软土地层中分隔型基坑变形特性及应力路径

Deformation Characteristics and Stress Path Analysis of Divided Foundation Pit in Soft Soils

摘要:

以上海软土地区紧邻历史保护建筑的超大地下综合体施工为例，建立考虑土体小应变特性的三维有限元计算模型，研究分隔型基坑变形特性及土体应力路径；将数值模拟结果与现场实测数据进行比对以检验模型参数取值的合理性，并基于此模型进行多工况计算分析。结果表明：仅靠优化基坑开挖顺序对基坑变形控制作用有限且不全面，有必要考虑地下结构回筑引起的基坑支护刚度和边界条件变化；先开挖小基坑并完成其地下结构回筑后再施工较大基坑的施工方案效果最优，与较不利工况相比可减小紧邻敏感建筑物侧围护结构最大侧向变形30 %；不同开挖顺序、地下结构施工引起的边界条件变化，以及不同排水条件使得坑内外土体应力状态复杂多变，其中坑内土体在不同工况下呈现不同应力路径，坑外土体则呈现多次卸荷的应力路径；当应用伺服式预应力钢支撑时，坑外土体表现出先水平卸荷而后加荷的应力路径，围护结构变形控制在开挖深度的0.1 %内，可有效保护周边敏感建筑物的安全。

Abstract:

Based on a case of a super large underground complex construction adjacent to a historical building in Shanghai， a three-dimensional finite element model considering small strain characteristics of soil was established to examine the deformation characteristics and stress path of soils during excavation. The field measurement data were utilized for the validation of model parameters， and six different construction conditions were calculated and analyzed. The results showed that the effect of only optimizing the excavation sequence on controlling the deformation of the foundation pit was limited and not comprehensive. It was necessary to consider the influence of integral stiffness of underground structure and boundary condition change during the whole construction process. The comparison that the optimal construction scheme was to excavate smaller pits first， followed by the completion of the underground structure and the construction of larger pits， which could reduce the lateral displacement of the retaining structure close to the sensitive building by 30 % compared with the adverse condition. In addition， different excavation sequences， boundary conditions， and drainage conditions made the stress state of soils inside and outside the pit more complex and varied. The soils inside the pit presented diverse stress paths in different unloading conditions， while the soil outside the pit showed a multiple unloading stress path. The application of the servo prestressed steel support could control the deflection of the retaining wall close to the protection object within 0.1% H_e （excavation depth）， and the retaining wall might move towards the outside of the pit under the combined action of steel supports at each level. Moreover， the soil outside the pit would exhibit the stress path of horizontal unloading first and then loading.