喜马拉雅东构造结西缘地应力特征及其对隧道围岩稳定性的影响

Characteristics of in-situ stresses on the western margin of the eastern Himalayan syntaxis and its influence on stability of tunnel surrounding rock

拟建川藏铁路林芝—通麦段位于喜马拉雅东构造结西侧,构造和应力环境非常复杂,查明其现今地应力特征对于铁路线路工程地质条件评价和隧道科学施工有重要的指导作用。采用水压致裂法在林芝—通麦段实施总计20个钻孔原地应力测量,根据实测数据对该段的现今地应力特征进行研究,并对其围岩稳定性和工程灾害效应进行评价。结果表明:(1)林芝—通麦段实测主应力随埋深的增大而增加,但整体应力积累水平相对偏低,500 m以深发育走滑型应力,表明水平主应力占主导,与区域动力学环境和主要构造活动方式相一致;(2)实测SH平均方位角为N61°~72°E,与震源机制解、GPS和壳幔各向异性资料揭示的区域构造应力方向具有较好的一致性,表明其受区域构造运动方式控制;(3)林芝—通麦段隧道Ⅱ和Ⅲ级围岩以中等~极强岩爆为主,初始岩爆埋深约为400 m,岩爆风险随埋深增加而增大,其中拉月隧道发生强烈~极强岩爆比例最高;(4)基于Hoek-Brown准则预测显示,围岩脆性破坏深度随埋深增大而线性增加,其中拉月隧道最大埋深处围岩脆性破坏深度可能将达1.4倍隧道半径。

The planned Nyingchi—Tongmai section of the Sichuan—Tibet Railway,located on the west side of the eastern Himalayan syntaxis,is in a complex tectonic and stress environment.Investigating the characteristics of in-situ stresses has an important guiding role for the evaluation of the engineering geological conditions of the railway and the scientific construction of the tunnels.Hydraulic fracturing stress measurements of a total of 20boreholes were carried out in the Nyingchi—Tongmai section.Based on the measured stress data,the present-day in-situ stress characteristics of this section were studied,and the stability of the surrounding rock and the engineering hazards were evaluated.The results show that:(1)The measured principal stress of the Nyingchi—Tongmai section increases with increasing the buried depth,but the overall stress accumulation level is relatively low.Strike-slip faulting stress develops below 500 m,indicating that horizontal principal stress is dominant,which is consistent with the regional dynamic environment and the tectonic activity pattern.(2)The measured SH average azimuth angle is N61°–72°E,which is in good agreement with the direction of the regional tectonic stress revealed by the focal mechanism solution as well as GPS and crust-mantle anisotropy data,indicating that it is controlled by the mode of the regional tectonic movement.(3)The surrounding rocks of GradeⅡand GradeⅢtunnels are dominated by moderate to extremely strong rock bursts.The initial buried depth of the rock bursts is approximately 400 m.The risk of rock bursts increases with the burial depth.The Layue tunnel has the highest proportion of strong to extremely strong rock bursts.(4)The prediction based on the Hoek-Brown criterion shows that the brittle failure depth of the surrounding rock increases linearly with increasing the buried depth,and the brittle failure depth of the surrounding rock at the maximum buried depth of Layue tunnel may reach 1.4 times the tunnel radius.