Because of the angularity

crushability

and high void ratio

calcareous soil has peculiar mechanical properties. Large-scale direct shear laboratory tests are conducted on calcareous gravelly soil taken from coral reefs

South China Sea. This paper intends to study the shear characteristics of calcareous soil under the conditions of various gradations

water contents

densities and mineral compositions. The results show that the extremely different mechanical properties of calcareous gravelly soil compared to the common non-cohesive soil. (1) Calcareous gravelly soil has a greater cohesion

larger internal friction angle

and lower softening value than quartz sand. (2) The internal friction angle increases with increase of dry density

while its apparent cohesion increases with increase of the median particle size (D50). (3) After shear failure

the apparent cohesion decreases from the peak value significantly; while its internal friction angle decreases slightly. The experimental results can be provided for the engineering constructions on coral reefs.