Determination of In Situ Stress by Inversion in a Superlong Tunnel Site Based on the Variation Law of Stress - A Case Study

摘要

The Sejila tunnel, part of the Sichuan-Tibet railway, is in a complex geostress environment because of its deep burial depths and tectonic movement. Based on the measured stress data combined with the structural history and features, the stress characteristics of the Sejila area are preliminarily identified. Then, a three-dimensional numerical model that can provide real topographic features is established, and a distribution law of stress boundary conditions is proposed according to compilations of much measured stress data. By means of support vector regression (SVR), the stress field of the whole Sejila region is determined and finds a reasonable accordance with the measured stress data. Results show that the vertical stress in deep buried stratum can be approximately regarded as one of the principal stresses, and it is reasonable to apply the lateral stress to the model boundary according to a linear function with burial depth. The in situ stress in the tunnel site exhibits that σ_H > σ_V> σ_h, and the σ_H direction deflects when it encounters faults or strata interfaces; the larger that the intersecting angle between fault strike and σ_H is, the smaller the deflection. Compared to the entrance, the rear of the tunnel is subjected to a high maximum principal stress with a high angle; moreover, most sections of the tunnel are estimated to suffer from severe rockbursts, except for a range of 5 km away from the tunnel entrance and 2.5 km away from the exit, according to the Russense criterion. This paper can provide the basis for the prediction and prevention of rockbursts in the Sejila tunnel.