Research On Bearing Characteristics Of Large-diameter The Cast-in-place Pile In Karst

Karst is a serious geological disaster in pile foundation engineering.Due to the existence of karst caves,the strength of the surrounding rocks,especially the tensile strength,is reduced,which weakens the initial bearing capacity of the foundation,and leads to the crush of the roof of the cave and the collapse of the bottom of the pile.And other issues,posing a threat to construction.At present,the research on the bearing characteristics of the pile in karst is not complete,and the calculation theory of ultimate bearing capacity of the pile in karst needs further study.Based on the geology of beaded caves in northern Guangdong and a bridge project,this paper carries out theoretical analysis and numerical simulation on the bearing characteristics of the large-diameter cast-in-place pile in karst,analyzes the load transfer mechanism of the pile,and proposes the theoretical calculation method of the ultimate bearing capacity of the pile.The main research contents are as follows.(1)The load transfer mechanism of the cast-in-place pile in the karst is deeply analyzed.The transfer mechanism,calculation model and failure mode of the soil friction on the side of the pile,the friction of the rock-socketed section and the supporting force under the plie are studied respectively.Based on the elasticity circular sheet deflection theory,with the help of matlab,the theoretical calculation formula of the supporting force of the bottom of the pile in karst is derived.The theoretical algorithm for the ultimate bearing capacity of the pile in karst formed by combining the pile specification.And the calculation method for the stability of the ultimate bearing capacity of the pile is introduced.(2)Using MIDAS GTS to establish models to analyze the factors affecting of the bearing characteristics of the large-diameter cast-in-place pile in karst.The study found that the smaller rock-socketed depth of the pile,the thinner the thickness of the cave roof and the larger the span of the cave will cause the smaller the bearing capacity of the pile.When rock-socketed depth of the pile increases from 1d to 5d,the attenuation of the bearing capacity of the pile decreases from about 40% to about 5%.When the rock-socketed pile depth exceeds the critical value for4 d,the attenuation gradually stabilizes at about 5%,and the rock-socketed pile in the karst can be regarded as an ordinary rock-socketed pile for calculation.When the thickness of the cave roof increases from 0.5d to 2d,the attenuation decreases from about 40% to about 5%.When the span of the cave increases from 2d to 8d,the attenuation degree increases from 2% to about55%.(3)Research on the settlement,stress,axial force and plastic zone distribution characteristics of the pile when single pile and pile group pass through beaded caves.Research shows that the first layer of beaded caves has the largest settlement and stress.The roof failure cone was first formed above the first layer of caves.The plastic zone appeared firstly around the first layer of caves.When pile group pass through the beaded caves,in the case of the same pile length,the settlement of the piles that have not passed through the cave is less than 1/2 of the piles which have passed through the cave.(4)Taking Php Storm as a development platform,using HTML,CSS,Java Script for frontend development,PHP for back-end development,based on pile specifications and circular sheet theory in elastic mechanics,developed a pile bearing characteristics calculation program in karst(PCCPK)independently.The calculation results of the PCCPK program are compared with the numerical simulation calculations of MIDAS GTS to verify the rationality of the PCCPK program calculations.