Rock Mechanics Challenges for the Excavation of a Deep Shaft in Anisotropic Ground

摘要

The #4 Shaft at Hecla’s Lucky Friday Mine, excavated to a depth of 2,923 m below surface, was completed in 2017. Although the shaft did not extend to record-breaking depths, it did experience extraordinary ground stresses due to an unusually high in-situ stress ratio given its depth. Due to the high in-situ stresses and, more importantly, the anisotropic nature of the argillaceous host rock, the shaft experienced very large time-dependent deformations (in excess of 30 cm radial convergence in unlined areas) as a result of buckling of the vertically-oriented foliation. This led to large loads on the shaft liner and, in some places, severe liner damage. Ultimately, this prompted a change in design, with the shaft excavation cross-section shape being changed from circular to an approximately elliptical or “notched” profile perpendicular to the strike of the foliation. The change in shaft geometry reduced the equilibrium ground displacements of the rockmass by nearly an order of magnitude and allowed continued shaft sinking without further liner failure. This paper documents the challenges associated with the #4 Shaft project from a rock mechanics perspective and provides an analysis of ground and support behavior. Major results from monitoring and modeling efforts are presented, and results obtained using different modeling approaches are compared. Practical recommendations and conclusions relevant to future deep excavation construction are also presented.