高地应力软岩隧道预留变形量设计方法

Design of Reserved Deformation for Soft Rock Tunnels With High Geostress

在弹塑性岩体中开挖隧道,若围岩无明显的流变性,支护施作太早将承受非常大的荷载,但支护结构并非越晚施作越好。如果支护结构刚度和强度设计不足,即使预留了变形空间,隧道仍可能因支护反力不足而发生大变形破坏。为确定合理的支护刚度和支护时机,基于GZZ强度准则采用大应变分析理论,考虑隧道扩挖影响,修正高地应力软岩隧道的围岩特征曲线。修正后的特征曲线与原始曲线有共同的起点,但随着变形增大逐渐偏离原始曲线;增大开挖半径会使特征曲线更高,这意味着支护结构需要提供更大的反力。因此,在工程设计时需要根据修正后的围岩特征曲线进行支护结构设计,避免因为支护刚度不足而发生大变形破坏。其次,基于初始地应力、围岩强度等参数对修正围岩特征曲线形态的影响,提出高地应力软岩隧道围岩最佳预留变形量的设计方法。当地应力较低时,特征曲线有明显的“最低点”,该点对应的支护反力最小,为最佳支护时机;在高地应力条件下,即使变形很大,围岩特征曲线也仍然未达到最低点,这是因为形变压力占主导,松散压力远小于形变压力。因此,在高地应力条件下,采用应力释放措施是有必要的,可通过特征曲线的曲率寻找最佳预留变形量。

Early support implementation can lead to an excessive load-bearing burden if the surrounding rock lacks apparent rheology during the construction of a tunnel in elastic-plastic rocks.This underscores the importance of timely support.In instances where the stiffness and strength of the support structure are inadequate,large deformation failure induced by insufficient support reaction force becomes a distinct possibility,even with reserved deformation in place.To ascertain optimal support stiffness and determine the appropriate supporting duration,the ground reaction curve(GRC)for soft rock tunnels experiencing high geostress is modified.The modification is based on the generalized Zhang-Zhu strength criterion and large strain analysis theory,accounting for the effects of tunnel expansion.The modified and original GRCs share a common starting point but gradually diverge due to increasing deformation.The modified GRC exhibits an upward trend as the excavation radius expands,resulting in a higher reaction force.Consequently,the modified GRC serves as a valuable guide for designing support structures that minimize the substantial deformations arising from inadequate support stiffness.Following the analysis of the impact of initial geostress and rock strength on the modified GRC,a method for designing optimal reserved deformation in the surrounding rock for soft rock tunnels subject to high geostress is proposed.In cases of low geostress,the optimal support timing aligns with the"lowest point"on the GRC,corresponding to the smallest support reaction force.Conversely,under high geostress conditions,the GRC fails to reach the"lowest point",even with significant deformation,attributed to predominantly small loose pressure than deformation pressure.Therefore,stress release becomes imperative under high geostress conditions,and the optimal reserved deformation can be determined from the curvature of the GRC.