上软下硬岩质地层浅埋大跨隧道松动压力计算

针对现有方法在上软下硬岩质地层中浅埋大跨隧道松动压力计算方面存在的问题,通过离散元数值模拟和正交试验,分析了围岩的松动破坏机制、风化层厚度和岩体中节理分布状态对隧道松动区形状和范围的影响规律及敏感性。结果表明:影响松动区的主要因素依次为风化层厚度、隧道埋深、贯通倾斜节理的倾角和间距、风化层水平节理间距;松动区可分为拱形和塌穿形两种类型,其边界曲线分别可用抛物线和乘幂函数近似表示。针对塌穿形松动区,通过回归分析建立了地表松动宽度和拱顶松动土条水平长度的计算公式。按照应力传递原理,在松动区内以梯形微分岩土条为对象,通过分层积分,推导出可反映节理产状和风化层厚度影响的浅埋大跨隧道围岩松动压力计算公式。工程实例计算表明,与现有方法相比,所提方法计算的松动压力值最小。

Calculation method of relaxation pressure of shallow large span tunnel in up-soft/low-hard rock stratum

In accordance with the problems of existing methods for surrounding rock pressure of shallow large-span tunnel in up-soft/ low-hard rock stratum, by using DEM (distinct element method) and orthogonal array testing method, the law and sensibilities of the failure mechanism of loose zone and the thickness of overlying weak rock stratum and the joint occurrence influencing on shape and scope of the loose zone are analyzed. In sequence, the sensibilities of factors are as follows in the magnitude order: thickness of overlying weak rock stratum, tunnel depth, angle and distance of continuous inclined joint, distance of horizontal joints in weak rock stratum. Through comparative analysis two failure modes of arch collapse and caving collapse are obtained; the boundary curves are analyzed and fit with linear and non-linear models by using the software ORIGIN, the test results show their boundary curves are power function curve and parabola curve. In accordance with caving collapse loose zone, new formulae for the length of earth surface crack and the soil strip width above the vault are formulated by using the liner least squares method. The errors of the empirical formulae are analyzed. According to the stress transfer principle, a differential trapezoid geotechnical strip is chosen as research object, the formula of surrounding rock pressure of loose zone which considering the joint occurrence and thickness of overlying weak rock stratum is deduced. Case studies show that the results of proposed method are smaller than the results of the existing representative methods.