天津地区基坑外既有隧道竖向变形机理及影响区研究

Study on vertical deformation mechanism and affected area of existing tunnel outside foundation pit in Tianjin

天津地区基坑开挖普遍引起坑外既有隧道隆起,但其机理缺乏明确研究,且数值计算较难模拟预测出隆起变形。针对此问题,在分析多个基坑开挖引起隧道隆起案例的基础上,运用有限元模拟和HSS本构模型,对隧道隆沉机理进行了研究。结果表明,隧道竖向变形由基坑围护结构水平变形引发的沉降与基坑卸荷回弹导致的隆起叠加而来,隧道隆沉取决于两方面影响因素的相对大小。不排水短期条件模拟时,基坑开挖卸荷在坑底以下及坑外引发了较大的负孔压,限制了土体的回弹变形及隧道隆起。数值模拟中应根据实际工程施工周期长短、黏性土层渗透性与排水路径长短综合选择土体的排水类型及模拟方法。天津地区粉土黏土互层的地层中孔隙水压力消散较快,施工时间长达数月以上的大型基坑采用排水模拟或能够考虑孔隙水压力消散时间效应的固结模拟,能够更为准确的反映基坑卸荷回弹的影响,预测出隧道的隆起变形。对于基坑外隧道变形的影响区,长期排水条件下隧道隆起区和变形过渡区的位置相比短期不排水条件均向上大幅提高,隆起区范围大幅增加。对于深18 m的基坑,沉降区与过渡区分界线可上升9~17 m,过渡区与隆起区分界线可上升14~16 m。

The foundation pit excavation in the area of Tianjin is generally to lead to upwarping of the existing tunnel outside the pit, but there is lack of clear study on the mechanism, while it is difficult to simulate and predict the upwarping deformation through numerical calculation. Aiming at this problem, the upwarping and settlement mechanisms are studied herein by means of finite element simulation and HSS constitutive model on the basis of analyzing several cases of tunnel upwarping caused by excavation of foundation pit. The results show that the tunnel vertical deformation comes from the upwarping superposition caused by the settlement and the unloading rebound of foundation pit induced by the horizontal deformation of the foundation pit retaining structure, and then the upwarping and settlement depend on the relative sizes of the influences from both the aspects. In case of the simulation of undrained short-term condition, larger negative pore pressure is to be induced by the foundation pit excavation unloading below the bottom of the pit and outside the pit, which is to limit the rebounding deformation of soil mass and the tunnel upwarping. In numerical simulation, the draining type and simulation method of soil mass are necessary to be comprehensively selected according to the construction period, the permeability of cohesive soil layer and the length of draining path. As the pore water pressure dissipates rapidly in the formation of silt-clay interbedding within the area of Tianjin, the adoption of drained simulation or the consolidation simulation that can consider the time effect of pore water pressure dissipation for the large foundation pit with construction period of more than several months can more accurately reflect the impact of foundation pit unloading rebound and predict the upwarping deformation of the tunnel. For the affected area of tunnel deformation outside the foundation pit, both the positions of the tunnel upwarping zone and the deformation transition zone under the long-term drained condition are significantly rise up if compared with those under the short-term undrained condition, while the range of the upwarping zone is to be greatly increased as well. For the foundation pit with the depth of 18 m, the boundary between the settlement zone and the transition zone can rise by 9~17 m, while the boundary between the transition zone and the upwarping zone can rise by 14~16 m.