Experimental study on dynamic characteristics of calcareous sand-gravel mixtures from islands in the South China Sea

In the construction process of reef islands in the South China Sea, the upper reclamation foundation materials exist in the form of a combination of large calcareous gravels and small calcareous sand mixed in random proportions. Such composition state makes the foundation exhibit complex mechanical properties under dynamic loads such as earthquakes. A series of undrained cyclic triaxial tests under different conditions of gravel content, relative density, confining pressure and initial shear stress are carried out to study the dynamic response of calcareous sand-gravel mixtures. The test results show that the mixtures display a lower axial strain growth and pore pressure rise rate than the pure calcareous sand under cyclic loading, regardless of loose and dense states. It indicates that the calcareous sand-gravel mixtures exhibit higher liquefaction resistance than the calcareous sands. The liquefaction resistance of the calcareous sand-gravel mixtures increases significantly with the gravel content, relative density and initial shear stress. In dense state, the liquefaction resistance of mixtures decreases with increasing confining pressure, but not for the loose samples, which probably relates to the coupled effects of the confining pressure and density. The effects of the gravel content on the liquefaction resistance of the calcareous sand-gravel mixtures are controlled by the grain skeleton structure. The grain-scale structure is dominated by coarse particles (calcareous gravel) or small particles (calcareous sand). The binary media characterization is an important factor for the study on mechanical properties for the calcareous sand-gravel mixtures.