Abstract:
Geotechnical design and ground control management strategies required to excavate open pits through historical underground mine voids can be relatively common in some mining districts (e.g, Abitibi, Timmins) with numerous case studies published (e.g, Henning, 2005). Most published case studies are within a hard-rock setting where void behaviour in an exposed bench or slope can be structurally controlled and geometrically defined by bounding features.
This paper demonstrates weaker rock characterization and domaining workflows for the design of an open pit through historical underground voids at an open pit for a project located in western United States. For the project site, voids can represent stopes, shafts, adits, and horizontal development. Importantly, the deposit is situated within tertiary aged volcanic rock complexes, intense alteration assemblages, and regional listric faults systems that have contributed to the penalization of the in-situ rock mass (prior to underground mining). This historical mining context, along with other early 19th century methods, are tabulated to demonstrate the possible range of void geometries, collapse behaviour, and back-filling scenarios that could be expected in a weaker rock setting. These characteristics are compared with the expected behaviour in hard-rock settings. Each historic mining aspect is further examined with regard to the design decisions on expected void influence in an already weak rock mass.
Specifically, this paper focusses on the adopted workflow to characterize materials near the historical workings, and subsequently define materially important but simplistic domains for slope design (and future ground control). The geotechnical classification scheme and the observed materials are summarized regarding the decision-making process to sub-divide historic mining zones based on geotechnical risk and stability behaviour in design.
Authors:
- Ed Saunders | Principal Rock Mechanics Engineer | SRK Canada
- Andrew LeRiche | Senior Rock Mechanics Engineer | SRK Canada
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