Observations of the soil particle movement during direct shear tests on soil-geosynthetic interfaces

The shear strength between soil-geosynthetic interface has been well studied by conducting large scale direct shear tests. However, the documents of the development of shear band and the interaction of soil particles and geosynthetics under direct shear mode are still limited. Consequently, a series direct shear testing program is conducted aimed at providing detail image records of the soil particles against geosynthetics during direct shear tests. The direct shear tests involved several soils and several geosynthetics (geogrid, geotextile, and geomembrane). The movement of soil particles in the shear box is observed through the transparent glass on one side of the direct shear box. During shear test, high resolution photo images of soil particles are taken. The results show that the soil particles between geogrid apertures interlock quite well. Therefore the shear band can extend from the shear surface to adjacent soil particles. The upward movement of soil particles is also consistent with the dilatant behavior recorded during shear test. However, the shear band width for soil-geotextile interface and soil-geomembrane interface is much smaller than the width for pure soil under direct shear mode. The shear behavior is simply the soil particles slide on the geosynthetics. Therefore the shear band is limited to the soil particles lying on planar geosynthetics. The test results support the general finding from the direct shear test results that the shear strength between soil and geogrid interface is similar to the shear strength of pure soil while the shear strength between soil and geotextile and geomembrane interface is smaller than the shear strength of pure soil.