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.

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The paper deals with the behavior of a geomembrane installed in 1963 for the internal waterproofing of a concrete pipe in a hydroelectric project. The type of geomembrane used was polyvinyl chloride (PVC), directly glued to the concrete support. The PVC geomembrane was directly exposed to the water; the water temperature in the pipe is usually 4°C in winter and 9°C in summer. The maximum waterspeed is 3 m/s, and the maximum water pressure is 0.6 MPa. A first sampling of the PVC was carried out in 1973. A second sample was removed in 1987. The first sample was stored in a room for 14 years in controlled conditions. Thermal analysis on both geomembrane samples was performed in 1987 to assess the performance of the material over the 24-year period since installation. Thermal-gravimetric analysis (TGA), differential scanning calorimetry (DSC), thermomechanical analysis (TMA), and dynamic mechanical analysis (DMA) were performed. Only small differences between the two samples were recorded, by TGA and DSC. However, TMA showed an increase in the softening temperature from 27°C in 1973 to 34°C in 1987. In addition, DMA showed an increase of the dynamic loss modulus of about 24% at the installed geomembrane operating temperature (from 4 to 9°C).