Enhanced borehole stability analysis for geological waste disposal under conditions of in-situ stress uncertainty: The case of Yamin Plain, Israel

This work presents a comprehensive parametric study of unsupported and supported vertical boreholes in deep geological deposits. The parametric study is demonstrated for the case of Yamin Plain area, which is the designated site for radioactive waste disposal in Israel, where uncertainty exists regarding the in-situ stresses. The Yamin case is suitable for this study as the target rock layers are of marginal stability under the applied stresses, and therefore a detailed analysis is called for. A parametric factor of safety analysis is utilized for the evaluation of the borehole stability, with and without structural support. To this aim, the estimated stress field is evaluated using both analytical calculations and advanced numerical analyses, incorporating a new spring-boundary condition that yields far greater accuracy than with the commonly used boundary conditions. The parametric study for the borehole stability problem is examined with respect to the in-situ stresses and the structural support. We find that a steel liner is safe under any in-situ stress scenario; HDPE liner is found to be insensitive to liner thickness for the stress scenarios examined, because of the compatibility between the stiffnesses of the rock mass and the liner material; because of the relatively high difference between compressive and tensile strengths of concrete, we find that beyond a certain liner thickness the stability of a concrete liner in fact decreases.