| Karst area in China accounts for about 1/3 of the land.At the same time,China is an earthquake-prone country,more than 80% of the land is Ⅵ ~ Ⅸ degree seismic region,which the karst area accounted for alarge part of.With the development of China and the implementation of the western development strategy,bridge construction in karst area is increasing day by day.Because of the special geological conditions in kart area,the small earthquake magnitude can cause serious damage to bridge pile foundation.The current design theory of pile foundation in karst area is still not perfect,it is short of operability for design and construction.Although there are many experts and scholars to study the influence of karst cave on the pile foundation and the stability of karst cave,most of research results aime at the pile foundation acting on the roof of single karst cave at the static effect.In fact, the karst cave distribution is complex in karst area,it usually appears in the form of String-Beaded Karst Cave,which brings more complex effects.The current design code of pile foundation has also failed to provide effective design calculation method. Therefore, the research on the surrounding rock’s stability of beaded hidden cave under the load of pile foundation and safe thickness of karst cave roof have an important theoretical significance,engineering reference value,reducing economic investment and speeding up the construction progress. This paper uses the method of the theoretical analysis combining with the numerical simulations, has concluded a systematic and thorough research of the stability of the seismic stability of bridge pile foundation in string-beaded karst cave. The paper’s main research contents and results are as follows:(1) Seismic damage characteristics of “small magnitude and high intensity”in karst area are analyzed,and the inversion of seismic wave in karst area has been down with the EERA.The time history curve of seismic wave and the SRSTMS curve after inversion are compared with the original curve,and the causes of the phenomenon of“small magnitude and high intensity”are discussed.(2) Based on the stability evaluation method of karst cave-pile foundation system under static load,the rock mass strength in different earthquake intensity and depth are analyzed combining the mohr-coulomb yielding criteria with the wave theory.According to the stress calculation formula of elliptic hole in elastoplastic mechanical,the stressvalue of karst cave is calculated.Then,the three failure modes of the karst cave roof including punching,shearing and bending are analyzed.We can get the theoretical formula for the safe thickness of karst cave roof under three kinds of failure modes.And the single factor sensitivity analysis is carried out for the independent variables in each equation.(3) The stability of single cave-pile foundation system under earthquake is analyzed with Abaqus software.The influence of safe thickness of karst cave roof and vertical displacement of pile by hole diameter,pile diameter,pile load,rock mass cohesion,seismic wave is studied,and get the fitting curve formula between the single factor and the safe thickness of the cave roof and vertical displacement of pile.Then,the regression model of the safe thickness of the cave roof and vertical displacement of pile is obtained by the method of multiple linear regression,and optimize it.Finally,the regression model of the safe thickness of the cave roof and theoretical calculation formula are compared,it is used to verify the validity of the numerical simulation.(4) The stability of string-beaded karst cave-pile foundation under earthquake is analyzed.The influence of pile foundation acting on the roof and the bottom of the cave by changing the vertical double cavity distance and size is studied.The effects of two kinds of variables on the stress state and displacement distribution of foundation are compared.The relative displacement of the pile under earthquake and the influence of the karst cave on the safety thickness of the top plate are analyzed. |
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