Keller constructed a low permeability grout curtain surrounding the CSO (combined sewer overflow) reservoir to protect the surrounding groundwater.
A part of the Tunnel and Reservoir Plan (TARP), the north lobe of the Thornton Quarry was converted into a reservoir with the capacity to retain nearly 8 billion gallons of CSO prior to treatment. To reduce the permeability of the perimeter rock formation, thus mitigating leakage into the groundwater, a double-row grout curtain was designed along the 9,800 LF perimeter.
Keller was contracted to reduce the permeability of the bedrock to an approximate value of 1 Lugeon by drilling holes with strict alignment requirements and injecting a suite of balanced and stable grouts.
Keller drilled approximately 800,000 feet of double-row grout curtain holes to depths ranging from 20 feet to 575 feet. Drilling was performed at angles varying from 0 degrees to 65 degrees from vertical, using a drill rig equipped with a water-actuated down-the-hole hammer. Upon completion of drilling, Keller grouted the rock in 20-foot stages.