Numerical spalling assessment methods in crystalline rock during the design of ONKALO rock characterisation facility

In deep underground tunnels and facilities, high in-situ stress magnitude can cause brittle failure in hard rock. As mines and nuclear waste disposal facilities pursue deeper, the stress magnitude increases with depth, causing the failure probability to grow. The understanding and the reliable prediction of the failure process is the key to optimal layout design and adequate rock support measures that enable cost-effective and safe construction of deep facilities. The spalling phenomenon occurs as a strong compressive pressure induces crack growth behind excavated surfaces and as buckling of thin rock slabs occurs. The spalling is initiated in the region where maximum tangential stress occurs, and it results in a V-shaped notch. This paper describes how sophisticated spalling prediction methods can be applied in order to determine adequate tunnel support design in Posiva’s ONKALO. The general methods and a case study are described. The results of different prediction methods are compared to achieve an understanding of the outcome range and to make a comparative analysis. The main analysis method is a state-of-the-art three-dimensional finite element method (3-D FEM), and the area of interest is divided into several models to achieve the best accuracy possible. The model is enhanced with quadratic elements to gain the accuracy of elements half the size or even smaller. The validity of the result is studied with the 3-D BEM and a displacement discontinuity method (DDM), which is used to describe the spalling process. In addition, a statistical reference analysis was done by using a Monte Carlo spalling simulation. The prediction results will also be compared later on with in-situ behaviour to conduct back-analysis. The support design of the area of interest is studied for the design process.