Direct Shear Testing of Sand – Geotextile Interfaces

The effect of shear box size, geotextile type and properties and sand grain shape and size on the sand – geotextile interaction was investigated experimentally by conducting interface tests with conventional and large-scale direct shear equipment. Four clean uniform sands, one with subangular grains and three with rounded grains of different sizes were tested in dry and dense condition. Seven non-woven polypropylene geotextiles of various types and properties and seven woven geotextiles with or without apertures were used in the tests. The large-scale direct shear tests were conducted according to ASTM Standard D5321, using a 300 mm square shear box. The tests with the conventional 100 mm square shear box were performed using the same normal stresses and comparable shearing rate with the large-scale tests. The interface friction coefficient values obtained from the tests with the conventional and the large-scale shear box are in good agreement. Therefore, the 100 mm shear box is satisfactory for testing materials like those used in the present investigation. The values of friction coefficient, tanδ, at the sand – geotextile interface are affected by the geotextile type and range from 71% to 104% with respect to the internal friction coefficients, tanφ, of sands. Although the interface friction coefficient values are larger in the sand with subangular grains, the efficiency (tanδ/tanφ ratio) values for the sand with subangular grains are lower in comparison with the sand with rounded grains of equal size. For interfaces between sands of different grain sizes and geotextiles without apertures, the obtained efficiency values generally increase with decreasing sand grain size.