Drilled Shaft Installation at an Abandoned Underground Mine Site

A seven-story concrete-framed office building and a three-story concrete-framed parking garage were built on the footprint of several abandoned coal mines. The mines were excavated using the room-and-pillar method, initially leaving pillars of coal in place to support the mine roof. During retreat mining, some of the pillars were removed and replaced by temporary support measures. Based on these ground conditions permanently cased drilled shafts were chosen to support both structures. The drilled shafts were founded in the bedrock below the underground voids and coal seams at the mine levels and are cased with a permanent liner through the voids and overburden soils. BAUER Foundations Corp. (BFC) performed probe drilling at each of the drilled shaft locations in order to confirm the existence and location of the void. Following this operation, BFC installed 126 drilled shafts with diameters of 36, 42, and 54 in. and depths up to 80 ft. BFC successfully managed the excavation of the unweathered sandstone above and below the mine levels. Based on the encountered conditions of the rocks above the mine, BFC changed the initially planned method and used only a temporary starter casing and drilled the remainder of the rock above the mine level without additional support. Subsequently, the liner was seated and sealed into the bedrock and the shaft was drilled to final depth. The vast majority of the shafts were dry, so that it was possible to pour the concrete using the free-fall method. The paper focuses on the description of the experiences made as well as on the lessons learned during both the probe drilling, drilling through hard sandstone, voids and collapse zones, and concreting of the drilled shafts.