Sand–geotextile interface shear strength by torsional ring shear tests

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The torsional ring shear apparatus was developed to evaluate the residual shear strength of soft soils. It has the advantage of allowing large displacement to take place while shearing. In using geotextiles for geotechnical engineering applications, the soil–geotextile interface shear strength is an important design parameter. Hence this paper describes the evaluation of the soil–geotextile interface shear strength using the torsional ring shear test. Dry medium sand and nonwoven polypropylene geotextile were used. The test program consisted of three series of tests for parametric study as well as the fourth series that compares the results from the direct shear and ring shear test. This test program was the first in a series aimed at studying the feasibility of measuring the soil–geotextile interface shear strength using the ring shear apparatus. The test results show that the peak friction angle and residual friction angle of the sand–geotextile interface is not significantly affected by the nominal mass of geotextile. The results also show that the sand–geotextile interface peak and residual friction angles are not greatly influenced by the rate of shear. However, the results show that the tests should not be conducted at a strain rate higher than 10 deg/min, beyond which erratic readings were observed. The results also show that the friction angle of the sand–geotextile interface decreases as the overburden pressure increases. Finally tests were conducted using the ring shear and direct shear operating at similar conditions. The results show that measurements of sand–geotextile interface friction angle measured by both equipments is almost identical at small shear displacement of less than 3 mm. Beyond the 3 mm small shear displacement, the direct shear test indicates higher friction angle than the ring shear test. The direct shear test is unable to measure interface shear strength at a displacements larger than about 15 mm, whereas the ring shear test can measure residual interface shear strength at displacement of more than 200 mm.