Abstract: The hollow-inclusion(HI) strain gauge is used for the first time to monitor the geostress of rock mass in fault zone. A cloud monitoring system based on half-bridge measurement circuit of Wheatstone bridge was established with the hollow-inclusion strain gauge buried at the depth of 24 m in the large-diameter borehole drilled at appropriate position by using centering device and cement paste grouting method during tunnel excavation. Only the elastic parameters of cement paste and hollow inclusion material were considered to calculate the correction coefficient. The three-dimensional stress increment was calculated after the initial time of strain calculation was determined by analyzing the law of monitoring data. Results showed that, with September 16, 2021 as the initial calculation date, the range of the first principal stress (σ₁) in the fourth and fifth period was 10.3~15.0 MPa with a gentle dip W direction; the second principal stress (σ₂) ranged between 3.1 MPa and 4.6 MPa, with a steep dip angle from NEE direction to SEE direction; the third principal stress (σ₃) varied between 0.2 MPa and 1.8 MPa, and the direction is gentle dip angle near S direction. The components of maximum horizontal principal stress (σ_H) and minimum horizontal principal stress (σ_h) varied from 10.1 MPa to 13.9 MPa, and 0.2 MPa to 1.8 MPa, respectively. The direction of maximum horizontal principal stress (α_H) was nearly EW. The results demonstrate that the initial calculation time determined by analyzing monitoring data is more rational. The stress monitoring of hollow inclusion strain gauge is affected by multiple factors including excavation-caused stress disturbance and one-time stress and deformation adjustment of the monitored segment. The obtained σ_H and α_H are close to in-situ stress test results, and the monitoring result reflects the stress field characteristics of the Longpan-Qiaohou fault zone.

Key words: geostress, hollow-inclusion strain gauge, stress monitoring, stress increment calculation, Central Yunnan Water Diversion Project, Longpan-Qiaohou fault