Large-scale direct shear test of the interface between coral sand and geogrid

Large-scale direct shear tests were conducted to investigate the shear behavior of the interface between coral sand and geogrid. Polypropylene biaxial geogrid was embedded in the coral sands with two grain size distributions, which were in-situ coral sand (ISG) and uniformly graded coral sand (UG) from the Paracel Islands in the South China Sea. The results revealed the strain-softening behavior of both coral sands. The peak shear strength of the ISG coral sand was higher than that of the UG coral sand since the relative density of the ISG coral sand was higher. A bilinear relationship of peak shear stress versus normal stress was observed, with a dividing point of 100 kPa normal stress. This is because the shear displacement of the coral sand transferred from shear dilatancy to shear contraction when the normal stress reached 100 kPa, which enhanced the cohesion. The irregular shape of coral sand particles and the strong interaction of the geogrid contribute to a higher interface shear coefficient of coral sand, compared with silicious sand. The relative particle breakage was found to increase as the growth of the normal stress, and breakage was more significant in the ISG coral sand. The particle breakage rate of the reinforced and unreinforced coral sand was very close to each other, indicating that the application of geogrid in coral sand has little effect on the particle breakage rate.