Experimental Study On Dynamic Shear Modulus Ratio And Damping Ratio Of Calcareous Sand Under Strain Control

With the development of the South China Sea and the implementation and promotion of the construction of reclamation islands and reefs,the coral sand and calcareous sand covered by the South China Sea Islands and reefs have attracted extensive attention from the engineering and scientific circles in recent years.Because of its special sedimentary environment and mineral composition,irregular shape,easy to break,and easy to produce cementation among particles,the dynamic characteristics and seismic performance indexes of calcareous sand are significantly different from those of general continental and marine sediments.At present,the research of calcareous sand by scholars at home and abroad mainly focuses on physical and mechanical properties,strength characteristics,liquefaction characteristics and There are few researches on the engineering characteristics of foundation bearing capacity,and there are no empirical values for two important dynamic characteristics,dynamic shear modulus ratio and damping ratio.Therefore,there are potential risks and uncertainties for the engineering geological conditions and seismic safety of South China Sea Islands and reefs.In view of this,in this paper,we use the GDS cyclic vibration triaxial system to carry out a series of dynamic characteristic tests on calcareous sand in an island reef of the South China Sea under the condition of strain control,investigate and analyze the effects of relative density,effective confining pressure and dynamic shear modulus ratio and damping ratio of calcareous sand,and use three dynamic constitutive models commonly used in engineering to analyze the shear stress-strain curve and dynamic shear modulus ratio curve of calcareous sand The optimal fitting scheme is proposed,and the test values of dynamic shear modulus ratio and damping ratio under eight groups of typical shear strain conditions commonly used in engineering are given.The specific research contents and preliminary conclusions are as follows:(1)The influence of relative density on dynamic shear modulus ratio and damping ratio of calcareous sand is analyzed.The test results show that the relative density has little effect on the shear stress and strain curves;the test points with different relative density are concentrated in a narrow range of change,and the test points with dynamic shear modulus and shear strain are scattered when the strain is small;the test points are more compact with the increase of shear strain;the damping ratio increases with the increase of shear strain,and finally reaches stability.(2)The effect of effective confining pressure on dynamic shear modulus ratio and damping ratio of calcareous sand is analyzed.The test results show that the slope of shear stress and shear strain curve decreases with the increase of effective confining pressure;the dynamic shear modulus increases with the increase of confining pressure,but the dynamic shear modulus decreases with the increase of effective confining pressure;the damping ratio decreases with the increase of confining pressure;when the shear strain reaches a certain value,the damping ratio has a downward trend.(3)Three kinds of Hardin-Drnevich model,Matasovic model and Davidenkov model commonly used in engineering have some differences on the shear stress-strain curve of calcareous sand.According to the difference analysis based on the fitting R,Davidenkov model is better than Hardin-Drnevich model and Matasovic model in the stress-strain fitting of calcareous sand.For the dynamic shear modulus ratio of calcareous sand,Hardin-Drnevich model and Matasovic model are better than Davidenkov model.