Post-grouting subsidence of the overburden in goaves at major engineering sites: Fiber optic monitoring and deformation mechanisms

BackgroundThe characteristics and mechanisms of residual deformations in goaves after engineering reinforcement (i.e., grouting filling) concern the design, construction, and long-term safety of major engineering projects. Furthermore, they are identified as the key to the efficient development of abandoned land in goaves within old mining areas. Therefore, it is necessary to explore methods and techniques for monitoring post-grouting subsidence of the entire strata in goaves. This will help acquire high-quality, long-term monitoring data on deformation in the field and reveal the mechanisms behind deformation associated with post-grouting subsidence in goaves.MethodsThis study investigated the Xiaohegou Bridge site (including goaves) in the Heyang-Tongchuan Expressway. To this end, this study constructed a monitoring system for the entire deep strata, which uses surface vertical boreholes as deformation monitoring channels and dense fiber Bragg gratings (FBGs) as sensors and adopts in situ modulation and demodulation of optical signals and long-range wireless transmission. This system exhibited a spatial resolution of up to one data point per meter and a sampling frequency of one sampling point per hour, achieving the long-term (3 a) dynamic monitoring of the post-grouting subsidence of entire strata (0‒297 m: from the surface to 10 m below the coal seam floor) at major engineering sites.Results and ConclusionsThe results indicate that the deformations of soil masses varied significantly with season at depths of less than 10 m from the surface, the seasonal deformations of soils weakened gradually with depth at a depth range of 10‒200 m, and the cumulative vertical deformations of strata showed minimal periodic seasonal variations at depths greater than 200 m. The post-grouting subsidence in goaves exhibited a maximum deformation amplitude of 1.34 mm and a maximum deformation rate of 0.013 mm/d. The strata in the goaves showed gradually converged deformation rates, with both deformation amplitude and rates meeting engineering design requirements. These results indicate that the bridge site is stable. The deformations associated with post-grouting subsidence are identified across the entire strata in the goaves rather than occurring only in caving zones and hydraulically conductive fracture zones adjacent to stopes. Notably, the opening and closing of mining-induced fractures that are not completely grouted across the entire strata are identified as the root cause of subsidence-associated deformations. The shallow to middle strata primarily underwent subsidence (compressive strain), with discontinuous transitions between tensile and compressive strains occurring locally. In contrast, deep strata experienced uplifts (tensile strain) and weak deformations, which are inferred to be induced by changes in the groundwater environment after grouting filling in the goaves. The dense FBG-based sensing technology demonstrates high suitability for the long-term monitoring of deformations at a millimeter to sub-millimeter scale after grouting subsidence in goaves, holding great significance for formulating treatment schemes, optimizing construction methods and techniques, and evaluating treatment results for goaves at major engineering sites that are extremely sensitive to deformations. This technology will enhance the disaster prevention and mitigation levels of mining areas and improve the capacity for efficient development and utilization of abandoned land resources.

背景 工程加固（即注浆充填）后采空区（goaves）残余变形的特征与机理，关乎重大工程项目的设计、施工与长期安全，同时也是老旧矿区采空区废弃土地高效开发的核心关键。因此，亟需探索采空区全地层注浆后沉降的监测方法与技术，以获取高质量的野外变形长期监测数据，揭示采空区注浆后沉降相关变形的内在机理。方法 本研究以合铜高速公路小河沟大桥场址（含采空区）为研究对象，构建了一套全深部地层监测系统：以地表垂直钻孔作为变形监测通道，以密集光纤布拉格光栅（FBG）作为传感元件，采用光信号原位调制解调与长距离无线传输技术。该系统空间分辨率最高可达1个数据点/米，采样频率为1个采样点/小时，可实现重大工程场址采空区全地层（0~297米：从地表至煤层底板下10米）注浆后沉降的长期（3年）动态监测。结果与结论 研究结果表明：地表以下10米范围内的土体变形随季节呈现显著变化；10~200米深度范围内，土体的季节性变形随深度增加逐渐减弱；200米以下地层的累计垂直变形则几乎无周期性季节变化。采空区注浆后沉降的最大变形幅值为1.34毫米，最大变形速率为0.013毫米/天。采空区地层的变形速率逐渐趋于收敛，变形幅值与速率均满足工程设计要求，表明大桥场址处于稳定状态。研究发现，采空区注浆后沉降相关变形并非仅发生在采场附近的冒落带与导水裂隙带，而是遍布采空区全地层。值得注意的是，全地层中未完全注浆的采动裂隙开合，是沉降相关变形的根本诱因。浅部至中部地层主要表现为沉降（压应变），局部存在拉应变与压应变的不连续过渡；而深部地层则呈现抬升（拉应变）与微弱变形，推测其由采空区注浆充填后地下水环境变化所引发。基于密集FBG的传感技术，可精准适配采空区注浆后沉降毫米至亚毫米级变形的长期监测，对制定变形敏感型重大工程场址的采空区治理方案、优化施工工艺、评估治理效果具有重要意义。该技术可提升矿区防灾减灾水平，增强废弃土地资源的高效开发与利用能力。