Dilatancy of the foundation filling material of island-reefs in the South China Sea

Calcareous gravelly sand (CGS) is the primary building material used in the foundation filling of island-reefs in the South China Sea. In this study, multiple consolidation-drained triaxial shear tests were conducted on CGS, in order to reveal the effects of relative density (Dr) and effective confining pressure (sigma '(3)) on the CGS dilatancy. The applicability of classical dilatancy equations to CGS was tested. An empirical dilatancy equation applicable to calcareous sand was proposed and the rationality of this dilatancy equation was validated. According to the results: (I) Phase-transition stress ratio (eta(p)) and failure dilatancy ratio (d(f)) are important indices for assessing the CGS dilatancy. Physical models were built to predict the eta(p) and d(f) of CGS based on its Dr and sigma '(3), respectively. (II) The dilatancy equation of the modified Cam-clay model could not reflect the CGS dilatancy. Rowe's dilatancy equation could reflect the CGS dilatancy to some extent, but it underestimated the CGS dilatancy when sigma '(3) = 50 kPa and overestimated the CGS dilatancy when sigma '(3) = 400 kPa. (III) A good linear relationship was identified between dilatancy ratio and stress ratio-related parameter ((eta(2)(f)-eta(2))eta(0.5)). Therefore, the empirical dilatancy equation built on this basis is highly applicable to calcareous sand.