浅埋隧道覆岩变形沉降的分布式光纤监测与分析

针对在浅埋隧道下穿既有设施的施工过程中上覆层变形难以实现分布式监测这一问题,基于脉冲预泵浦光时域分析(PPP-BOTDA)分布式光纤传感技术,以云南某浅埋隧道连续下穿二级公路和钢筋加工车间场地为例,提出分布式光纤的布设方案,对隧道上覆层沉降变形进行监测,分析上覆层的变形特征。结果表明:上覆层中光纤应变的时空演化过程与岩土体物理力学性质、掌子面推进过程、初衬支护作用以及水文地质条件有关;掌子面前方和后方上覆层分别处于拉应力和压应力控制区,掌子面的位置对应着拉—压应变过渡带。同时,将分布式光纤和地表水准仪沉降监测进行比较,两种方法测得沉降结果趋势一致,验证了分布式光纤在隧道施工过程中对上覆岩土体纵向沉降监测的可行性。分布式光纤监测由于能够获取覆岩的整体变形时空演化特征,具有较明显的优势。

Monitoring and analysis of stratum deformation and subsidence overlying a shallow tunnel using distributed optical fiber sensing technology

It is difficult to monitor the distributed deformation of overlying stratum during the construction of a shallow tunnel. To address this problem, the distributed optical fiber sensing technology was proposed based on pulse-prepump Brillouin optical time domain analysis (PPP-BOTDA). This technology was applied to a tunnelling project, where a shallow tunnel traverses a highway and a steel workshop located in Yunnan Province. The layout of the monitored pipe equipped with distributed optical fibers was introduced. The monitoring results show that the strain and stress distribution the optical fiber in the overlying stratum are dominated by the properties of rock and soil, the advancement of the tunnel face, the supporting effect of initial lining and the hydrogeological condition. The optical fiber in front of and behind the tunnel face is subjected to tension and compression, respectively. The location of the tunnel face corresponds approximately to the transition zone between the compression and tension zones. Combining with the results obtained by levelling instrument, the feasibility of the distributed optical fiber sensing technology was investigated. The results obtained from these two methods exhibit the same subsidence trend, which demonstrates its feasibility. The distributed optical fiber sensing technology can capture the spatiotemporal evolution of the overburden deformation, and therefore has conspicuous advantages over point-wise monitoring methods.