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Seepage Barriers/Grout Curtains


Washington, PA: Dam Foundation Pressure Grouting, Grout Curtain Installation

dam-seepage-barriers-grout-curtains-washington-pa

The project was performed for a confidential client to mitigate seepage from the dam foundation and abutment. The dam was constructed approximately 30 years ago and was intended to create an approximately four-acre impoundment for recreational use. The Dam is licensed as a Category 3 Dam by the PADEP. Seepage was first observed at the downstream toe of the Dam approximately 20 years ago. While several interim repair measures were taken to reduce seepage, it has steadily increased over the years.

Grout borings were installed at a 30-degree angle (from vertical) to maximize the likelihood of intercepting high-angle fractures and joints. Casing was installed through the overburden using appropriate casing-advance methods and was grouted in place. Borings were drilled with a rotary drill rig utilizing a destructive bit with water-only flush to approximately 66-feet below ground surface. This drilling method produces a smooth walled hole to facilitate pressure testing and grouting operations.

Water pressure tests were conducted in 20-foot ascending stages to assess the permeability of the rock. Water passed through a header system configured with flow control valves, and flow/pressure transducers, prior to delivery to the test stage. The pressure and flow transducers were linked to a specialized real-time monitoring system. Information was transmitted from the header to a central computer system, which displayed, in real-time, the pressure and flow readings in graphical format. The system also recorded and logged the pressure/flow readings in a file archive. The test stage interval was isolated using a water packer apparatus that consisted of two straddle-style pneumatic packers separated by perforated pipe. Borings were water tested with single and multiple pressure five-minute tests in 20-foot ascending stages. Water flow rates and total takes were recorded and used to generate total Lugeon values for each stage.

Grouting of the bedrock progressed in 20-foot ascending stages. Geo-Con and the engineer established a grouting program that included progressive grout viscosity adjustment and flow rate limitations in response to real-time assessment of grouting parameters. The goal of the program was to achieve efficient refusal/closure of fractures while maintaining a limited grouting reach in each boring. 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 an aluminum pipe. Difficult or high-take stages were addressed with a Medium Mobility Grout containing Portland cement, bentonite and fly ash. Grout flow rates, apparent lugeon, pressure, and total gallons of grout pumped were recorded and displayed by the real-time monitoring system.

Geo-Con worked closely with the engineer to respond to individual boring Lugeon values and grout takes and make real-time decisions regarding additional boring locations to address the most problematic areas within the abutment to maximize the effect of the curtain and remain within the intended budget.



 

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