Guidelines for permitting, construction, and monitoring of retention bulkheads in underground coal mines.
Many mining operations rely on retention bulkheads to provide a barrier between impounded water and active mine workings. However, bulkhead failures can cause catastrophic flooding that puts the underground workforce at risk. Underground observations and evaluations of existing bulkheads indicate that a systems approach is required when building an underground water or slurry retention system. In addition to engineering the bulkhead, the designer must ensure the quality control of materials and workmanship of the bulkhead, the reaction of the mine strata when exposed to water under pressure, and methods to monitor the performance of the retention system. Researchers from the National Institute for Occupational Safety and Health (NIOSH), with the assistance of the Mine Safety and Health Administration (MSHA), conducted an extensive review of bulkhead permits submitted to the MSHA Technical Support office in Bruceton, PA. In addition, the researchers visited accessible bulkhead installations at underground mining operations to gather information related to construction practices, maintenance issues, and monitoring and emergency response procedures. Several key items were identified that must be considered when permitting the installation of an underground fluid retention system. 1. In general, bulkheads are installed across mine openings to create an area for an underground impoundment. At many operations, the underground impoundment becomes a sealed area once the bulkheads are installed. Current MSHA ventilation regulations require seal structures to withstand pressures of 50 psi if the atmosphere behind the seal is monitored and maintained inert. If the atmosphere is not inert, the seal structure must withstand pressures up to 120 psi [72 Fed. Reg.4 28795 (2007)]. The calculations and design for these structures can be complex and should be completed by a registered professional engineer with a strong background in structural design and a working knowledge of underground mining operations. 2. The underground impoundment should be located at an elevation that will still permit personnel to evacuate to the surface if a breach occurs. If the facility is to be used as a live fluid handling system, it must have adequate storage capacity to handle anticipated peak inflow rates, which includes fluid being directed into the impoundment from the operation and anticipated inflow of fluid from adjacent mine works and surface sources. The system should provide a minimum reserve capacity to allow for electrical interruptions, mechanical failures, and other conditions that prevent discharging fluid from the impoundment. The barrier pillars that form the perimeter of the impoundment must have the proper strength and geological qualities to remain stable for the life of the impoundment. Calculations and testing must be conducted to verify these strength and geological characteristics. 3. The bulkhead design must consider all sources of fluid that could increase the pressure on the structure and design for the maximum anticipated fluid level. The selected bulkhead construction material must be compatible with the fluid impounded. To control leakage, contact pressure grouting of the bulkhead/strata interface is recommended for long-term installations, and all conduits that pass through the bulkhead must contain antiseep collars and be corrosion-resistant. If cementitious material is selected for bulkhead construction, steps must be taken to control the heat of hydration. 4. Strata supporting the bulkheads must be tested and deemed competent to remain stable for the life of the impoundment. Roof, rib, and floor material that is affected by water, damaged, or fractured by mining must be removed from the mine floor beneath the bulkhead. Ring pressure grouting is recommended for all long-term installations to control leakage through the strata surrounding the bulkhead. Mining underlying seams in the vicinity of the impoundment should be minimized and conducted in such a manner that will not reduce the capability of the bulkheads to retain fluid. Standard precautions associated with mining under bodies of water must also be taken. 5. A detailed construction plan that covers each phase of the construction process from site preparation through completion of the installation must be developed. This plan should include quality control procedures to ensure that the construction materials meet standards required by the design, the proper construction techniques are followed, the required procedures are followed for materials mixed on site, and the photographic and written documentation is obtained and recorded during construction of the installation. 6. Routine monitoring the performance of the underground fluid retention system is essential and will be required for the life of the impoundment. Pressure transducers connected to monitoring systems that continuously record the static pressure exerted on the bulkhead are recommended. System designs should include provisions for monitoring inflow and discharge rates, pumping operations, and measurable leakage rates. For impoundments that cannot be monitored through underground workings, surface monitoring wells must be established to track fluid levels. The guidelines in this report are to be used as a tool to identify areas or conditions that could impact the long-term stability of an underground fluid retention system and to give direction in addressing these situations. Although an attempt has been made to identify the common design considerations, it should not be considered an all-encompassing list. Each operation has unique features that could impact the integrity of an underground impoundment, and the responsibility of identifying these site-specific conditions rests with the permit applicant. The goal is to assist the permit applicant through the process of developing a complete permit package for a fluid retention system that is adequately designed for the operation and provides a safe environment for the workforce.