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Slope Design at Cuajone Pit, Peru

SRK News | Issue 54: Rock Engineering and Slope Stability

A4   |   Letter

SRK Chile’s Rock Mechanics team completed a study of the slope design at Cuajone pit. The Cuajone porphyry copper deposit is located on the western slopes of Cordillera Occidental, in the southern Andes of Peru. The current pit measures about 2.5km east-west, 3.0km north-south, and has a maximum depth of 950m. Mining by open pit methods commenced in 1976 and has continued since then. Ore production is 90ktpd.

SBlock software (Esterhuizen, 2004) was used for the bench berm design. This software uses the Key Block principle (Goodman & Shi, 1985) and joint set statistics to simulate a large number of potential blocks in open pit benches. For a given structural domain, several slope orientations were conducted and the results matched with the bench behaviour. A 3D plot of the benches located failures with SBlock software. Figure 1 presents a visual correlation with the observed frequency and type of failure noted in the pit benches.

The failure volume and bench widths predicted by SBlock agree sufficiently with the observed instabilities of the slope benches. This initial study validated the methodology, and further bench stability evaluations will be undertaken using SBlock, where the minimum berm width required for single benches (15m) ranges from 6.5 to 8.0m.

At Cuajone, past failures provide an excellent basis for designing the proposed push-back or final pit design. The geological and structural information holds good data for confidence in design. Since the past raised doubts about the design, analysing these failures was essential to provide the confidence needed to proceed with the next push-back.

Cuajone’s history displays a number of classical failure mechanisms: large scale wedge failures and circular arc failures in highly altered soft rock. In mid-February 1999 a failure of approximately 12 million tonnes occurred (Figure 2). The failure is typical of a soft rock circular arc failure mode, with shear strength controlled by cohesion rather than friction. There is strong evidence that groundwater pressures played a significant role. Immediately before the failure, there was a period of heavy rainfall.

As part of the slope design program and slope optimisation for the next push-back, the past and present performance of the pit slopes were evaluated to assess the potential behaviour of future pit expansion. A geomechanical assessment is underway to evaluate the stability of the walls for expansion over the next 15 years. A series of geotechnical studies were performed to analyse slope stability, based on limit equilibrium methods, finite element bidimensional numerical models and discrete element tridimensional numerical model 3DEC.

Esteban Hormazabal:

SRK Africa