In Situ Stress Field along the Axis of Deeply Buried Tunnel in Southwest China Employing the Segmented Single-Borehole Inversion Method

Investigating the in situ stress field is one of the primary tasks of the tunnel disaster prevention and design stage in a high geostress environment with a strength-stress ratio lower than four. Due to the limited measured data, inversion is the most mainstream method to obtain the in situ stress field in engineering areas. After discussing the influence of the tunnel longitudinal dimension effect on the inversion accuracy, this paper proposed a segmented single-borehole inversion (SSBI) method that uses multiple regression models to characterize the in situ stress field of the tunnel. This method was applied to four deep-buried (467-1,780 m) and super-long (7.2-16.3 km) tunnels in Southwestern China, and the distribution characteristics of the in situ stress field along the tunnel axis in the fault stratum and intrusive rock stratum were analyzed. The results showed that the SSBI method with several regression models had a higher inversion accuracy than the traditional multiple-boreholes inversion (MBI) method with one regression model, e.g., the inversion error decreased from 12.7% to 6.0% in the Muzhailing tunnel, and the inversion error decreased from 30.7% to 6.3% in the Lanjiayan tunnel. Furthermore, the average relative error between the calculated values and the measured values not involved in the inversion was 11.0%, verifying the reliability of the proposed method. The inversion of the in situ stress field in two tunnels with special strata showed that the magnitude and orientation of the in situ stress at the boundaries of the intrusive rock and the fault had changed drastically, and its distribution characteristics had a certain regularity.