This article demonstrates the effects of regional hard rock mining in several case studies of shallow stope cave-in failures. Applicable numerical modeling methods indicate that destressing of the rock mass is in effect under such extensive mining, which facilitates the occurrence of block caving, raveling and plug failures in effectively fissured rock masses.
1. INTRODUCTION
Canada's history is intimately related to mining. The mine workings that remain today are both a reminder and a legacy of this activity. Shallow stopes of hard rock (metal) mines, abandoned as well as active, can be subject to several gravitydriven failure mechanisms [1], originating within a variety of geological terrains and controlled by a range of rock mass quality and disposition of discontinuities.
Three case studies of large or regional mining operations are used to highlight failure occurrences such as raveling (peripheral block by block rock mass failure), block caving (rock mass break-up and gravity flow mobilization of blocks towards and into an opening) and plug failures (sudden fall of the surface crown pillar as a unit, delineated by well-defined boundary plane discontinuities), and the controlling factors.
2. HOLLINGER MINE, ONTARIO
The area under, and adjoining, the city of Timmins (located in northern Ontario 670 km north of Toronto) represents a striking example of intense undermining of infrastructure and dwellings. Seven mines operated in this immediate area in which gold was primarily recovered. Of note, is the very large extent of the number of underground stopes of the Hollinger Mine (Figure 1), (from surface to 1,800 m deep) and the McIntyre Mine (from surface to 2,500 m deep). The shallow stopes cover an area 1.6 km x 1 km. Historically, the area has also been subject to numerous and widespread rock mass and fill failures from shallow stopes to surface. The Hollinger Mine has been the focus of considerable site remediation to reduce the impact to population safety and to infrastructure. It is presented here in order to establish the rock mass effects from intense mining, and explain the gravity failures in effect here and in the other case studies presented.
The mines in the Timmins area are situated in terrains dominated by basalt and dacite lava flows. Imposition of folding brought about faulting and shearing of these units and created open fracture conditions generally conducive for intrusions and formation of vein systems. Figure 2 provides a view of the geology in the mine area. Their widths normally ranged from 0.3 m to 22 m and averaged ~3 m. The host Precambrian lava flows are schistose and altered. Major jointing patterns for this area have not been reported. Some investigative work [2,3] indicates normally up to three jointing families occurring in shallow drill core samples: two families oriented NE-SW, one dipping South 45 -55?°, the other dipping North 45 -55° ?, and a third, vertical, occurring with less regularity. Added to these families are occasional discontinuities related to the geological units themselves. Lava formations vary from massive, to brecciated where blocks occur 0.1 m to 1 m in width. Sharp and gradational contacts exist between flow units. Schistocity is in close proximity or hosts to the fractured-filled deposits. It occurs variably and gradationally. These discontinuities have effectively created a blocky rockmass.
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