Sublevel mining methods are very popular in Canadian metal mines, which enable maximum recovery due to pillar-less mining. Ore is recovered by backfilling stopes and mining pillars after curing of backfilled stopes. Backfill material must be engineered to avoid backfill failure leading to dilution of precious ore. Current practice of designing backfill includes only gravity loading (static) on backfill. In reality, backfill is loaded by static and dynamic loads during extraction of adjacent pillars. The static load is due to weight of the backfill and dynamic loading is due to blasting and sudden loss of confinement. It is believed that dynamic loading on backfill must have an influence on backfill stability. This paper presents FLAC3D modelling results of static loading due to gravity and dynamic loading on backfill due to loss of confinement and blasting. The CRF stope is subjected to blast vibrations. The numerical modelling results are compared with the cavity monitoring survey (CMS) profiles of a case study thus validating the modelling technique.
Many Canadian metal mines have employed sublevel stoping method with delayed backfill or one of its variations, such as blasthole stoping, vertical crater retreat (VCR), or vertical block mining (VBM) for the extraction of steeply dipping ore bodies (Terzaghi 1961). In sublevel stoping methods the ore body is divided into blocks or stopes, which are mined out while following a pyramidal mining sequence in transverse-retreat directions. The extraction is followed by backfilling the excavation with waste material. Cemented or consolidated rockfill (CRF) is a type of backfill, made by mixing cement slurry with rock aggregates from either development waste or nearby surface quarry (Yu and Counter 1983; McKay and Duke 1989; Annor 1999). CRF has a number of advantages over other backfill materials. Its stiffness can support tall stopes; it requires raw material that is readily available; the placement operation is simple; the curing rate is fast; and its capital cost is low (McKay and Duke 1989; Farsangi 1996; Annor 1999; Yumlu 2001; Kurakami et al. 2008). When good practice is in place, CRF enables achieving good ground control by reducing footwall and hanging wall slough while mining adjacent stopes. It also provides a solidworking surface for mining upper levels. Stiffness and strength of CRF by and large govern its stability. Aslight reduction in CRF properties may lead to backfill failure. Factors like cementation, curing time, aggregate grading, aggregate shape, interlocking of aggregates, placement and mixing method, and quality control dictate the strength and stiffness of CRF (Yu 1989; Farsangi 1996;Yumlu 2001; Kurakami et al. 2008). Several authors have reported that blast vibrations of secondary stopes adjacent to previously mined and filled primary stopes are amongst the major causes of CRF failure thus leading to dilution of precious ore (Aithchison et al. 1973; Yu and Counter 1983;Yu 1989; Farsangi 1996;Yumlu 2001; Caceres Doerner 2005).