Project Update by by Ashim Gajurel and Tom Pennington, PE
The East Bay Municipal Utility District (EBMUD) has completed the first phase of its planned upgrades to existing chemical facilities at the Pardee Reservoir in Valley Springs, California. The project includes construction of two 250-foot-deep (76 m), 16-inch-diameter (405 mm) stainless steel chemical feed shafts that penetrate the crown of the existing 8-foot-diameter (2.4 m) Pardee Tunnel. In addition to design services, McMillen Jacobs Associates has been providing engineering services during construction to EBMUD. The construction contract was awarded to Force Drilling LLC of Mt. Morris, Pennsylvania.
The project faced a unique challenge in that the Pardee Tunnel, one of the primary conveyance facilities in EBMUD’s raw water transmission system, could not be taken offline or unwatered because of high daily demand for water supply and the high cost and complications of obtaining water from alternate sources (such as other local reservoirs). This required that the new chemical feed shafts be installed remotely from the ground surface and with virtually no personnel access to the Pardee Tunnel where the shafts were planned to penetrate, 250 feet (76 m) below. Complicating the installation was a tight construction window that required shaft installation be performed within a reduced flow period in the tunnel (from January 2 to February 28). These flows were reduced from approximately 130 MGD (492 ML/day) to 40 MGD (151.4 ML/day), which allowed for diver access to remove debris from the tunnel and provide close-up inspection of the shaft penetrations. This reduced flow also allowed for near-continuous inspection of the shaft penetrations in the tunnel by live video feed from a remotely operated vehicle (ROV) installed within the tunnel.
Construction of the shafts began in December 2020. First, an 8-inch-diameter (205 mm) pilot hole was drilled by directional drilling techniques to within 5 feet (1.5 m) of the tunnel. Once the bore was verified to be within tolerance, it was then successively reamed to diameters of 15 inches (380 mm) and 22 inches (560 mm). Prior to shaft penetration, the area just above the tunnel was grouted to seal up potential voids and improve ground quality at the shaft and tunnel interface. Then, the tunnel was penetrated utilizing a custom contractor-designed 22-inch core barrel to ensure that the rock and concrete core could be retrieved from the shaft and to minimize the amount of debris that would enter the tunnel. Once penetration was made, the 16-inch (405 mm) casing was installed in 20-foot (6.1 m) sections. When the final termination depth was achieved, a packer was inflated to create a seal and the annular space between shaft and casing backfilled with grout.
After drilling, the maximum deviation of the shafts was about 0.008% (0.245 inch) from the planned alignment, which was well within the 0.1% vertical tolerance requirement. Both shafts penetrated the Pardee Tunnel within 2 inches (50 mm) of tunnel centerline. The location of the shafts with respect to the tunnel is crucial as it will allow the future chemical feed lines to operate effectively and ensure proper mixing during full tunnel operation.
Despite the successful outcome, the team faced several construction challenges. During penetration of Shaft 1 into the tunnel, it was discovered that the tunnel concrete lining contained reinforcing steel. This was unexpected and required that construction divers enter the tunnel to remove debris and cut the rebar before shaft installation could be completed. Additional challenges were faced during construction of Shaft 2, where much weaker ground was encountered above the tunnel. As a result, the construction sequence was modified and additional consolidation grouting performed to fill potential voids and improve the integrity of the ground. Unexpectedly, grout leakage was observed into the tunnel during grouting nearly 30 feet (9 m) above the tunnel, and ROV footage confirmed that a small 3-inch (75 mm) hole in the tunnel lining was the leakage source. The hole was patched by construction divers, and shaft installation continued. After lowering of the shaft casing, a void, approximately 3-feet-high by 3-feet-wide (0.9 m x 0.9 m), was discovered just above the tunnel lining. To fill the void, construction divers installed oakum and epoxy sealant, in addition to the inflatable packer installed at the bottom of the shaft. Following testing and multiple inspections by construction divers, the packer was inflated successfully and the shaft casing grouted in place, filling the void without any further leakage of grout into the tunnel.
Backfilling of the shafts was completed and the chemical feed lines were successfully flow tested in March 2021. Future improvements to the site include upgrades to the existing lime and CO2 facilities, which are scheduled to start construction in 2022.
Read More: Pardee Chemical Feed Shaft Project