Standing Support Alternatives in Western United States Longwalls (1900)

Mines in the western United States have had limited choises for standing support. Wood crib were often too soft andunstable. The introduction of the Can support in the early 1990s provided an effective alternative, and it remains the dominant form of tail­ gate support. Water-filled prestressing cells are now used to cap the Can and preload it to provide an active roof load. The cluster Prop, consisting of three timber wedge props bundled to­ gether, provides more capacity than an equivalent sized Can support and improves transport efficiency. However, the Clusler Prop is less stable and does not maintain a consistent load throughout its loading profile. Pumpable roof support are another alternative support, but they have not been proven in high deformation environments. This paper compares the performance characteristics of these various support systems. Introduction Gate roads and longwall recov­ ery rooms require proper support to ensure successful longwall opera­ tions regardless of geographic region. But various circumstances make this particularly difficult for western U.S. longwalls. Nowhere in mining is the compatibility of secondary supports more critical than in western U.S. mines, where convergence is often an order of magnitude greater than in most eastern U.S. mines. In particu­ lar, the stiffness of the support must be compatible with the ground reaction, to avoid premature support failure or unintentional damage to the roof and floor when the support is too stiff and catastrophic failure of the roof when the support is too soft (Fig. 1). Historically, timber cribs and posts were the domi­ nant form of secondary support. However, the demands placed on these supports by longwall mining, coupled with the low-strength. short supply and increasing cost of mine timber in western U.S. mines. have necessitated the development of alternative support systems. Beginning in the mid-1990s. the National Institute for Occupa­ tional Safety and Health (NIOSH) in conjunction with various support manufacturers promoted the devel­ opment of innovative support sys­ tems. A report published in the Proceedings of the 15th International Conference on Ground Control in Mining documented these initial support developments (Barczak and Molinda, 1996). The first alternative to the wood crib in western U.S. mines was the Confined Core Crib (3C) support (Fig. 2), The 3C support was devel­ oped by John Frederick at Southern 1 Utah Fuel Company (SUFCO) Mention of any company name or . product does not constitute endorse­ ment by the National Institute for Occu­ tional Safety and Health. and was successfully employed for years in the SUFCO mines (Frederick, 1994). The 3C support is a corrugated steel container filled with minus 76­ mm (3-in.) pumice rock that was readily available at the Utah mine site. The steel container provides confinement to the pumice rock as it was loaded by the convergence of the mine entry. Initially, the 3C is a very soft support due to the void ra­ tio of the pumice fill. However, de­ spite this soft response, the support performed adequately in the twoentry yield pillar tailgates in the SUFCO mines where moderate floor heave, occurring prior to the longwall panel front abutment. caused suffi­ cient convergence to compact the fill material and thereby stiffen the sup­ port for improved performance as the face approached. The 3C support was the predeces­ sor of the Can support. The Can support, as shown in Fig. 3, has replaced wood cribbing and continues to be the dominant form of standing support in western mines to­ day. The Propsetter support (Fig. 4) was developed in the mid-1990s by Strata Products. It has become one of the most commonly used longwall supports in eastern U.S. mines. But its use in western U.S. mines has been lim­ ited, in part, by its limited yield capability and stability in the plus 3-m (10-ft) mining height ap­ plications. This paper discusses enhance­ ments to the Can support technology and recent support innovations that provide alternatives to the Can sup­ port for western tailgate applications. Specifically, the use of a new pre­ stressing technology for Can sup­ ports will maximize its stiffness and help to offset t he deficiencies caused by topping the Can with timbers. A new timber support called the Clus­ ter Prop was developed by Strata Products USA. It has been success­ fully used in several eastern U.S. mines as an alternative to the Can support. A comparison of the perfor­ mance capabilities of the Cluster Prop with the Can support is in­ cluded. Pumpable roof supports is