黏土地层浅埋盾构隧道开挖面三维稳定性上限分析

Upper-bound Solutions for Three-dimensional Excavation Face Stability of Shallow Shield Tunnel in Cohesive-frictional Soils

基于隧道开挖工程实际的破坏形态,提出了更接近于实际的开挖面三维失稳破坏模式,由开挖面前椭圆对数螺旋体和拱顶上部半椭圆球体组成土体松动区,为方便计算将前者划分为多个椭圆锥体滑块,利用极限分析上限法推求出隧道开挖面上极限支护压力的目标函数,并通过与已有文献结果对比验证了方法的正确性。研究结果显示,土体内摩擦角对松动破坏区有显著影响,内摩擦角的增加会导致松动区明显缩小。对于整体失稳和局部失稳两种情况,极限支护压力随埋深比C/D、地层容重和地面超载均呈线性增加关系,而地表超载的影响相对较小;但局部失稳时,当埋深比超过一定值时,极限支护压力不再受埋深比C/D的影响;两种破坏模式计算所得极限支护压力随黏聚力和内摩擦角的增加均分别呈线性减小和指数衰减的趋势;当内摩擦角介于5°~30°之间时,整体和局部失稳两种模式结果差距较大,且埋深比C/D越大差距越大;相比土体的黏聚力和容重,内摩擦角对开挖面支护压力的影响更为显著。研究成果进一步完善了开挖面稳定性的评价体系,可为浅埋盾构隧道施工中开挖面支护压力的设计提供借鉴。

Based on the actual failure form of tunnel excavation face,a new three-dimensional failure mechanism for tunnel face stability in cohesive-friction soils was improved.The loose failure zone was composed of an elliptic logarithmic helicoid in the front of excavation face and an upper half ellipsoid.The elliptic logarithmic helicoid was divided into multi-collapse cones for the convenience of calculations.The objective function of the limit support pressure on the tunnel excavation face was derived using the upper-bound method of limit analysis.The effectiveness and correctness were verified by comparing with the reports in the existing literature.The results indicate that the internal friction angle has a significant effect on the soil loosening failure zone.The loose failure zone is obviously reduced with the increase of the internal friction angle.For both the cases of overall failure and local failure,the limit support pressure exhibits linear increasing relationship with the increase of soil density,ground overload and ratio between tunnel cover and the tunnel diameter(i.e.the dimensionless parameter C/D),while the influence of ground overload is relatively small.However,in the case of local failure,the limit support pressure is no longer affected by with the increase of the parameter C/D when the parameter C/D is larger than a certain value.The limit support pressures calculated by both failure modes show a linear decrease and exponential decay with the increase of soil cohesion and internal friction angle.When the internal friction angel is between 5°and 30°,significant difference exists between the results of overall and local failure modes.The greater the dimensionless parameter C/D,the greater the difference.Compared with the cohesion and density of the soil,the internal friction angle has a more significant influence on the limit support pressure of the excavation face.The research conclusions have further improved the evaluation system of excavation face stability and serve as reference and guidance for the design of excavation face support pressure in shallow shield tunnel face construction.