Study On Dynamic Response And Deformation Mechanism Of Coral Sand Under Earthquakes

As Xi Jinping put forward in the report of the 19 th National Congress of the Communist Party of China that “Persist in land and sea coordination and accelerate the building of a maritime power” and the development of the “One Belt and Road”maritime silk road cooperation concept,reclamation islands and engineering construction in countries along the “One Belt and Road” and the South China Sea are developing rapidly.The coral islands dotted in the South China Sea are one of the most important geological,geomorphic and ecological features of the South China Sea.However,due to the active earthquakes in the South China Sea,the seismic stability of coral islands is worthy of attention.The types of coral sites mainly include horizontal sites,inclined sites,and vertical revetment sites in harbor basins.To reveal the seismic performance of coral sand sites under earthquake action,the combination of shaking table model tests and numerical simulations is used to analyze the dynamic responses of three typical types of coral sand sites,which can provide theoretical support for the seismic resistance of the coral sites.The main research results obtained in this paper are as follows:(1)The dynamic response of the acceleration,dynamic pore pressure,and the laws of liquefaction and residual deformation are revealed through the shaking table comparison test between the horizontal coral sand site and the quartz sand site.The acceleration response of the horizontal coral sand site under the action of the earthquake is smaller than that of the quartz sand site.The excess pore pressure ratio of the horizontal coral sand site is smaller than that of the quartz sand site.The smaller the input peak acceleration,the greater the difference in excess pore pressure ratio.The micro-scanning test showed that compared with the quartz sand particles,the coral sand particles have more irregular shapes,stronger inter-particle occlusion,and the coral sand particles are more porous.The pore water pressure of the coral sand site rises slowly under the action of an earthquake,which is more difficult to liquefy than the quartz sand site.Due to the low liquefaction and high shear strength of the coral sand site,the acceleration response and residual settlement in the coral sand site under the same conditions are smaller than those in the quartz sand site.Under the input peak acceleration of 0.2 g vibration intensity,the residual settlement of the coral sand site is 51.5% smaller than that of the quartz sand site.(2)A shallow buried rectangular underground structure-horizontal coral sand site coupling shaking table test was carried out,revealing the dynamic response law and settlement characteristics of the site.With the increase of the groundwater level,the acceleration response of the site weakens,the acceleration difference between the structure and the soil increases,the excess pore pressure ratio of the site and the degree of site liquefaction increase,and the strain and bending moment of the underground structure decrease.As the groundwater level rises,the ground settlement increases.However,due to the floating force generated by the liquefaction of coral sand,the settlement of the underground structure decreases with the increase of the water level,and the uneven settlement of the ground becomes more obvious.The surface settlement is the largest at a horizontal distance of about 0.8 times the width from the side wall of the underground structure.When the peak acceleration is 0.2 g,the excess pore pressure ratio of the coral sand site is about 42.61% to 67.35% of that of the quartz sand site,and the settlement of the underground structure of the coral sand site is about 81.8% of that of the quartz sand site.(3)The dynamic response and lateral expansion mechanism of the site are revealed through shaking table tests on inclined coral sand sites with different inclination angles.As the slope angle increases,the acceleration amplification factor increases.Due to the difference between the initial confining pressure and initial shear stress of the site,the excess pore pressure ratio of the coral sand site decreases with the increase of the inclination angle.When the peak acceleration is 0.4 g,when the slope angle increases from 5° to 15°,the excess pore pressure ratio of the coral sand site decreases by 0.20%-7.55%.Under the vibration input intensity of 0.2 g,no obvious liquefaction occurred in the coral sand field,while under the vibration input intensity of 0.4 g,sand blasting occurred in the field,and the coral sand site had obvious lateral flow.When 0.4 g,the lateral flow distance and settlement of the inclined site are significantly greater than 0.2g.Although the excess pore pressure ratio of the inclined field is smaller than that of the horizontal field,it is prone to lateral deformation.Especially when the inclination angle is 15°,the flow distance of the coral sand field increases sharply.The lateral flow distance near the water level is significantly larger than other locations.(4)Based on the shaking table test,the horizontal lateral displacement and dynamic stability of the vertical revetment supporting coral sand site were studied.The results show that with the increase of the peak acceleration of the vibration input,the peak acceleration,structural strain,horizontal displacement,excess pore pressure ratio and horizontal dynamic earth pressure in the coral sand site increase,and the vibration acceleration difference between the structure and the surrounding soil is more obvious.The horizontal dynamic earth pressure of the revetment structure and the coral sand contact surface is distributed along the depth in a “Z” shape,which is consistent with the horizontal coral sand site dynamic response when the underground structure exists.The revetment structure is not obviously damaged in small earthquakes,and can produce a large slip distance in strong earthquakes,while the underground structure in the horizontal coral sand site mainly produces vertical displacement during the vibration process,and the horizontal displacement is small.When the vibration input intensity is0.4 g,the coral sand site is locally liquefied near the groundwater level,while the quartz sand site is almost completely liquefied.The horizontal displacement of the revetment structure of the coral sand site is smaller than that of the quartz sand site,which is about86.8% of that of the quartz sand site.(5)The influence of key parameters such as relative density,input acceleration peak value,permeability coefficient and initial groundwater level on the dynamic response and deformation of the coral sand site is further revealed through the finite difference software FLAC3 D.Due to the high relative density of the coral sand site,the peak site acceleration,excess pore pressure ratio and deformation during the vibration are all small.When the relative density of the soil increases from 55% to 85%,the final horizontal displacement of the inclined site decreases from 10.43 cm to 1.60 cm,with a decrease of 84.7% when the peak acceleration is 0.4 g.With the increase of peak acceleration,the site gradually develops from partial liquefaction to complete liquefaction,the degree of site deformation increases,and the uneven settlement of shallow underground structure-horizontal coral sand site is more obvious.As the permeability coefficient increases,the acceleration response of the coral sand site weakens,the degree of liquefaction decreases,and the site deformation decreases.The initial water level has a significant impact on site deformation characteristics,when the initial water level rises from 0.2H to 0.8H,the horizontal displacement of the revetment structure increases by 69.21%.