动态逆作开挖下有侧移钢管桩组的整体稳定性能研究

Research on overall stability of steel pipe pile group with lateral displacement under dynamic top-down excavation

有侧移静压钢管桩组支承的承载性能对于逆作增建地下室时既有建筑的安全保障至关重要。为获得逆作开挖下有侧移钢管桩组的整体稳定性能,提出一种考虑初始几何缺陷影响的钢管桩组非线性稳定承载力求解方法;基于线性特征值分析,就桩顶锚于原有基础(方案1)、桩顶锚于转换承台(方案2)两种方案,探讨土体水平抗力、原有基础抗弯刚度等因素对侧向约束的影响,求得稳定荷载和计算长度;继而引入一致模态和基坑侧移变形缺陷,采用非线性极值点分析初始缺陷的不利影响,基于此提出计算长度加大系数,进而求得修正后的非线性承载力;最后以单钢管桩组和整体模型为研究对象,对两种方案的侧向刚度差异及实际风载下的整体侧向位移进行分析。研究表明,相较方案1,方案2中有侧移钢管桩组在逆作开挖下的稳定承载性能有显著提升,不同土体水平抗力临界荷载比约为3.30~3.40;初始缺陷对钢管桩组的非线性承载力有不利影响,缺陷幅值越大,承载性能越差,工程设计中按包络法建议加大计算长度至1.15倍;风载作用下,两种方案中有侧移钢管桩组顶部的最大整体侧向位移约为50 mm,其引起的侧移缺陷对非线性承载力的不利影响不可忽略。

The bearing capacity of steel pipe pile group under static pressure is very important to the safety of existing building when the basement is added by reverse construction.In order to obtain the overall stability performance of steel pipe pile group with lateral displacement under top-down excavation,a nonlinear calculation method for the stability bearing capacity of steel pipe pile group was firstly proposed considering initial geometric imperfection.Secondly,based on the linear eigenvalue analysis,the influence of lateral constraints such as the horizontal resistance of soil on two plans(pile top anchored on existing foundation(plan 1)or transfer platform(plan 2))was discussed in depth.The stability bearing capacity and the calculation length were also obtained.Then,through introducing the uniform mode and lateral displacement imperfection of foundation pit,the nonlinear extreme point analysis was applied to obtain the adverse effect of initial imperfection,and the amplification factor of calculation length was proposed to obtain the modified nonlinear ultimate stability bearing capacity.Finally,taking the steel pipe pile group and the overall model as examples,the lateral stiffness difference of the two plans and the overall lateral displacement under actual wind loading were analyzed.The results show that the stability bearing capacity of plan 2 for steel pipe pile group under top-down excavation is significantly improved compared with that of plan 1,and the critical load ratio is about 3.30-3.40 for different horizontal resistance of soil.The nonlinear stability loading factor considering initial imperfection is smaller than that of linear analysis,and the bearing capacity decreases with the amplitude of the imperfection.In engineering design,it is suggested to enlarge the calculation length to 1.15 times according to the envelope method.Under the horizontal wind loading,the maximum overall lateral displacement of the two plans for the top of steel pipe pile group is about 50 mm,and the adverse effect of the lateral displacement imperfection on the nonlinear bearing capacity should not be ignored.