Stability analysis of deep-buried hard rock underground laboratories based on stereophotogrammetry and discontinuity identification

In a tunnel, instabilities in the surrounding rock mostly occur within the sidewalls and crown. After acquiring the rock mass structure, a combination of laboratory experiments, numerical simulations, and in situ monitoring data can permit a more reasonable stability analysis of the surrounding rock and engineering support design to ensure a safer engineering project. To overcome the shortcomings (e.g., inefficiency, high labor costs, and safety risks) of traditional methods for mapping the rock mass structures of the sidewalls and crowns of tunnels, this study proposes a safe, rapid, and efficient method that can acquire a 3D digital elevation model (DEM) of the sidewalls and crown of a tunnel and the corresponding rock mass structures by using digital photogrammetry (DP). The proposed method was then tested in an engineering tunnel. Error analysis of check points and discontinuity orientations showed that the errors were within a reasonable range. The method was further applied to traffic tunnel #1 of the China Jinping Underground Laboratory Phase II (CJPL-II), and the spatial coordinates and orientations of the joints were obtained. A 3D quasi-deterministic discrete model was subsequently established by converting the coordinates and orientations of the joints from a geological coordinate system to a local coordinate system in discrete element software. The quasi-deterministic model was then used to confirm that the joint persistence has an important influence on the stability of the surrounding rock of a tunnel and, thus, affects the support installation. Finally, the joint persistence value was determined by the size of the onsite unstable block. The results of this study provide a reference for the design, construction, and support of similar deep-buried jointed hard rock tunnels.