Effect of shearing rate and cyclic load on shear strength of coral sand

Coral sand is characterized by non-uniform gradation, angular and elongated particle shapes, and a hollow, fragile structure. Although the shear strength behavior of coral sand has been investigated in previous studies, the effects of shear rate and repeated shear displacement on its shear behavior have not yet been fully clarified. This study investigates the shear behavior of coral sand using direct shear tests conducted under saturated conditions, in which the effects of shear rate and repeated displacement are evaluated independently. A total of 74 direct shear tests were performed on coral sand specimens prepared at two relative densities (Dr). To investigate the effect of shear rate, tests were carried out under three normal stresses (σ = 50, 100, and 200 kPa) and nine shear rates ranging from 0.01 to 5 mm/min. In a separate test series, quasi-static cyclic displacement-controlled direct shear tests were conducted on medium-dense and dense specimens to examine the influence of repeated shear displacement. In these tests, each cycle corresponds to one complete forward and backward shear displacement with a prescribed amplitude. Five displacement cycle levels (10, 20, 30, 40, and 45 cycles) with different displacement amplitudes were considered. The experimental results show that a characteristic shear rate (SRso), corresponding to the minimum measured shear force, can be identified for each relative density in the shear-rate-controlled tests. The variation of internal friction angle with shear rate generally follows the trend of shear force, whereas the apparent cohesion at Dr = 75% exhibits a different tendency. In the quasi-static cyclic displacement tests, medium-dense coral sand tends to exhibit a larger increase in peak shear force than dense coral sand. These observations provide experimental insight into the shear behavior of coral sand under loading conditions relevant to coastal and island engineering applications.