Background The 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.

Methods This 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 Conclusions The 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.