岩溶区穿越溶洞基桩屈曲临界荷载计算方法与试验研究

Method for calculation of buckling critical load of pile foundation crossing karst cave in karst area

基桩穿越大型溶洞时可能发生屈曲失稳,屈曲临界荷载即为基桩极限承载力。针对岩溶区穿越单层溶洞基桩屈曲问题,依据能量法原理建立了穿越单层溶洞基桩总势能方程。根据基桩屈曲的突变特性,引入尖点突变理论,建立了基桩在顶部弹嵌、底部嵌固情况下失稳的尖点突变模型,导出了该体系的分岔集方程,进而通过分析其失稳条件,提出了穿越单层溶洞基桩屈曲临界荷载计算方法。为验证理论方法的合理性,进行了基桩穿越不同高度溶洞的室内模型试验,研究表明:(1)基桩主要发生桩身压缩破坏和屈曲破坏,洞高小于6d时基桩发生材料压缩破坏,洞高大于等于6d时基桩发生屈曲,位移最大点位于临空段的中点;(2)基桩屈曲临界荷载随洞高增大呈线性减小,理论方法适用于基桩发生屈曲的情况,且与试验结果吻合较好;(3)洞高越小、顶板越厚、基桩弹性模量越大、桩-岩界面越粗糙,则基桩屈曲临界荷载越大,反之则越小。

Buckling instability may occur when pile foundation passes through large caves, and the critical buckling load is the ultimate bearing capacity of pile foundation. To solve the buckling problem of pile foundation crossing single layer karst cave in karst area, a total potential energy equation of pile foundation crossing single layer karst cave is established according to the principle of energy method. According to the catastrophe characteristics of pile foundation buckling, the cusp catastrophe theory is introduced,and a cusp catastrophe model of pile foundation instability under the condition of elastic embedding at the top and fixity at the bottom is established. The bifurcation set equation of the system is derived, and then by analyzing its instability conditions, the method of calculation of buckling critical load of pile foundation crossing single-layer karst cave is developed. In order to verify the rationality of the proposed method, laboratory model tests of pile foundation crossing karst caves with different heights are carried out. Some conclusions are drawn. 1) The compression failure and buckling failure are main forms. When the height of the cave is less than 6d,the foundation pile fails due to material compression failure. When the height of the cave is greater than or equal to 6d, the buckling occurs in pile foundation. The maximum displacement point is observed to be located at the midpoint of the karst cave section. 2) The critical buckling load of pile foundation decreases linearly with the increase of cave height. The proposed method is suitable for the buckling of pile foundation, and it is in good agreement with the experimental results. 3) The small cave height, thick roof, great elastic modulus of the pile foundation, and rough pile-rock interface can result in a great critical buckling load, and not vice versa.