The stability of a pump station cavern excavated in Chagrin shale was evaluated. During the excavation of the top heading, rock along the roof centerline displaced downward more than was anticipated and more than is typically observed in rock cavern construction. Large deformations during construction were resulted from the excavation sequence that had disturbed the rock. Some time-dependent rock deformations were observed during and after excavation. Several opinions have been expressed on the source of this time-dependent deformation including stress changes induced by excavation, strain softening along the bedding planes, and creep of the shaley rock under compressive stresses. Finite Element Analysis (FEA) was performed to simulate the excavation sequence and analyze the rock behavior observed during excavation. To address concerns regarding long term stability of the cavern, the FE model was utilized to estimate long-term deformations by incorporating time-dependent constitutive behavior of the rock mass and discontinuities.

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