Authors: Eggers, MJ
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Porphyry copper-gold deposits are situated in some of the most dynamic and complex geological environments, which makes engineering for open pit slope design a demanding task. The key to tackling this challenge is engineering geology, in particular understanding the key geological factors that are likely to control slope instability mechanisms in the pit wall. Engineering geology should underpin all engineering decisions throughout the investigation, modelling, analysis and design process. Engineering geological controls can be grouped into the regional to district-scale setting of the deposit and deposit-scale factors. Knowledge of the regional setting should be summarised into a geological history that explains all elements of the model making sure the field relationships between lithology and structure are adequately described. At the deposit-scale, brecciation, alteration, weathering and structure are important elements. These processes serve to change the rock mass quality compared with the original rock due to degradation in the mechanical properties. Hydrothermal brecciation is an important mechanism for change in rock mass character while the telescoped vertical alteration zonation system typical in porphyry-style deposits can modify rock conditions particularly in areas of increased overprinting of alteration higher in the porphyry system. Slope performance experience from operating mines in Southeast Asia strongly indicates that structurally controlled failure mechanisms operating at the inter-ramp slope scale dominant slope stability conditions. Larger scale failures involve a composite mechanism of interacting structures sometimes with a rock mass component in the centre or toe of the failure geometry. While structure is the clear primary control, rock mass condition is also a major contributor to controlling the style of movement. Antecedent rainfall is a major influence on instability signifying that drainage measures are an important component of the slope design package. The dominance of structure in controlling slope performance is an important conclusion as geotechnical studies for pit slope design in porphyry-style deposits often focus substantially on rock mass. More attention is required in understanding the regional geological setting, how this influences district-scale features and what the controls are on deposit-scale structural patterns.


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