| With the proposal of the Belt and Road cooperation initiative and the marine economic powerful nation strategy,especially the 21 st Century Maritime Silk Road,ocean development has entered a new stage.The construction of marine projects along the South China Sea and the Belt and Road is in full swing.At the same time,the South China Sea and many other areas along the Belt and Road are located at the junctions of the Pacific plate,the Indian Ocean plate and the Eurasian plate.Affected by complex geological tectonic movements,earthquakes occur in this area from time to time.With the rapid progress of engineering construction,the seismic safety of the South China Sea and other areas along the Belt and Road is particularly important.In this paper,through a combination of high-precision industrial CT scanning test,shaking table model test and numerical simulation,the interaction mechanism of coral sand-foundation-superstructure under earthquake was studied.And the intensity of earthquake,relative density and sand type were discussed.The difference and similarity of the liquefaction characteristics and dynamic response of coral sand sites and Fujian sand sites were compared and analyzed.The results of the study provide a certain reference for the seismic design of coral sand sites.The main achievements of this paper are as follows:(1)Three-dimensional non-destructive scanning and reconstruction experiments of samples based on high-precision industrial CT technology were carried out.Through the sampling of the soil foundation before and after the shaking table test,a sample that could reflect the actual particle arrangement during the test was obtained.The reconstructed sample model was verified based on the porosity and particle gradation distribution curve measured in laboratory experiments,and the particle and pore distribution characteristics of the samples at different earthquake intensities and different sampling locations were analyzed.The study found that the coordination number of coral sand particles is greater than that of Fujian sand,which is about 1.14 times that of Fujian sand.After the input of the seismic excitation,the coordination number of coral sand and Fujian sand particles both increase.And the increase in the coordination number of the two kinds of sand under the 0.2 g earthquake intensity is greater than that at 0.1 g.The particle coordination number of coral sand at the shallower depth is smaller than that at the deeper depth after the liquefaction.The pore diameter distribution of coral sand is more uneven than that of Fujian sand.Compared with the initial state,the coral sand pore diameter unevenness coefficient decreases by 3.46% under the 0.2 g input acceleration,Fujian sand decreases by 6.94%,and the coral sand pore diameter unevenness coefficient decreases less than Fujian sand.(2)Based on the design method of similarity ratio,the shaking table model tests of the three-layer frame structure on the coral sand with different densities under earthquakes were designed and carried out.Combined with the changes in the coordination number,pore diameter distribution and pore distance in the microscopic CT test,the pore pressure,acceleration,seismic subsidence of sand and the acceleration,bending moment,horizontal and vertical displacement of the model structure of the two sand sites were studied.It was found that the excess pore pressure ratio of coral sand site is about 80%?100% of that of Fujian sand site.The acceleration response of coral sand site is greater than that of Fujian sand site when the ratio of excess pore pressure exceeds1.0,and this difference gradually weakens with the increase of the depth.The final settlement of the coral sand site under the excitation of 0.2 g sine waves is smaller than that of the Fujian sand site,which is about 52%-65% of the Fujian sand site.At 2 s,the superstructure settlement of the coral sand is smaller than that of the Fujian sand,which corresponds to that the increase in the coordination number of coral sand particles is smaller than that of Fujian sand.(3)Three coral sand sites with different groundwater levels were prepared through the self-made water level measuring tubes.The shaking table model tests of soilfoundation-superstructure interaction under different groundwater levels were carried out.The seismic response of soil and superstructure were tested and tested.The study found that the excess pore pressure ratio of the soil gradually increases with the increase of the groundwater level.The increase of the groundwater level has a reduced effect on the acceleration transmission of the soil,and the settlement and horizontal displacement of the superstructure increase with the increase of the groundwater level.The strain of the superstructure column gradually decreases as the groundwater level rises.This corresponds to the acceleration response of the superstructure at different groundwater levels.(4)Based on the finite difference software FLAC3 D,a numerical analysis model of the dynamic response of the coral sand foundation-shallow foundation-superstructure system under the earthquakes is established.The reliability of the numerical model is verified through the results of the shaking table model test.On this basis,the dynamic response characteristics of the coral sand foundation and superstructure are analyzed.The hysteretic damping algorithm is used to characterize the changes in the shear modulus and damping ratio of coral sand with cyclic shear strain,which can better describe the energy absorption characteristics of coral sand under earthquakes.The numerical model can reflect the dynamic response characteristics of the coral sand foundation-shallow foundation-superstructure system under earthquakes.Compared with the static state,in the vertical direction,the influence range of the superstructure on the vertical stress of the foundation under the earthquake is significantly increased.In the horizontal direction,when the horizontal distance from the superstructure is 1.5 times the width of the raft foundation,the influence of the superstructure on the vertical stress of the soil foundation is almost negligible.(5)Shaking table tests of soil-pile-superstructure interaction under earthquakes were carried out.The seismic response comparison of coral sand and Fujian sand site,four-pile and nine-pile foundation,and 0.1 g and 0.2 g earthquake intensities were set up to discuss the the influence of sand type,number of piles and earthquake intensity on the seismic response of the soil-pile-superstructure dynamic interaction.It was concluded that the peak value of excess pore pressure ratio of coral sand between piles and beside piles is about 0.24 ? 0.99 times that of Fujian sand.The horizontal displacement,column and pile bending moments in the coral sand site are smaller than those in the Fujian sand site.The increase in the number of piles will promote the development of the excess pore pressure ratio of the coral sand site and increase the bending moment response of the superstructure columns.The development rate of the excess pore pressure ratio of the coral sand of the nine-pile foundation under the peak acceleration of 0.2 g is greater than that of the shallow foundation,and the acceleration amplification factor of the nine-pile foundation is smaller than that of the shallow foundation.The final settlement and horizontal displacement of the superstructure of the shallow foundation are greater than that of the nine-pile foundation.(6)Based on the shaking table tests,a finite difference program for simulating the interaction of the coral sand-pile-superstructure was developed using FLAC3 D.The attenuation of the shear modulus of the coral sand was considered during the earthquake.Based on the validated FLAC3 D numerical model,the effects of pile lengths,pile diameters,coral sand permeability,relative density and earthquake intensities on the coral sand-foundation-superstructure seismic response system were analyzed.The results show that the pile length and pile diameter have a great influence on the seismic response of the coral sand site.The final settlement of the superstructure decreases with the increase of pile length and pile diameter.When the relative density increases from 32% to 82%,the final settlement of the superstructure decreases from 12.71 mm to 6.36 mm,with a decrease of 50.0%.As the intensity of the earthquake is greater,the peak excess pore pressure ratio is greater,resulting in more severe seismic damage to the superstructure. |
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