钙质土因其颗粒形状不规则、易破碎、高孔隙比等特征，其力学性质较为特殊。采用室内大型直接剪切试验设备，对取自南海珊瑚礁和三亚岸礁的粗颗粒钙质土进行了直剪试验，研究了粗颗粒钙质土在不同含水率、不同密度和不同矿物组成条件下的钙质土剪切特性。结果表明，粗颗粒钙质土表现出与常规无黏性土截然不同的力学性质，即（1）与石英砂相比，表观黏聚力较大，内摩擦角较高，软化性较弱；（2）表观黏聚力随着平均粒径的增大而增大，内摩擦角随着干密度的增大而增大；（3）与峰值强度相比，土体剪切破坏后其残余强度的表观黏聚力锐减而内摩擦角仅略有减小。研究成果可为岛礁工程建设提供借鉴，也可为其他粗颗粒土的研究提供参考。

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.