Distribution characteristics and inversion analysis of in-situ stress field at tunnel site of an extra-long and ultra-deep tunnel in Wumeng Mountain area

In-situ stress is a fundamental geological parameter dominating the design and construction safety of deep mountain tunnels. With the rapid expansion of transportation infrastructure in southwest China, numerous extra-long and deeply buried tunnels inevitably pass through complex tectonic zones with intense horizontal tectonic compression, where severely high in-situ stress easily triggers large deformation of surrounding rock and rockburst hazards. The Qiaojia Tunnel on the Ludian-Qiaojia Expressway, located on the northeastern margin of the Wumeng Mountain tectonic belt, is a separated double-line extra-long tunnel with a maximum burial depth exceeding 1 600 m. Restricted by field construction conditions, only limited borehole measuring points can be arranged for hydraulic fracturing tests, which fail to reflect the overall spatial distribution of the three-dimensional in-situ stress field across the entire tunnel site. To fully characterize the initial in-situ stress distribution and develop a reliable inversion framework applicable to ultra-deep tunnels under strong tectonic extrusion, this paper carries out systematic field measurement and numerical comparative research.

First, six sets of hydraulic fracturing in-situ stress data ranging in depths from 346-637 m were collected from borehole K67+910 m in the tunnel exit section and were used to analyze the variation patterns of the three principal stresses with burial depth. The finite element software MIDAS GTS NX was utilized to establish a three-dimensional geomechanical model with a plane size of 2 400 m × 2 400 m. Multiple sets of mechanical parameters for dolomite, limestone, and mudstone are assigned to the model according to official geological investigation documents. Four types of boundary schemes, namely displacement constraint boundary, stress loading boundary, hybrid boundary, and the boundary scheme derived from initial strain energy theory, were separately implemented for comparative inversion tests. The strain-energy-based scheme was selected as the optimal boundary condition due to its superior fitting performance with the measured stress data. After applying the optimal boundary condition to the full-domain model, the inversion results at each measuring point were extracted and compared with field test values, with ±20% defined as the allowable relative error threshold for evaluating inversion accuracy.

Field test results demonstrated that the magnitudes of the three principal stresses increased approximately linearly with burial depth. The magnitude sequence of in-situ stress ranked as follows: maximum horizontal principal stress (S_H) > vertical overburden stress (S_v) > minimum horizontal principal stress (S_h). The dominant azimuth of the measured S_H was NW32°, proving that the regional stress field was dominated by horizontal tectonic stress. The numerical inversion results indicated that the simulated S_H azimuth was concentrated between NW30° and NW35°, which achieved high consistency with field measurement. The relative errors at most measuring points were controlled within 15%, and all errors fell within the permissible range. In addition, the simulated growth gradient of principal stress with depth was consistent with field measurements, verifying the stability and reliability of the strain-energy-based inversion method.

This study systematically clarifies the applicable scope and inherent limitations of four commonly used boundary schemes for in-situ stress inversion. Compared with traditional single-boundary inversion methods, the proposed strain-energy-based approach can simultaneously reconstruct both gravity-induced stress and regional tectonic stress fields without obvious distortion of stress contour distribution. The distribution patterns and quantitative stress data obtained in this research can provide solid theoretical support and practical engineering references for the in-situ stress inversion of analogous ultra-deep extra-long tunnels, and can guide the optimization of surrounding rock support parameters and the early warning of high-stress geological disasters such as rockbursts and large deformation caused by extrusion.