Research On Safety Evaluation Method For Piled-Raft Foundation Of Tall Buildings In Karst Region

In recent years,with the development of economy and society in China,more and more high-rise buildings have been built in karst areas,how to ensure the safety of karst foundation becomes an important issue in engineering construction.Beaded cave is a common karst geology with high hole rate and connected holes,which reduces the bearing capacity and stability of foundation and brings risks to construction.In view of the engineering construction problems,the finite element analysis of single pile,pile group and whole structure model on karst site is carried out based on the mature static test technology of pile foundation.A practical evaluation method of the safety of the roof of the piled raft foundation of high-rise building is presented through summarizing the stress characteristics of the bottom bedrock and the main factors affecting the safety of the roof of the cave are discussed.The main contents of this paper are as follows:(1)Combined with the theory of rock strength,a practical evaluation method for verifying the safety of the bottom of the pile-tip cave roof is put forward which makes that the pile-tip bedrock in a certain range of the pile own sufficient bearing capacity without damage through single pile and group pile test,if the bedrock force state at the bottom of pile are not less than that of overall structure after construction,we can ensure that foundation is safe.(2)Geological datas of karst area and survey reports of engineering projects are collected,research of safety of cave roof relevant scholars are compiled to determine the parameters needed for finite element analysis.(3)The finite element models of single pile,pile group and the whole structure on karst site is established by using finite element software ABAQUS.The additional stress distribution of the rock at the bottom of pile is analyzed.(4)By considering the effects of upper soil layer,pile length,position of the pile in the whole structure,load of the upper structure,when the additional stress of the bedrock in certain range of the pile bottom of the single pile and pile group is the same as that of the bedrock in the same range of the pile bottom of the whole structure in different geologic conditions and different structural arrangements,the loads of single pile and pile group are determined for engineering application reference.(5)In order to solve the problem that the construction quality of some piles at the local position of the building is flawed,the finite element model of overall structure with missing piles are established to study the variation law of rock stress at the bottom of pile.Under very unfavorable conditions,the maximum Mises stress and vertical stress of rock increase by no more than 80 kPa and 130 kPa.Rock stress increment is small relative to the strength of limestone,which has little effect on the safety of overall karst foundation.