Laboratory Investigations on the Shear Strength of Geogrid Reinforced Soils

A new apparatus for direct shear and pull-out tests was conceived to study the soil-geosynthetic and the geosynthetic-geosynthetic interaction: it consists of a testing box, having a contact area of 0.1 m2, with vertical and horizontal loads applied by an electronically controlled two-axis testing machine. Different types of geogrids (both uniaxially and biaxially oriented) were tested. Two different types of soil were used: silty sand and gravel. Particularly,direct shear tests were run using different testing rates (varying from 0.1 to 5.0 mm/min). The second part of the research concerned triaxial tests, using a very large cell (0.82 m height and 0.35 m diameter) and the same gravel soil. These tests were performed with and without the geogrid reinforcements. The direct shear tests have demonstrated that the rate of displacement does not influence significantly the soil-geosynthetic interaction. On the other hand, the triaxial tests have emphasized that the geosynthetic inclusion give a not negligible cohesion to the whole mass and determine peculiar modes of failure.