Study On Bearing Mechanism Of Pile Penetrating Karst Caves And Cavern Treatment Technology In Karst Area

In the design and construction of pile in karst area,if the roof thickness of karst cave does not meet the requirements of design and specification,the pile should penetrate the karst cave and embed into the bottom plate of karst cave at a certain depth,that is karst cave pile.Due to the existence of karst cave,the failure mode and load transfer mechanism of the stratum-pile-cave system are different from those of conventional rock socketed pile.The determination of critical thickness of karst cave roof,bearing mechanism and calculation method of karst cave pile are not perfect.In order to ensure the smooth construction of the karst cave pile,the cavern treatment effect is not ideal.In view of the above problems,this paper systematically studies the bearing mechanism of karst cave pile and the cavern treatment technology based on a certain engineering construction project in karst area.The main research contents and conclusions are as follows:(1)This paper established the punching failure model of the cave roof,puted forward the "critical thickness-diameter ratio(the ratio between the roof thickness of karst cave and the pile diameter)" to determine the safe thickness of karst cave roof,deduced the formula of the relation between the thick-diameter ratio and the soft and hard degree of rock and the quality of rock mass,calculated the critical thick-diameter ratio of different rock mass basic quality levels.the critical thick-diameter ratio of the ?level of the rock mass basic quality is 2.5,the critical thick-diameter ratio of the ? level of the rock mass basic quality is 2.5?3.0,the critical thick-diameter ratio of the ? level of the rock mass basic quality is 3.0?3.8,the critical thick-diameter ratio of the ? level of the rock mass basic quality is 3.8?5.0.(2)Through the numerical modeling calculation,the failure modes of the stratum-pile-cave system were analyzed.The failure modes of the pile penetrating single karst cave are:tensile failure of the cave roof,shear failure of the soil around the pile,and crushing shear failure of rocks at pile tip,the first two failure modes occur before the last one.The failure modes of the pile penetrating string-beaded karst caves exhibits progressive failure characteristics,which can be classified into three types:single-layer roof failure type,multi-layer roof failure type and all-through failure type.(3)The tensile failure of the cave roof is a symmetric rotating body with wide at the bottom and narrow at the top,and the width and height of the tensile failure zone are mainly affected by the cave span,free surface will only be available for forming tensile failure zone when the cave span reaches to a certain extent.(4)During the bearing process of the pile penetrating karst caves,the tensile failure of the cave roof forms collapse arch,the calculation of the lateral frictional resistance of the pile should ignore the contribution of rock or soil mass within height of such collapse arch,arch effect caused by collapse arch leads to increase of the lateral frictional resistance imposed by covering rock and soil to pile,so an arch effect coefficient bigger than 1.0 shall be taken into account,considering actual situation of engineering,it is suggested to take 1.0 as safety reserve.(5)In order to ensure the smooth construction of the karst cave pile,this paper put forward different cavern treatment technology for the different karst development situation.Aiming at the cavern with a height less than 1.0m,the controllable cavern grouting and filling treatment technology was developed,and the compound grout with controllable grouting range and grouting amount was prepared,the grout materials are water,cement,fly ash and mineral powder,the additives are Na2CO3 and sodium bentonite,and the key technical indexes of quality control were put forward.Aiming at the eavern with a height of not less than 1.0m,a kind of treatment technology of stone slag concrete poured in the cavern was developed,and stone slag concrete with controllable pouring range and pouring amount was prepared.The raw materials are water,stone slag,cement,fly ash and mineral powder.The key technical indexes of quality control under different cavern conditions were put forward.