Research on in situ stress inversion of deep-buried tunnel based on pressure/tension axis mechanism and geological structure

The investigation of the in situ stress distribution has always been a key condition for engineering design of deep tunnels and analysis of surrounding rock stability. In this paper, a comprehensive judgment method coupled with pressure/tension (P/T) axis mechanism and geological structure is proposed to invert the in situ stress in the Duoxiongla tunnel in Tibet. In the process of TBM tunnel excavation, 3887 groups of microseismic events were collected by means of microseismic monitoring technology. By studying the temporal and spatial distribution of 3887 groups of microseismic events, 42 groups of microseismic data were selected for in situ stress inversion. Then the focal mechanisms of 42 groups of microseisms were inverted. Combined with the analysis of the previous geological survey, the inversion results of the in situ stress were analyzed. According to the focal mechanism of the tunnel area, the linear in situ stress inversion method was used to invert the in situ stress in the source area. Finally, according to the PTGS (pressure/tension axis mechanism and geological structure) comprehensive judgment method proposed in this paper, the in situ stress of the tunnel microseismic region was determined. The results show that there are mainly three groups of fissures and joint surfaces in the tunnel area, and the in situ stress is dominated by the horizontal tectonic stress; the main driving force of the rupture surface in the excavation process of Duoxiongla tunnel is the horizontal tectonic stress; the distribution of the maximum and minimum principal stress obtained by the inversion is consistent with the distribution of the P/T axis; combined with the linear in situ stress inversion method and the comprehensive judgment of PTGS, the azimuth and dip angles of the three principal stresses are finally determined as N90.71°E, 4.06°, N5.35°W, 3.06°, and N8.10W, 85.32°, respectively. The study verifies the feasibility of microseismic inversion of in situ stress.