A Research On Body Wave Characteristics Of Coral Sand Based On Bending Element

Coral sand is a common natural engineering material in the South China Sea.The research on the engineering characteristics of coral sand plays an important role in the construction of reclamation projects in the South China Sea.At present,the research on coral sand is mainly focused on the conventional mechanical test,and the research on its body wave characteristics is less.The longitudinal(P)wave and shear(s)wave of body wave play an important role in liquefaction identification,anisotropy evaluation,seismic response analysis and so on.In addition,Poisson’s ratio is an important parameter of soil numerical calculation,which can not be obtained directly by conventional tests.The physical and mechanical properties of coral sand are very different from those of terrigenous sand.It is inevitable that there will be large errors in selecting the empirical value of Poisson’s ratio of coral sand.The Poisson’s ratio of coral sand can be obtained by P and S wave characteristics.Therefore,it is of great engineering value to study the body wave characteristics of coral sand.In this paper,coral sand in the South China Sea area is taken as the test object.According to the actual grading range of coral sand reclamation project,four groups of coral sand with the maximum particle size of 10 mm are configured.By using the bending element wave velocity test system,the processing of the bending element excitation signal and receiving signal and the method of improving the test accuracy are explored.The wave characteristics of coral sand body under different test conditions such as confining pressure,relative density and water content were studied.The main work and achievements are as follows:(1)The optimization method of excitation and reception signal quality of coral sand bending element is explored,and the variation trend of excitation signal and reception signal under different confining pressure and frequency is analyzed.It is found that the P-wave receiving signal of bending element always keeps high quality,and the wave velocity result is not affected by the excitation frequency;while the shear wave signal is obviously interfered by the near-field effect,increasing the excitation frequency can weaken this interference,but it will not disappear,too high excitation frequency or too high confining pressure will lead to the deterioration of the received signal.(2)The first break identification method of bending element receiving signal is explored.The inversion method,cross-correlation method,zero crossing method and peak point method are used to identify,process and compare the coral sand shear wave bending element receiving signal.The results show that the first inversion method has the highest applicability.The increase of excitation frequency will increase the shear wave velocity to a certain extent,but the effect is not obvious when the frequency exceeds 10000 Hz.(3)The bending element test system is used to test coral sand under different test conditions,such as confining pressure,relative density and water content.The test results show that the wave velocity of coral sand always increases with the increase of confining pressure and relative density;the body wave velocity increases obviously with the increase of water content at the beginning,but decreases after reaching a certain degree;the bending element longitudinal wave velocity of saturated coral sand increases with the increase of confining pressure and relative density Two kinds of P-waves can be observed in the test.(4)According to the form of Hardin’s empirical formula,the experimental data of body wave velocity are fitted,and the empirical formula for calculating body wave velocity based on void ratio and confining pressure is given.In addition,the results show that the body wave velocity of coral sand is related to the coarse grain content.(5)The Poisson’s ratio of coral sand is calculated by using the obtained body wave velocity.Due to the existence of wave velocity measurement error,the Poisson’s ratio can not be obtained very accurately,but the value range can be determined.From the overall trend,it can be seen that the Poisson’s ratio of coral sand decreases with the increase of confining pressure and relative density,and the influence of water content on Poisson’s ratio is the same as that of body wave.