| The southwest of China is located in the Himalayan seismic belt,where the topographic structure is mainly plateaus and mountains.In addition,the complex groundwater systems and a large number of underground karst caves bring a lot of difficulties to the engineering projects there.Especially in recent years,the high-speed railway have been developing rapidly in China,the pile foundation for bridge is likely to be built above the cave,which puts forward a higher requirement on the stability of the pile foundation.Therefore,it is necessary to study the minimum safe thickness and stability of the roof under the influence of earthquake.Based on the existing researches,this paper investigates the seismic stability of the pile foundation in karst area by the centrosymmetric shaking table model test.(1)This paper introduces the current detection and treatment modes of cave at first.The Zhoujiawan Bridge of Chongqing-Guizhou railway is chosen as an prototype for the model test,and the geometric similarity ratio,density similarity ratio and acceleration similarity ratio are assumed to be 1/100,1/1.2 and 1/1 respectively.According to the similarity theory and the separation similarity principle,the similar constants of the main parameters and the mechanical parameters of the materials can be determined.Finally,designing orthogonal experiment of mixture ratio to develop the geotechnical material,which meets the experimental requirements,is presented in detail.(2)The static load test is carried out in two kinds of models.In the first model,the roof thickness is 2 times of the pile diameter,while the cave diameter varies from 2 to 5 times of the pile diameter.In the second model,the size of the cave is 3 times of the pile diameter,the thickness of the roof varies from 1 to 4 times of the pile diameter.By doing these model tests,the Q-S curve and the failure mode of the roof under two different conditions are obtained and analyzed.According to the experimental date,the minimum safe thickness of the cave roof under the static load is obtained.(3)According to the result of static load test,the dynamic load tests of the model under the two conditions above are carried out.The EL-Centro wave,Wenchuan wave,and a synthetic seismic wave are selected in this paper,and the input amplitude of acceleration increases from 0.05g,0.1g,0.2g,0.3g in turn.When the cave diameter is less than twice of the pile diameter,shear failure occurs in the cave roof;when the cave diameter is greater than twice of the pile diameter,punch failure occurs.It is shown that the existence of cave has obviously reducing effect on the seismic wave in the time domain analysis of the acceleration of each model.With the increase of the upper load,the acceleration of the pile bottom is obviously reduced,and the amplification factor of the acceleration of each measure point is reduced.The Fourier transform of the model acceleration shows that the fundamental frequency of the model decreases throughout the test,and the existence of the cave enlarges the spectrum of the seismic wave.(4)According to the failure mode and experimental data of cave roof under earthquake action,the stress around the karst cave is similar to the distribution of orifice stress in the two-way infinite plate.Based on the quasi-static analysis method,the safe thickness of the roof in the shear failure and punching failure mode are obtained when the karst cave is under the influnece of earthquake of 6-9 seismic intensity. |
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