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The spatial distribution of rock mass stress in goaf seriously affects the stable operation of underground reservoir. Measurement of stress distribution in the goaf of coal mines is difficult due to the complex internal environment and other restrictive conditions of goaf. This paper presents a new method of monitoring the deformation of key stratum in overburden by optical fiber sensing, and then characterizing the pressure of goaf. The internal mechanism of key stratum breakage and the variation of goaf stress is investigated by physical model with the geometric dimension of 3 (length) × 1.3 (height) × 0.4 (width) m), in which the Brillouin Optical Time Domain Analysis (BOTDA) fiber-optic sensing technology and Fiber Bragg Grating (FBG) technology are used to monitor the stability of key stratum and the non-contact Digital Image Correlation (DIC) technique is employed to provide real-time deformation measurements. Combining the fracture mechanics model of key strata with the parabolic function model of subsidence trajectory, the stress evolution model of coal seam mining space is established for the first time, the goaf pressure change and the corresponding relation between the key layer breakage was quantitatively characterized. The research results show that: The fracture location of the key strata is consistent with the peak stress location of the goaf. The breaking limit strain threshold value of the key strata based on optical fiber sensing detection is 4000 μe. Through the inversion of optical fiber monitoring results, the fracture degree and range of overburden rock in goaf are obtained, thus reflecting the position and change of the peak pressure in goaf. The consistency between key layer deformation and goaf pressure variation is verified.

The destruction of the water-resistant key strata is the root cause of water resource destruction in ecologically fragile areas and mining water hazards. Accurate and effective scientific monitoring of the stability of water-resistant key strata is an important basis for water-preserved coal mining and rock formation control. At present, drilling detection, theoretical analysis and numerical simulation are often used to calculate the height of water-conduction fractured zone to indirectly determine the aquifer permeability. With the development of optical fiber sensing technology, fiber Bragg grating (FBG) and distributed optical fiber (BOTDA) methods are used to monitor the stability of mining overburden, which provides a new research idea for water-preserved coal mining. The research show: In shallow seam, when the thickness of bedrock is only 60–67 m, the water-conduction fractured zone will develop directly to the ground surface, which will inevitably lead to the destruction of bedrock aquifer, the fracture line and its longitudinal cracks nearby prove to be the main seepage channel; the strain detected by optic fiber presents double peak characteristic due to the stress concentration of strata in fracture position, and the peak position corresponds to the fracture line. It's possible to prediction the fracture line position by optic fiber sensor ; combining with the limit equilibrium theory calculation, the breaking limit strain threshold value of the water-resistant key strata based on optical fiber sensing detection is given as 2000 μe. The FBG sensor can accurately monitor the deformation of rock mass at a specific location. The detection of instability position of the water-resistant key strata is basically consistent with the actual observation and distributed fiber optic detection results.