Experimental study on anisotropy of saturated coral sand under complex stress conditions

Anisotropy is the inherent property of coral sand. A series of undrained monotonic shear tests are carried out on the saturated Nansha coral sand by using the GDS hollow cylinder torsional shear apparatus. The effects of consolidation stress direction angle α₀ and monotonic loading direction angle α_m on the undrained response of saturated coral sand are investigated. The test results show that α₀ and α_m have significant influences on the undrained response of saturated coral sand. The undrained response characteristics of coral sand will become more complex under the coupling effects of α₀ and α_m. For all the test conditions considered, the excess pore water pressure u_e of saturated coral sand presents contraction first and then dilatancy trend. The change of the shear resistance (S_PT) and its effective angle (${\phi '_{{\text{PT}}}}$) mobilized at the phase transformation state along with α_m are significantly different for various α₀. There is a unique linear correlation between S_PT and the dimensionless parameter β, which is a cosine function with α₀ and α_m as variables. With the increase of the generalized shear stress q_g, obvious strain hardening phenomenon can be observed for the samples.