Estimation of in situ stress along deep tunnels buried in complex geological conditions

The headrace tunnels of the Jinping II Hydropower Station are 16.7 km in length and have a maximum depth of 2525 m. Many difficulties have been encountered in estimation of virgin rock stresses along the tunnels. One method was presented including the macroscopic analysis of the region geological structures and topography and the integrated analysis of in situ stresses based on the multi-source information in local tunnel sections. The characteristics of geological structures and tectonic movement were studied to determine the orientations of the tectonic stresses within the Jinping II tunnel project region. The influences of the topography on the distribution of in situ stress were analyzed using numerical methods. The virgin stress regimes were preliminarily estimated based on the geological and topographical analysis and the World Stress Map. Under the guidance of these results, multi-source information was collected regarding rock brittle failures, EDZs, core disking and large-scale deformation of soft rock. Several techniques were adopted, including multivariate regression, numerical simulation, and stereographic projection. An iterative method for the orientations and magnitudes of in situ stresses is presented. The in situ stress regimes, orientations, and magnitudes in tunnel sections of different depths were obtained and verified. They can provide the basic conditions for analysis of the stability of surrounding rock mass during excavation.

► One method was presented for estimating the virgin rock stress in deep tunnels.
► The macroscopic influence of the region geological condition on in situ stress was analyzed.
► The multi-source information revealed in local tunnel sections was included in this method.
► This method was applied in the headrace tunnels with a maximum depth of 2525 m.