Denver, CO: U.S. Department of the Interior, Bureau of Mines, TN 435, 1994 May; :1-2
Objective: Provide the National Park Service (NPS) with guidelines for closing abandoned mine openings such as shafts and adits. Approach: The NPS needed alternative methods of closing a variety of abandoned mine openings on National Park lands. Because the U.S. Bureau of Mines (USBM) has a long history of mining-related experience, the NPS requested that the Bureau provide them with technical mine closure information. USBM engineers compiled information on various types of mine closures and prepared technical illustrations of those closure techniques. Multiple closure options, by type of opening, were prepared as one-page field guides to assist NPS field personnel in determining the most appropriate closure method for a given situation and detailing the construction of the closure method. Results: Guidelines provided to the NPS included a description of the method, use applications, construction techniques, technical illustrations, and technical support information. The following provides a brief description of 11 mine closure techniques identified for use by NPS personnel. Most of the closure methods, except the backfill methods, allow for construction that can provide access to the mine. However, providing access makes the closure susceptible to vandalism. Most closure methods require periodic monitoring for subsidence and corrosion or deterioration of construction materials. Adit Backfill-Backfill closure consists of filling the portal area of an adit with on-site or imported unclassified fill and riprap. If the adit drains water, a noncorrosive drainpipe must be installed. This method provides permanent closure of the mine opening. Bat Gate-Because many species of bats live in abandoned mines, it is often necessary to provide a means for bat access while preventing human access. To accomplish this, a steel grate supported by a rigid steel frame is cemented or bolted into rock at the mine portal. Sites-specific sized openings in the steel gate allow passage of bats and other wildlife and provide natural ventilation. This closure method is used where the abandoned mine provides habitat for threatened and endangered species of bats and where the portal is in competent rock that will support the steel structure. Bat gates are effective at preventing accidental or intentional entry by the public. The gates, however, can be breached by persistent vandals and must be monitored. Cable Net Closure-Steel cable is formed into a mesh with locked intersections to prevent spreading of the mesh. The steel net is anchored in competent rock surrounding the mine opening with rock bolts or rebar anchors. Cable net closure is adaptable to large, odd-shaped openings, including those with historic resources. Steel netting allows bats and other small wildlife to pass and can be modified to allow passage of larger animals. This type of mine closure is intended for temporary closure. Steel nets can be cut with cable cutters but not with bolt cutters. Concrete Block Bulkhead-A bulkhead of concrete masonry blocks and cement grout is built into the adit portal or inclined shaft collar. This application is used where no access is required for wildlife and there is not a nearby source of competent stone for backfilling the opening. The portal must be in competent rock, except for the floor. Native rock can be mortared to the exterior wall to lend a natural appearance. Concrete Bridge Channels Cap-Precast concrete bridge sections are laid over the shaft collar and backfilled to approximate original contours at the site. This method is used where the shaft collar has 3.6 meters (12 feet) or less of soil cover over competent rock and can be used to cover large shaft openings. Concrete Shaft Cap-A concrete cap is constructed over the shaft collar. The cap is backfilled to approximate the original contours at the site. This closure method is used where the shaft collar is in competent rock. Hollow-Core Shaft Closure-A steel-reinforced concrete shaft wall is constructed that will serve as a foundation for a shaft cap and an optional corrugated steel culvert access. This closure method can be used where the shaft opening is in soil up to 12 meters (40 feet) deep over bedrock. It can be used where the soil collar has collapsed, forming a funnel-shaped surface opening. Native Rock Bulkhead-A bulkhead of native rock and cement grout is built into the adit portal or inclined shaft collar. This method is used where no wildlife access is needed and a natural appearance is desired. The portal must be in competent rock, except for the floor. PUF Shaft Closures- Polyurethane foam (PUF) shaft closures consist of installing a bottom form, spraying PUF over the form to a required minimum thickness (dependent on the size of the opening), and backfilling with common fill PUF is a permanent closure. Lightweight construction materials are easily transported or backpacked to remote sites. The bottom form requires little structural strength. Backfilled openings do not attract vandals or casual visitors if returned to natural contours. Riprap Bulkhead-Riprap is placed inside and around an adit portal or inclined shaft collar to provide a permanent and complete blockage of the opening. If the portal drains water, a noncorrosive drainpipe must be installed. This closure method is used for permanent closures where no access is required for people or wildlife. Shaft or Stope Backfill- Backfill mine closures consist of filling shafts or stopes with on-site or imported unclassified fill, riprap, and clay barriers where it is necessary to isolate aquifers. Backfill closures are intended for use as permanent closures where no access is required for people or wildlife.
Denver, CO: U.S. Department of the Interior, Bureau of Mines, TN 435