The interlocking effect of calcareous soil due to irregular particle shapes increases the shear strength markedly under shearing. To investigate the interlocking mechanism of calcareous soil

a series of triaxial tests was carried out for different particle sizes under consolidated undrained and consolidated drained conditions. Particle shape analysis was also conducted to reveal the formation mechanism and manifestation of occlusal force of calcareous soil. The test results show that: 1) irregular shape is the prerequisite for the occlusion of calcareous soil particles; 2) the difference in the content of particles for different shapes in calcareous soil with different particle sizes results in different values of occlusal force; 3)the occlusal force of calcareous soil is obviously affected by the stress level. Under low confining pressure

the occlusal action causes dilatancy and increases the internal friction angle. Conversely

under high confining pressure

the occlusal action causes particle breakage

raises cohesion

but reduces the internal friction angle. The occlusion of calcareous soil has obvious enhancement on the strength. Therefore

the occlusion of calcareous soil should be considered in engineering design work of coral reef.