly affected by the boundary and testing conditions of the testing apparatus. The residual interface behaviour was investigated by multiple reversal direct shear tests. Since the use of multiple reversal direct shear tests on the proposed apparatus can impose a high degree of shear displacement and stress uniformity on the soil-geotextile interface, a more reliable definition of the residual interface friction can be obtained. The results indicate that woven-nonwoven geotextile interfaces exhibit a significant postpeak strength loss after a number of shear cycles. In the case of woven geotextiles, this is attributed to the opening up of the filaments associated with the physical damage caused during shear, whereas for nonwoven geotextiles it is due to the pulling out or tearing of filaments.Key words: geotextile, direct shear test, interface friction coefficient, peak shear strength, residual shear strength.