Analysis of shearing of a rock ridge

A rock ridge carrying a major roadway through a quarry in the greater Chicago, IL area sheared along sub-vertical regional joints causing significant distress of the roadway as a result of apparently limited adjacent quarry excavation of a glory hole. The large-scale movement rendered the road unusable. This paper presents a detailed analysis of factors contributing to this response. Post-event field observations of ridge movement revealed that lateral displacements were concentrated along a horizontal thin shaley seam below the bottom of the quarry. These observations show that the sheared ridge slid along the shaley seam and moved as a single block. Three-dimensional numerical analyses of the ridge and the quarry construction activities show that as the quarry was deepened, a buildup of large horizontal stresses occurred in the rock base above the shaley seam. The excavation of the glory hole through this rock base lead to arching and an increase in longitudinal stresses and a reduction in lateral confining stresses along the ridge. The analyses show how the unique combination of large horizontal stresses, the horizontal shaley seam, and stress redistribution due to glory hole construction lead to large shear displacements along the persistent sub-vertical regional joints.