SRK was contracted by Strata Earth Services to perform a numerical analysis of the McCook Quarry near Chicago, Illinois. The 3DEC analysis was performed to evaluate a series of 3DEC models previously constructed and compare numerical model results with slope monitoring data for different mining scenarios. The client had concerns that large scale blocks would work their way loose due to mining stress relief, resulting in dramatic increases in groundwater inflows to the quarry. The 50m deep quarry is situated between the Chicago River 100m to the north and the Shipping canal 100m to the south, with the Stephens Expressway immediately adjacent to the quarry walls.
The numerical model was designed to analyse sensitivities to joint orientation, joint strength, rotation of the stress field, and dip of a shallow-dipping clay seam. SRK studied the joint orientation from mapped key joints provided by the client.
The model results were used to estimate deformation of rock walls due to excavation and to evaluate their potential influence on adjacent infrastructure.
Local 3DEC models with 755 to 1,615 discrete blocks in critical sections of the walls were constructed and analysed by SRK for stability. One local-scale model was used to analyse the stability of the north wall of the quarry on the riverside (illustrated on Figures 1 and 2 above), and an additional model was built to assess the stability of the rock weir between quarry lobes. The magnitude of displacements at specific locations from 3DEC modelling is in general agreement with field observations. SRK is currently using the model to make forward predictions of rock wall behavior for the final excavation geometries, and for future use as a combined outfall stormwater retention facility.
Results from these analyses have allowed the client to make modifications to the mining sequence and provide evidence to the regulatory authorities that block movements are anticipated to have a negligible impact on inflows as mining advanced to the next benches.
