Experimental study on development model of excess pore pressure for saturated coral sand based on shear strain characteristics

A series of undrained cyclic torsional shear tests are carried out on the saturated coral sand specimens from Nansha Islands by using the hollow cylinder torsion shear apparatus. The aim of the tests is to investigate the effects of fine contents(FC), relative density (D_r) and applied cyclic stress ratio CSR on the generated excess pore pressure ratio R_u of the specimens. The test results show that: the development rate of R_u-N curve increases with the increasing FC and CSR, but decreases with the increasing D_r. In addition, the development rate of R_u-N curve of the saturated coral sands significantly exceeds that of the terrestrial quartz sands, and the traditional Seed pore pressure model is not fully applicable to characterize the development pattern of the excess pore pressure of coral sands. For the same D_r and FC, R_u of the specimens under different CSR is uniquely related to the shear strain amplitude (γ_a). A pore pressure evaluation model based on the shear strain characteristics is established, and the analysis shows that the pore pressure model parameter A is a soil-specific constant, and the density-corrected pore pressure model parameter B/(D_r)^1.5 has a single negative power function relationship with the equivalent skeleton void ratio e_sk^*.