Experimental Investigation On Shear Behavior Of Coral Reef Calcareous Sand In Triaxial Tests

With the development of the coral reef island in South China,calcareous sand becomes a filling material for various buildings and structures.Studies show that the shear behavior of calcareous sand is different from the quartz sand.Therefore,it is necessary to study the shear behavior of calcareous sand to provide technical support for the construction of these coral reef islands.The main contents and conclusions are as follows:(1)A series of un-drained triaxial tests are carried out on calcareous sand.Test results show that the effect of initial void ratio on the shear strength is more remarkable than confining pressures,and vice versus for the pore pressure.When the initial void ratio keeps constant,the initial tangent modulus increases with the increase of confining pressure and decrease of the relative density,while the peak friction angle decreases at the same time.The peak friction angle has a semi-logarithmic relationship with the average principal stress.The critical state friction angle is always smaller than the peak friction angle.For calcareous sand with a certain gradation,the critical state stress ratio is constant,regardless of the initial void ratio and confining pressure.(2)A series of drained triaxial tests are carried out on calcareous sand.Test results show that,calcareous sand first undergoes volume contract and then gradually develops into volume dilation.Compared with calcareous sand,Fujian standard sand has smaller shear shrinkage.At a certain relative density and confining pressure,the peak strength of calcareous sand is greater than that of Fujian standard sand.For calcareous sand,the greater the confining pressure is or the smaller the initial porosity ratio is,the greater the initial tangent modulus is;the initial tangent modulus under the drained condition is larger than that under the un-drained condition.The relative density has little effect on the friction angle in the critical state,and the effect of confining pressure is notable.The friction angle under various characteristic states can be expressed as a logarithmic function of confining pressure.(3)The dilatancy or softening of calcareous sand is analyzed.The peak dilatancy rate and the effective stress ratio have a linear relationship,while the peak dilatancy rate and the initial porosity ratio are in a logarithmic relationship.Compared with the peak state,the shear expansion rate in the critical state is less than the peak state.The brittleness index can be used to quantitatively describe the evolution of strain softening of calcareous sand.The plastic modulus of calcareous sand can also be described by the equation consisting of the critical state stress ratio,stress ratio and state function.During the shearing process,the plastic modulus first decreases with plastic deviatoric strain to 0,then increases,and finally tends to 0 in the critical state.The plastic modulus of the calcareous sand in the phase transformation state is logarithmic with the initial void ratio.