A New Numerical Model to Study Isolated Rock Blocks Around Underground Excavations Taking Into Account In-situ Stresses

When an underground excavation is cut in a discontinuous rigid rock mass, instabilities may occur mainly due to block failure. Isolated rigid block methods were developed since the 80's to locate critical blocks and evaluate their stability but have major drawbacks: ignoring in-situ stresses, mechanical behavior of joints and rotational movements. Other methods included those variables improperly and were limited to simple cases. This paper presents a critical review of previous approaches then a more complete model to study isolated rock blocks is proposed. It is based on the fact that stresses on the block faces are known before excavation and once a face is freed, the block moves as a rigid body in translation and rotation. Applying equilibrium and rock joint behavior equations, the stresses on the faces after excavation can be calculated and stability evaluated using a Mohr-Coulomb criterion. Any block geometry can be studied by partitioning the block faces into simple elements. Numerical integration is done on elements using Gauss points. The method is applied on a case study and comparisons are made with other simplified methods. Finally, a parametrical analysis shows the important influence of in-situ stresses and joint stiffnesses on the block's stability.