The Goderich Salt Mine, located on the shoreline of Lake Huron in Ontario, Canada, has been in operation for more than 60 years. The underground production rooms are roughly 52 ft tall and 60 ft wide and are located about 2,000 ft below the surface of the lake. Their mining method has changed in recent years from drill-and-blast to mechanical excavation, advancing the rooms progressively in four cuts.
Since 2014, the geomechanics team in Denver, Colorado, has provided support on numerous projects involving stress analysis, creep calculations, panel design, cut sequencing, ground support design, and drift rehabilitation. In October of 2018, Goderich requested assistance in determining the mechanism behind a series of buckling failures that were observed in the upper corners of the mechanically excavated, rectangular rooms. Buckling generally occurred in a layer of salt 4 to 10 inches thick after the room was opened to a height of 26 ft. The fractured slabs of salt posed a ground fall hazard and were identified by state regulatory agencies during a safety inspection.
Eric Poeck conducted a site visit to inspect the working sections of the mine and document the size and location of observed failures. The buckling did not occur in all rooms and could not be correlated with any geologic anomaly, gas pressures, presence of moisture, or proximity to mining activity. A series of quasi-2D numerical models were run in an effort to reproduce the failure. The model included explicit horizontal bedding seams spaced 0.5 to 4 ft apart and ground support elements in the roof. The investigation included variations in salt strength, stiffness, creep parameters, and bedding seam shear strength.
The results of the analysis revealed that a reduction in the assumed shear strength of the bedding seams accommodated horizontal displacement of the salt immediately above the roof. The increase in room height from 13 to 26 ft was accompanied by a significant increase in horizontal stress in the salt near the room corners. The shear strength of the bedding seams had never been tested, but the model identified the threshold at which horizontal stress concentrations were relieved by shear deformation, which in turn was relieved by the buckling of the lowest slab.
SRK worked with the engineering staff at Goderich to implement new machinery and improve its scaling methods. The mine now utilises a wheeled, telescoping-boom grader fitted with a hydraulically powered, toothed drum instead of a bucket. In addition to removing loose ground, the drum is used to excavate shallow cuts in the room corners, which prevent the transmission of horizontal stress through the thin slabs. The results of the numerical analysis and the modified scaling plan satisfied state regulators, and the risk of groundfall in the room corners has been greatly reduced.
