Experimental studies on effects of initial major stress direction angles on liquefaction characteristics of saturated coral sand

In order to investigate the liquefaction characteristics of saturated coral sand from Nansha Islands under different initial major stress direction angles (α₀), a series of undrained cyclic torsional shear tests for saturated coral sand are carried out by using the GDS hollow cylinder torsional apparatus. A remarkable finding of this study is that, α₀ has significant influence on the liquefaction characteristics of coral sand, the excess pore water pressure (u_e) for case α₀ ≠ 45° cannot reach the initial mean principal stress ${p^{\prime }}_0$, but the u_e for case α₀ = 45° can eventually reach do. The developments of strain components for various α₀ cases are different, but the specimens for all α₀ cases will produce larger general shear strain amplitude (γ_ga), and the correlation between the u_e to ${p^{\prime }}_0$ and γ_ga can be expressed approximately as tangent function. Based on the experimental data, the number of cycles (N_L) required to induce initial liquefaction decreases with the increasing α₀ for a given cyclic stress ratio (CSR). However, by defining the modified cyclic stress ratio (CSR_M) as a new index for cyclic stress level, a virtually unique relationship between CSR_M and N_L can be established for all α₀ cases. The applicability of CSR_M is verified by the original test data of saturated sands from the literatures.