浅埋盾构隧道端头土体稳定性极限平衡分析

LIMIT EQUILIBRIUM ANALYSIS OF SOILS STABILITY AT SHALLOW TUNNEL END UPON SHIELD EXCAVATION

盾构始发与到达时易引发端头土体失稳破坏,是盾构隧道施工中的高风险环节。针对浅埋盾构隧道,提出一种直线和对数螺旋线组合的土体滑动模式,运用极限平衡分析方法,对端头土体的稳定性进行分析,推导稳定系数计算公式,并对其影响因素进行分析。端头土体的稳定性取决于土体自身的强度、加固体的厚度和加固强度及洞门直径。端头土体自身的内摩擦角和黏聚力越大,稳定系数越大;加固体的强度越大,稳定系数越大;加固体厚度越大,端头土体稳定性越好;但随着洞门直径的增大,端头土体稳定系数逐渐降低。随后得出端头土体的加固厚度的计算方法和流程,分析其影响因素,并给出工程算例。将该方法应用于南京地铁盾构隧道端头土体稳定性分析及加固厚度计算,结果令人满意,被工程所采纳。该研究成果和方法可为同等条件下盾构隧道端头土体稳定性分析及加固设计提供借鉴。

A working shaft enclosed by the continuous wall must be built at two ends of the tunnel to be excavated for shield starting and arriving in shield tunneling. Soil body at two ends of the tunnel may collapse along a sliding surface and pose a high risk in shield tunneling. A combined sliding mode of straight line and logarithmic spiral line was proposed for shallow shield tunnel. A formula to calculate the stability of the soil at the ends of shallow tunnel was deduced by means of limit equilibrium analysis and the factors influencing the soil stability were analyzed with the derived formula. The soil stability at tunnel ends is dependent on the soil strength,the thickness of reinforced soil body,the strength of reinforced soil body and the diameter of tunnel outlet. The greater the internal friction angle and the cohesion of the soils are,the higher the soil stability. The greater the thickness and the strength of the reinforced soils are,the higher the soil stability. The soil stability at tunnel end decreases with the increasing of the diameter of the tunnel outlet. The method was successfully used to calculate the soil stability and the thickness of the reinforcement of soil body of a shield tunnel of metro in Nanjing.