Abstract: Calcareous sands deposited on coral reefs are different from the quartz sands in their physical and mechanical properties. In this paper, we conducted ring shear tests with a single cycle on the calcareous sand sampled from the South China Sea. The effects of relative density and vertical stress were compared with the quartz sand under the same particle size and test conditions. The results indicate that shear stress and displacement curves of calcareous sand in forward ring shear tests tend to soften with a clear residual strength, while that tend to harden in reversed ring shear tests. The strength in forward and reversed ring shear tests are equal. The curves of the shear stress with displacement of quartz sand in forward and reversed ring shear tests tend to soften. The ratio of residual strength to peak strength of calcareous sand ranges from 0.75 to 0.93, while that of quartz sand ranges from 0.89 to 0.96. Residual strength of calcareous sand is higher than that of quartz sand with same particle size distribution and test conditions and the ratio ranges from 1.05 to 1.3. The particle of calcareous sand, in the size of 0.5 mm to 2 mm, occurs breakage under the vertical loading of 100 kPa and 200 kPa, and breakage percentages of particle are 4% and 6% respectively.

Key words: calcareous sand, ring shear tests, residual strength, particle breakage