In June of 2020, the geomechanics group in Denver, Colorado, was contacted by a subsidiary of Waterton Global Resources to assess the stability of a slope at an open pit copper mine in North America.
The slope consisted of four 25-ft benches beneath a crushing station, which was located roughly 60 ft from the crest. The mine manager had observed the formation of tension cracks roughly 8 ft from the crest and was concerned about the overall
stability of the slope and the crusher’s foundation. In addition to the recent tension cracks, mine personnel had observed the progression of a wedgeshaped failure in another portion of the crest, which had consumed about 20 feet of the working surface between 2015 and 2020. Finally, large gaps between concrete slabs near the base of the crushing station highlighted the possibility that large-scale deformations were occurring, as the mine personnel did not know when the gaps formed or if they had recently increased in size. Because the mining property had
changed hands, the client’s engineering staff could not obtain detailed records
from construction of the crushing station or any geologic core within 500 ft of the
area, which made it difficult to conduct a meaningful analysis. Rock mass
strength properties were calibrated by reproducing failure of the bench crest in
a limit-equilibrium model, but application of these properties to the entire slope
resulted in unrealistic failure limits.
A geotechnical report from another consultancy was provided, but the rock type that was assumed prevalent throughout the slope had a fairly high strength, and was therefore not representative of the materials currently failing near the bench crest.
SRK recommended the installation of two piezometers, two inclinometers, and a series of prisms for surveying, as well as collection of geotechnical data from holes that would be drilled for the instruments. Eric Poeck travelled to the site in November 2020, logged the core, and directed the installation of the piezometers and inclinometer casing. Transitions in rock quality were clearly observed in the core. Highly weathered schist with a rock mass rating (RMR) in the order of 20 existed in the first 7 to 9 ft, while a transition to fresh schist, with an RMR in the order of 70, occurred at depths of roughly 50 ft. The depth of the weakest material corresponded with the observed thickness of failure near the crest, and the fresh core matched observations of the rock type in the geotechnical report.
Stability models were updated, and the results showed that the likelihood of large-scale failure in the slope was very low. Data from the monitoring program confirmed that deformations over a six-month period were below the noise threshold of the system. The mine can now seek remediation of the bench crest without worry of a larger failure undermining the work.
