Project Update: Pardee Chemical Feed Shaft Project

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Project Update by Pedram Bemani, PhD, PE, Jenn Sketchley, PE, and Tom Pennington, PE

The East Bay Municipal Utility District (EBMUD) is upgrading its water treatment plant facility at the Pardee Reservoir in Valley Springs, California. The Pardee Chemical Feed Shaft Project is part of the chemical facility upgrades that will increase raw water treatment capabilities and provide corrosion protection for 90 miles (145 km) of tunnels and aqueducts before treatment and distribution in the East Bay Region of Northern California. The project involves construction of two 255-foot-deep (77 m), 16-inch-diameter (405 mm) stainless steel chemical feed shafts that will penetrate the crown of the existing Pardee Tunnel near the chemical facility. McMillen Jacobs Associates is providing engineering services to EBMUD, including completion of the detailed design of the shafts and tunnel penetrations, risk management, bid support, and engineering services during construction.

Shaft construction will use top-down, blind bore drilling methods, penetrating the existing 8-foot-diameter (2.4 m) horseshoe-shaped tunnel while it is in operation. Construction measures are being planned to eliminate the need to access the tunnel during shaft construction, thereby reducing costs and logistical challenges of taking the tunnel out of service. Given the depth of the shafts and the requirement to intercept the tunnel within inches of the tunnel crown, the use of a directionally guided drilling methods, as well as closely monitored alignment checks, will be required to ensure the shafts intercept the tunnel at the planned location.

Targeted consolidation grouting will be performed around the tunnel from within the bottom of the pilot bore to fill potential voids that may be present. The last 5 feet (1.5 m) of the shafts will be drilled by coring methods to minimize the risks of overexcavation, tunnel lining damage, and the debris quantities that would fall into the tunnel. Once the shafts penetrate the tunnel, the shaft and feed line casings will then be lowered into the shafts and an annulus seal will be installed at the tunnel connection to permit annulus grouting of the shaft casing. The most critical stages of the construction will be monitored using remotely operated vehicles (ROVs) inside the tunnel. Debris removal and potential repairs of the tunnel connections, if needed, would be performed using construction divers. The RFP is anticipated to come out during the first quarter of 2020.