No abstract

Coal pillar design in longwall mining must balance the need to ensure roadway stability with economic extraction of reserves. In particular, the design must accommodate the effects of stress redistribution providing stability not only during roadway formation but also by the extraction of longwall panels. There are four types of pillars usually involved, as illustrated in Figure 1, namely: Pillars between main access roadways; Pillars between access roadways and a longwall panel (start/stop pillars); Pillars between adjacent longwalls (rib pillars); Boundary pillars between adjacent mines. This paper outlines the approach of the Cape Breton Development Corp. (CBDC), Nova Scotia, Canada, to meeting the challenges of designing pillars in modem longwall coal mining under the North Atlantic Ocean. In addition to complying with regulatory requirements, other influences must be considered, such as: Protection from interaction effect of working other seams; Protection of the seabed by maintaining strata mass integrity. After discussing the mainly empirical, current practices in pillar design, two longer-term strategies are discussed: (1) pillar reduction (smaller sizes), where appropriate, and (2) elimination of rib pillars completely, where possible. A key feature in developing these strategies is an innovative research and development program run jointly by CBDC and CANMET's Cape Breton Coal Research Laboratory (CBCRL). Several projects are summarized involving development of appropriate geotechnical monitoring and numerical modeling procedures.

No abstract

The Phalen Colliery in the Sydney Coalfield is extracting the 2.6 m thick Phalen Seam by the panel and pillar longwall retreat method. Abandoned workings occur in the Harbour Seam 140 m above and overlie the Phalen Colliery. Parts of these workings are flooded, and pose a number of questions relating to the safety of operations in the Phalen Colliery. In order to answer some of these questions, the Cape Breton Development Corporation re-entered an abandoned non-flooded roadway in the Harbour Seam directly above a retreat panel in the Phalen Seam to allow the deployment of instrumentation to monitor interaction effects. Fifteen monitoring sites, each 4.5 m apart, were installed in the 5 East Top Level after it had been re-entered and made safe. Each monitoring station included a vibrating wire strain gauge piezometer and a potentiometric extensometer. The piezometers were attached to rock bolts anchored into the floor and were connected by a flexible standpipe to a constant head apparatus. Variations in hydraulic head due to subsidence were monitored to determine subsidence. The extensometer units were mounted on rock bolts anchored into the floor of the roadway beneath the ribside and monitored ground strains. The instrumentation was capable of remote reading in the event that access to the site was restricted after installation. This paper describes the instrumentation, the installation programme, and some preliminary results.