Preliminary Study On The Practical Liquefaction Evaluation Method For Carbonate Sand Sites

Recent years,carbonate sand has been widely used for sea reclamation in the South Sea of China as main foundation material,a large number of carbonate sand sites have been formed.Due to its marine biological origin,carbonate sand is more irregular and fragile than silica sand.The liquefaction resistance of carbonate sand is increased by large friction and occlusion caused by irregular particle shape,which is obviously decreased by the particle breakage.Thus carbonate sand exhibits more complicated liquefaction characteristics than silica sand.The current liquefaction evaluation methods are mainly based on laboratory tests and seismic damage investigation of silica sand sites,it is an urgent problem to improve or modify the liquefaction evaluation methods based on silica sand sites for liquefaction risk analysis of carbonate sand sites in China.In this paper,monotonic triaxial shear tests,cyclic triaxial tests and light dynamic penetration tests are conducted to study the differences in physical properties,shear properties,dynamic behaviors,liquefaction properties and penetration resistance between carbonate sand and silica sand of the same gradation.The results of these tests are used for evaluating the applicability of the liquefaction evaluation methods for silica sand sites to carbonate sand sites.The main research contents are as follows:(1)Drained triaxial shear tests of carbonate and silica sands were carried out.Under monotonic shear condition,the peak deviatoric stress of both sands increases with increasing confining pressure,but the increases of carbonate sand is greater.Carbonate sand is found to have larger internal friction angle,but the value of which decreases with increasing confining pressure.The volume deformation follow the same rule of first shrinkage and then dilatancy for both sands,but the dilatancy of carbonate sand decreases significantly with increasing confining pressure.The irregular particle shape and particle breakage of carbonate sand conduct to the difference of shear properties between carbonate sand and silica sand.Besides,the results of isotropic consolidation tests and drain triaxial shear tests of carbonate sand indicate that the form of particle breakage is wearing of large particles and increasing of small particles,the degree of particle breakage increases with increasing confining pressure.(2)Undrained cyclic triaxial texts of carbonate and silica sands were carried out.Under cyclic shear condition,the development of axial strain and accumulation of pore pressure increase with increasing dynamic stress and decrease with increasing relative density for both sands,it was also found that under the same dynamic stress level,the number of cycles corresponding to initial liquefaction are significantly larger for carbonate sand.The irregular particle shape of carbonate sand conducts to the stability of granular structure,which indicates that carbonate sand has higher liquefaction resistance and deformation resistance than silica sand.(3)Light dynamic penetration tests of carbonate and silica sands were carried out.It was found that the penetration resistance increases with increasing penetration depth and relative density for both sands,and carbonate sand has higher penetration resistance than silica sand at the same relative density.Combining the results of liquefaction resistance tests and penetration resistance tests,it can be concluded preliminarily that the current liquefaction evaluation methods based on penetration tests of silica sand sites might be conservative for evaluating the liquefaction susceptibility of carbonate sand sites.