Liquefaction Characteristics Of Calcareous Sand In Coral Reefs And A Simple Evaluation Method

The fill material of the reclamation project in the South China Sea area is sediment from the sea floor,and the main component is coral reef calcareous sand.Calcareous sand is a special geotechnical material formed by in-situ or near-source deposits of skeletons of reef-building corals,coral algae and other marine organisms.It has the characteristics of irregular shape and easy to break,rich internal pores and strong compressibility,which make the liquefaction characteristics and strain behavior different from that of ordinary terrestrial silica sand under the same stress condition.In this paper,undrained cyclic torsional shear tests and cyclic triaxial tests were carried out on calcareous sand taken from Xisha Islands in the South China Sea.The development law of stress-strain curves and the characteristics of anti-liquefaction ability of calcareous sand were studied,and the inherent mechanism of pore water pressure changes before and after liquefaction was expounded.In addition,a series of comparative physical and mechanical tests were carried out on the coral reef calcareous sand samples and terrestrial silica sand of the identical grade,and the differences in shear stress-strain behavior,liquefaction resistance and penetration resistance between calcareous and silica sands were discussed.The main research contents are as follows:1.Taking the coral reef calcareous sand from the Xisha Islands of the South China Sea as the test material,a series of consolidated undrained cyclic torsional shear tests at a relative density of D_r=70%were carried out by using a hollow cylindrical cyclic torsional shear apparatus.The calcareous sand stress-strain behavior and the rule of variation of excess pore water pressure were studied and the inherent mechanism of the material changing from consolidation to liquefaction was revealed.At the same time,in order to study the influence of relative density on liquefaction,consolidated undrained shear tests under different relative densities were carried out by using a cyclic triaxial tester.Combined with the results of undrained cyclic torsional shear tests,the relationship between anti-liquefaction capacity and cyclic stress ratio was analyzed to reveal the influence rule of relative density on liquefaction capacity.With the decrease of the cyclic stress ratio,the cycle times of calcareous sand to achieve liquefaction continue to increase and the rate of increase is getting faster and faster.The anti-liquefaction ability is positively correlated with the relative density because the number of contact points between particles and the contact force can improve the anti-liquefaction ability under high density.2.Based on the hollow cylindrical torsional shear apparatus,the undrained unidirectional torsional shear tests,drainage cycle torsional shear tests and undrained cycle torsional shear tests under consolidation conditions were carried out,and the consolidation undrained cycle triaxiality was carried out based on the dynamic triaxial apparatus.In this experiment,the differences in shear stress-strain behavior and resistance to liquefaction of calcareous and silica sands samples with the identical grade were compared from multiple angles.It was found that the calcareous sand has higher shear strength,internal friction angle and liquefaction resistance than silica sand at the same relative density.The main reason for this difference was that calcareous sand had irregular shape and multi-angular features,which required higher resistance to be overcome when the particles moved with each other.3.In this paper,the comparative light dynamic penetration tests of calcareous and silica sands at four relative densities are designed.The test results show that the penetration resistance of calcareous sand foundation is higher than that of silica sand under the same relative density,and the penetration resistance increases with the increase of relative density.According to the specification recommended NCEER liquefaction criterion,the relationship between the cyclic resistance ratios and light penetration hammer number N₁₀ under the two sands were determined by comparison with the results of the light dynamic penetration test and the cyclic triaxial tests at different relative densities.Meanwhile,a simple liquefaction evaluation method for calcareous sand foundation is proposed...