Spalling prediction methods in high stress conditions

Ever deepening mines and deep nuclear waste disposal facilities with strict safety regulations face spalling problem that is not well understood. Spalling phenomenon occurs as a strong compressive pressure inducing crack growth behind excavated surface and buckling of thin rock slabs. In this paper we describe how sophisticated spalling prediction methods can be applied 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 outcome range and a comparative analysis. The main analysis method is state of the art Three-Dimensional Finite Element Method (3-D FEM). The 3-D FEM mesh density is enhanced with a control surface and its limits and optimization is discussed. The result validity is studied with Kirsch equations and with two-dimensional indirect boundary element program to examine the tangential boundary stress. Shaft shape and required support is studied for the design process. Also a statistical reference analysis was done using Monte Carlo Spalling –simulation developed in year 2009 by Derek Martin and Rolf Christiansson.