Study On Bearing Characteristics And Dynamic Response Of Pile Foundation Crossing Karst Caves Under Traffic Loads

China’s transportation system is currently developing vigorously,with more and more bridges and highways passing through karst areas.The existence of karst caves adds a lot of difficulty and uncertainty to construction.If not properly handled,many problems will arise.Pile foundation is widely used in karst areas,and there are two main methods for handling pile foundations:(1)If the thickness of the top plate of the karst cave meets certain requirements and is sufficient to bear the upper load,the pile foundation can be embedded in the top plate of the karst cave,known as the underlying karst cave type pile foundation;(2)Bored cast-in-place piles that cross karst caves and are embedded in the bedrock below the bottom plate are called karst cave crossing pile foundations.During the service stage,bridges and highway pile foundations in karst areas not only need to bear the self weight of the stratum,but also will bear the vertical cyclic load of vehicles passing by.In addition,the existence of karst caves reduces the safety of the foundation,inevitably leading to changes in the bearing performance and dynamic response of the pile foundation.The research results of domestic and foreign scholars on the safety thickness and stability of the roof of the underlying karst cave type pile foundation are relatively complete,providing a rich theoretical basis for the safety design of pile foundations in karst areas.However,the research on the crossing karst cave type pile foundation is relatively small,and the relevant results are not yet systematic,and focus on statics,its load response under traffic loads The bearing mechanism and the dynamic response of surrounding rock in karst caves need to be studied more systematically.In this paper,in response to the above mentioned problems such as the load response of pile foundations crossing karst caves and the dynamic response of surrounding rock in karst caves,a vertical cyclic loading model test of pile foundations crossing karst caves has been carried out,and a preliminary exploration has been conducted on the vertical load response,bearing characteristics changes,and dynamic response and load transfer characteristics of single rock-socketed piles crossing single-layer karst caves and beaded karst caves.The main work is as follows:(1)Based on the similarity law and the separation similarity design method,the design of limestone similar materials and the selection of the optimal ratio were carried out;A multifunctional model test system has been designed and processed to meet static,dynamic,and hammer loading requirements;Completed the design of test content such as the layout of monitoring points.(2)A model test was conducted to investigate the bearing characteristics and dynamic response characteristics of a single pile crossing a single layer karst cave under traffic loads.The variation laws of pile strain,axial force,bending moment,and unit side friction were studied,and the hysteretic characteristics of pile top displacement were analyzed;Based on the measured data of sensors,the dynamic response laws of dynamic earth pressure and acceleration around the karst cave are obtained,and then the impact of the karst cave on load transfer under traffic loads is analyzed.(3)A model test was conducted to study the bearing characteristics and dynamic response of a single pile crossing a beaded karst cave under traffic loads,and its dynamic response and pile rock interaction were studied;Comparative studies were conducted on the differences in pile strain,as well as the transmission of soil pressure and acceleration around karst caves when a single pile crosses different numbers of karst caves.(4)The calculation method of bearing capacity of pile foundations crossing single layer karst caves and beaded karst caves in karst areas is studied.The commonly used calculation methods for the vertical bearing capacity of rock socketed piles in the specifications are summarized,and the calculation steps for bearing capacity considering the reduction coefficient of karst caves are proposed.