Subsidence Mitigation Using Void Fill Grouting

Void fill grouting is a technique which has been extensively used by CTL/Thompson, Inc. to mitigate mine subsidence potential resulting from abandoned underground coal mine workings. The technique involves rotary drilling from the surface down into the mine workings and pressure injection of grout into underground mine voids and rubble zones to fill voids. Communities of Rock Springs, Glenrock and Hanna, Wyoming were built over abandoned coal mines. Over time, the rock comprising the roof of the mine begins to fracture and collapse into the open mine. This process continues until the space is either occupied by rubble or the caving reaches the surface, threatening public health and safety. To successfully mitigate the potential of subsidence reaching the surface, a thorough understanding of the subsidence mechanism is required and the proper mitigation method selected. The paper describes typical subsidence mechanisms and mitigation methods. The depth to mining is important when determining whether or not subsidence will reach the surface. Also important are; the thickness of the mined seam, rock bulking . characteristics and strength of the overburden rock. This information can be obtained by investigative drilling, laboratory testing on material samples and careful analysis. The paper presents brief descriptions of analytical methods to determine critical mine depth. Once the critical depth to the mine is determined, a pilot drilling and grouting program is undertaken to determine the optimal hole spacing for mitigation. During construction, holes are drilled on a grid spacing as determined in the pilot program and low strength grout is injected through the drill holes into the mine workings. Critical items during grouting are; grouting sequence, slump variation, pressures, cutoff quantities and verification drilling. CTL/Thompson, Inc. has successfully used this technique in Rock Springs and Glenrock, Wyoming for over 13 years with construction costs exceeding 35 million dollars. Verification drilling, and the lack of insurance claims or surficial subsidence features indicate that void fill grouting is an effective method to mitigate the effects of subsidence due to abandoned underground mines.