Water injection test in unconsolidated strata through deep borehole based on fiber Bragg grating monitoring

Abstract：In order to explore the behavior of dewatered unconsolidated strata during the process of injecting water through a borehole from the surface, a new type of fiber Bragg grating（FBG） sensing system was successfully used for the real-time strain monitoring of unconsolidated strata during a water injection process into a dewatered unconsolidat- ed strata of a coal mine. The unconsolidated strata water injection test was implemented through a borehole from the surface at a depth of ＋ 115.83 to ＋ 135.15 m. The employed fiber Bragg grating sensing system and monitoring change of the ground water level were introduced at the depth in the unconsolidated strata where the water injection was con- ducted through two boreholes from the surface. Water injection pressures during the test were 0.1,0. 2,0. 3 MPa,and the total water injection volume was 4 226.84 m3. The relationship between the water injection pressures,water injec- tion volume and the strain change of the unconsolidated strata, the stress and strain conditions of the unconsolidated strata before and after water injection were analyzed. The results show that the dewatering compression of the unconsoli- dated strata is reduced with increasing water injection pressures and water injection volumes. When comparing the condi- tions before and after water injection,it is found that the stress of the unconsolidated strata is in tension after the injec- tions. The FBG monitoring system is very sensitive and stable, and it can realize the real-time strain monitoring of the water injection process within a deep unconsolidated strata through a surface borehole. This FBG monitoring system and the test result presented are very important for understanding further treatment of settlement in unconsolidated strata.