既有建筑下挖增层桩顶约束对受荷桩桩身屈曲稳定临界荷载的影响

Analysis of different constraints at pile head on the critical buckling capacity for excavation beneath existing building

既有建筑物地下室增层开挖施工过程中,随着开挖深度的增加,桩侧土层对基桩的约束逐渐减小,从而可能引发屈曲失稳。不同的桩顶约束对桩基础的屈曲失稳影响不同。首先,以甘水巷3#组团地下室建设工程为背景,介绍了下挖增层改造的施工工艺;之后,引入了不同桩顶约束时的桩身挠曲变形函数,结合Winkler弹性地基梁理论建立了桩土体系总势能方程,利用最小势能原理,导得了既有建筑物下挖增层工况下,不同桩顶约束时,桩基础的屈曲稳定临界荷载;在此基础上,分析了桩顶约束对桩基础屈曲稳定临界荷载的影响。得知:随着半波数的增加,桩基础的屈曲临界荷载逐渐收敛,桩顶固定收敛速度最快,铰接次之,弹性嵌固最慢;不同的桩顶约束对桩基础屈曲稳定临界荷载比影响不同,在同一开挖深度下,桩顶固定时,临界荷载比最大,弹性嵌固次之,铰接最小;随着开挖深度的增加,桩顶固定时的临界荷载比缓慢减小,而桩顶嵌固及铰接时的临界荷载比急剧降低;甘水巷3#组团地下室建设工程中,托换桩不发生屈曲破坏。

During the construction progress of excavation beneath the existing buildings,the constraints between the pile foundation and soils around pile decrease with the increase of excavation depth,which may lead to buckling instability of pile foundation.The influence of critical buckling capacity caused by different constraint conditions at pile head is different.At first,the construction procedure of the No.3 section basement construction project of Ganshuixiang is introduced,then the different deflection functions with different constraint conditions at the pile head are obtained.Secondly,the total potential energy of pile-soil system is established by the Winkler s elastic beam theory.The expressions of critical buckling capacity of pile foundation under different constraint conditions are deduced by using minimum potential energy principle.Based on this,influence factors on the critical buckling capacity of pile foundation under different constraint conditions are analyzed.It shows that,the critical buckling capacity of pile foundation converges with the increase in half-wave number,the convergence rate of fixed pile head is fastest,the hinge takes second place and the fixed with sway is slowest;Constraint conditions at pile head affect critical buckling capacity,the ratio of critical buckling capacity of fixed pile head is largest,fixed with sway takes the second place and the hinge is smallest at the same excavation depth;The ratio of critical buckling capacity of fixed pile head decreases slowly with increasing excavation depth,however,the hinge and fixed with sway at pile head decrease rapidly;The buckling failure of underpinning pile would not happen in the No.3 section basement construction project of Ganshuixiang.