超大断面隧道软弱破碎围岩空间变形机制与荷载释放演化规律

摘要

为研究不同施工工法和工艺组合条件下超大断面隧道穿越软弱破碎地层的围岩稳定性问题,以兰渝线两水隧道为背景,开展铁路双线隧道在软弱破碎地层中超大断面开挖的大比尺三维模型试验,真实再现台阶法支护开挖、台阶法和全断面毛洞施工的全过程.首先,基于现场较为破碎的千枚岩岩样基本力学参数室内试验结果,以松香、铁晶粉以及聚四氟乙烯棒等材料为原料,研制兼具低强度和弱黏结特性的软弱破碎围岩相似材料和初喷混凝土、锚杆等支护结构的相似材料,在最新研制的可实现三面均匀同步加载的大型三维地质力学模型试验台架上模拟隧道台阶法支护、全断面支护和全断面无支护施工的全过程,并采用光纤光栅传感器、电阻式应变计、多点位移计以及微型压力盒全程监测洞壁及其整数倍(0～3倍)洞径范围内围岩的应力、位移以及近区荷载的变化信息,分析不同施工过程中隧道围岩受力和变形的三维空间演化规律.研究结果表明:(1)软弱破碎低强度和流变特性使围岩变形具有更强的时空效应,同时存在掌子面挤出变形、先行位移和后方位移3个时空演化过程；(2)软弱破碎围岩变形的三维扰动深度一般在3倍洞径内,台阶法支护开挖扰动范围最小,全断面支护开挖次之,但后续长距离推进极易诱发围岩坍塌;(3)台阶法较大断面开挖有利于控制收敛变形,减少10％～25％,沉降变形则相差不大,上台阶开挖过程是拱部围岩垂向荷载的急剧释放期,下台阶开挖是边墙围岩水平荷载的急剧释放期；(4)台阶法支护开挖掌子面前方岩体扰动范围为0.5～1.0倍洞径,多属荷载集聚区,是软弱破碎围岩稳定加固的重点区域和最小边界范围；(5)隧道断面围岩整体荷载释放过程存在3个典型变化阶段,即掌子面附近荷载集聚区、前方荷载弱集聚区和掌子面后方荷载释放区,及时施作支护可有效减弱掌子面前方围岩荷载的集聚程度和荷载峰值,使荷载峰值出现的位置滞后,有利于掌子面及其附近围岩的整体稳定.%A large scale 3D geomechanical model test is made based on the Liangshui tunnel of Lanzhou-Chongqing Railway to investigate the stability of surrounding rock masses of super-large section tunnel through soft broken formation under the condition of different construction methods and technologies. The model test simulates the super-large section excavation of twin-track tunnel in soft broken phyllite formation, which presents the whole process of bench methods with supporting. A kind of soft broken surrounding rock masses similar material is researched in terms of the test of basic mechanical parameters of phyllite. The similar material is made from iron powder, quartz sand, barite powder and rosin alcohol solutions, which owns the characteristics of low intensity and light sticky. And the similar material of initial shotcrete and bolt is also researched. The whole construction progress of bench methods with supporting, entire section methods with supporting and entire section methods without supporting is simulated by the 3D model frame. The model frame can load from three different directions synchronously. The changing information of stresses, displacements and loads in the scope of entire times of the tunnel diameter (0-3 times) can be monitored by fiber grating sensors, resistance strain gauge, multi-point extensometer and micro pressure cells. In comparison with the results of numerical simulation, the 3D evolution law of the stresses and strains of surrounding rock masses in different construction progresses is analyzed. The research results are as follows. (1) The characteristics of low intensity and rheology strengthen the time-space effect of the surrounding rock masses. And there coexist three time-space effects: extrusion deformations of tunnel face, procession displacements and rear displacements. (2) The 3D disturbance depth of the deformation of surrounding rock masses is within tripling diameter of the tunnel. The disturbance range of bench methods with supporting is the minimum and the entire section methods with supporting take the second place. But subsequent long distance promoting will result in collapse of surrounding rock masses. (3) Large section excavation of bench methods is in favor of controlling convergence deformation with a decrease of 10% - 25%. But it nearly has no influence on settlement deformation. The excavation process of upper bench is the rapid releasing period of vertical load of arch surrounding rock masses and the excavation progress of lower bench is the rapid releasing period of horizontal load of side walls. (4) The disturbance ranges before the tunnel face of bench methods with supporting are about 0.5 - 1.0 times of the diameter, most of which are load agglomeration areas. And these are also the stable strengthening regions and minimum boundary ranges of soft broken surrounding rock masses. (5) There are three typical changing stages for the load releasing progress of the surrounding rock masses including load agglomeration areas near the tunnel face, anterior load soft agglomeration areas and rear load releasing areas. The load agglomeration degree and load peak value of the surrounding rock masses in front of the tunnel face can be weakened by applying supporting in time. And the supporting will also make the position of peak load value lag. And this is in favor of the stability of the tunnel face and the surrounding rocks near it.