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Void Filling

Jefferson, PA: Grout Curtain By Pressure Grouting Old Dilworth Mine


Geo-Con was contracted to perform core drilling, water pressure testing and low-pressure grouting at the Dilworth mine site in Jefferson, Pennsylvania. The goal of the grouting program was to seal mining induced fractures in the underlying bedrock and prevent migration of surface groundwater to the mine. Overall, the objective was to enhance surface water flow in a nearby stream. A series of grout curtains were installed at specific locations and to specific depths to address subsurface flow of groundwater.

Core holes were installed using a CME 45-300 rotary drill equipped with a wire-line double core barrel system and HQ size (3.75-inch) drill bit. The holes were drilled at a 10-degree angle and in split-spacing pattern. Borings were drilled approximately 100 to 165 feet below ground surface in effort to target the highly fractured rock zones above the long wall mine.

Water pressure tests were conducted in 15 feet ascending stages to assess the permeability of the rock. Water passed through a header system configured with flow control valves, flow meter, and pressure gauge prior to delivery to the test stage. The test stage interval was isolated using a water packer apparatus that consisted of two straddle-style pneumatic packers separated by perforated pipe. Test pressure was set at ½ psi per foot of depth. Water flow rates, pressure, and gallons of water pumped were recorded once a steady state of flow was achieved and used to calculate the permeability of the rock.


Grouting of the bedrock progressed in 15-foot ascending stages and was accomplished with a High Mobility grout containing Portland cement and bentonite. Grout passed through the header system prior to delivery to the target grout stage. Stages were isolated with a grout packer apparatus that consisted of a single pneumatic packer attached to the bottom of a steel pipe.

Grout was injected at 1 psi of pressure per foot of depth until refusal of the stage was achieved. Difficult or high-take stages were addressed with a Medium Mobility Grout containing Portland cement, bentonite and fly ash. Grout flow rates, pressure and total gallons of grout pumped were recorded and used to generate real-time Lugeon values.

Geo-Con worked closely with the project team and a world renowned grouting expert to interpret the water pressure testing and grouting data as the project progressed, and to adjust the grouting program to most effectively grout the remaining fractures. The project was challenged by steep terrain in a heavily wooded area. Geo-Con was able to build access roads and cut benches into the existing contours to allow access for the drill rigs and grout mixing equipment.


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