A tunnel located in the southeastern margin of the Qinghai-Tibet Plateau faces exceptionally complex high in-situ stress issues during its construction due to intense regional geological tectonic activity combined with the influence of deep-cut valley landforms.By deploying eight boreholes in the engineering area and employing the hydraulic fracturing method,62 segments of in-situ stress data and 25 segments of oriented impression test data were obtained.These data,along with regional geological information,laboratory tests,and back-analysis of the initial in-situ stress field,were used to investigate and analyze the characteristics of in-situ stress in the tunnel engineering area.The research results indicate:(1)Field in-situ stress tests reveal that the engineering area is dominated by horizontal tectonic stress overall,with the preferred orientation of the maximum horizontal principal stress being NE～NEE,which is basically consistent with the regional in-situ stress background values.(2)Back-analysis of the initial in-situ stress field shows that the maximum horizontal principal stress SH along the tunnel axis ranges from 7.7 MPa to 26.28 MPa,the minimum horizontal principal stress Sh from 4.1 MPa to 18.6 MPa,and the vertical principal stress Sv from 5.2 MPa to 29.3 MPa.The stress field type in the engineering area is mainly strike-slip faults(SH>Sv>Sh),with some local sections featuring normal faults(Sv>SH>Sh).93.84%of the area along the tunnel is under high to extremely high in-situ stress conditions,possessing the conditions for high in-situ stress that can lead to large deformations.(3)Based on the double-index classification standard for large deformations in railway tunnels,the total length of the tunnel is 20 247 m,with a total length of 7 230 m experiencing soft rock large deformations,accounting for 35.71%.Among them,5 960 m are subject to mild large deformations,accounting for 29.4%;1 010 m are moderate large deformations,accounting for 4.99%;and 260 m are severe large deformations,accounting for 1.28%.The research conclusions can provide scientific basis and technical support for the preliminary survey and design of deep and long tunnels in western mountainous areas under similar geological conditions.
deep-lying long tunnelswestern mountainous areashydraulic fracturing methodin-situ stress characteristicslarge deformation
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