Study On Dynamic Properties Of Calcareous Sand In Dredger Fill Site Of Islands And Reefs In South China Sea

Calcareous sand is a special kind of biogenic marine soil with a calcium carbonate content of over 50%,it is highly porous,has a high grain angle,low strength,and is easily crushed.Calcareous sand has been used extensively as a filling material in the construction of the South China Sea's islands and reefs.The calcareous soil fill site has the risk of liquefaction and uneven settlement under an extreme load of earthquakes and waves,which seriously threatens the safety of the reef project.At present,the liquefaction of calcareous sand is quite controversial,moreover,there are few studies on the volumetric deformation of calcareous sand after vibrations.Therefore,it is necessary to study the liquefaction properties and volumetric properties of saturated calcareous sand,providing empirical support for the construction of the South China Sea's islands and reefs.In this paper,a series of cyclic triaxial tests were carried out on calcareous sand in a dredge fill site of islands and reefs in the South China Sea.In the experiment,the effects of relative density,effective confining pressure,and cyclic stress ratio on liquefaction properties and volumetric properties of saturated calcareous sand were considered.Based on this,the influential factors of cyclic strength of saturated calcareous sand were analyzed,and a pore pressure development model of calcareous sand was constructed.The calculation method of the volumetric strain of calcareous sand after vibration was established,and the relevant test results,obtained based on the cyclic triaxial test,were compared and verified.The main research work and conclusions are as follows:1.The test results show that with the increase of pore pressure,the cyclic stress of the calcareous sand sample decreases,which leads to an increase of measured cyclic vibration times,which may lead to an overestimate of the cyclic strength of calcareous sand.The calculation method used to obtain the cyclic vibration can be used to eliminate the influence.Under the condition of loose to medium density,the cyclic strength of calcareous sand increases with the increase of effective confining pressure,but in the dense state,the cyclic strength of calcareous sand decreases with the increase of effective confining pressure.Due to the presence of fine and better gradation,the cyclic strength of calcareous sand in the dredge fill site of islands and reefs in the South China Sea is higher than that of other sand types.2.Based on the results of cyclic triaxial tests,pore water pressure development models of calcareous sand in a dredge fill site of islands and reefs in the South China Sea were constructed.(1)With the increase of relative density,the normalized curve shape of pore water pressure development of calcareous sand is gradually convex from concave to upward,and then the total pore pressure model of calcareous sand is provided.(2)The experimental results show that the curves of pore water pressure increment ratio and cyclic shear stress ratio of calcareous sand are approximately straight lines,and the slope of the straight line is related to the relative density of calcareous sand.Therefore,the incremental pore pressure model of calcareous sand can be obtained.(3)The experimental results show that the relationship between the cyclic pore pressure ratio and the cyclic strain ratio of calcareous sand is a hyperbolic curve,and a strain model suitable for describing the development of pore water pressure of calcareous sand is proposed.3.The experimental results show that the volumetric strain of calcareous sand increases with the decrease of relative density,the increase of effective confining pressure and the cyclic stress ratio,and the effective confining pressure has a greater influence on the volumetric strain of calcareous sand.Also,the volumetric strain of calcareous sand after vibration is related to the pore pressure ratio and maximum shear strain.Based on this,the calculation method of volumetric deformation of calcareous sand in the dredge fill site of islands and reefs in the South China Sea after vibration is given.4.Two-layered shear model boxes for testing on a shake table were developed.The dynamic characteristics of calcareous sand and quartz sand were compared by shake table tests.The method of calculating the volumetric deformation of calcareous sand after vibration was verified.Under the condition of similar working conditions,calcareous sand particles have higher liquefaction resistance due to their mutual mosaic effect,but in loose conditions,the volumetric deformation of calcareous sand after vibration is higher than that of quartz sand.The results show that it is feasible to solve the volumetric deformation of calcareous sand in the dredge fill site of islands and reefs in the South China Sea by maximum shear strain.However,the key to calculating the sand settlement is to improve the accuracy of the maximum shear strain of soil calculation.