Servicing fast-growing Loudoun County, a suburb of Washington, DC, Loudoun Water started its Potomac Water Supply Program to provide customers a safe, reliable, and sustainable supply of drinking water. One of the program’s key components is the banking of raw water in retired quarries for future use. Quarry A is the first such location, and Schnabel is lead geotechnical engineer and tunnel designer on the project.

The project involves converting Quarry A to a pumped storage reservoir that will hold over 1 billion gallons of water. Elements include a new pumping station, a plug for an existing tunnel connecting two quarry cells, and two raw water transmission mains. The 36-inch and 42-inch-diameter transmission mains will cross under a scenic trail and creek adjacent to a historic bridge. To house them, we designed a 560-foot-long tunnel at depths of up to 80 feet, with two shafts connecting the pipelines back to shallow grades.

We also designed a 260-foot-deep intake shaft and three intake tunnels for the pump station, the tunnel plug, permanent slope stabilization around the quarry rim, and an instrumentation program to monitor rock mass conditions during construction and reservoir filling. To inform the design of pump station foundations and related site improvements, as well as tunnels, shafts, and slope and rock wall stability, we oversaw a multiphase subsurface investigation involving both shallow and deep borings.