Central Contra Costa Sanitary District (CCCSD) and the City of Concord (City) teamed with Carollo Engineers, DCM-GeoEngineers and Bennett Trenchless Engineers to design and manage the construction of a new interceptor sewer that will eliminate the need for a 50-mgd pumping station.
The A-Line Relief is a six-mile long gravity relief interceptor sewer located in the northern portion of CCCSD’s service area and will be constructed in phases over the next 15 years. The current phase of design and construction includes approximately 3,050 ft of 96-in., 870 ft of 72-in., and 540 ft of twin 48-in. RCP under Walnut Creek. This gravity tie-in will eliminate the largest wastewater pump station in the county. The new pipelines were constructed through a variety of surface conditions including a municipal golf course and driving range, under the approach landing path of a municipal airport, across a major regional recreation trail, busy city streets in a fully developed commercial neighborhood and under the earthen Walnut Creek flood control channel.
The pipelines were installed by three different methods including pipe jacking by earth pressure balance machine (EPBM) tunneling (approximately 3,050 ft), microtunnel boring machine (MTBM) tunneling (approximately 870 ft) and conventional open-cut trenching (approximately 540 ft) across the Walnut Creek flood control channel. The EPBM pipe jacking of 96-in. concrete pipe occurred in two drives of 1,350 and 1,700 ft from a common jacking shaft, and as many as five to seven intermediate jacking stations were installed in each of these two drives. The MTBM pipe jacking for the 72-in. concrete pipe occurred in two drives of 410 to 460 ft from a common jacking shaft.
Ironically, due to the limited amount of pipe cover within the 350-ft wide Walnut Creek channel, the channel crossing and tie-in to the existing Concord pump station was constructed by open-cut construction. Normally trenchless construction would be a preferred option for an environmentally sensitive creek crossing. Construction of the A-line Phase 2A Relief Interceptor was awarded to general contractor Mountain Cascade of Livermore, Calif., for $20.65 million. The tunneling subcontractors were Pacific Boring and Nada Pacific of Caruthers, Calif., for the EPBM and MTBM tunneling, respectively.
“Ultimately, the project’s achievements were a result of a complete and sound design by the engineer, the expertise and skill of an experienced contractor and the dedication and cooperation among all parties in resolving the unexpected,” said Alex Rozul, project manager for CCCSD.
The project has several key accomplishments that make it outstanding. The single most critical construction challenge occurred when the second 72-in. microtunnel drive was installed at the wrong grade (off by 2.8 ft).This error occurred when the laser guidance system was set incorrectly by a factor of 10.
The corrective method chosen was to pull back the entire 72-in. RCP while backfilling the void space created with a cellular concrete with a wet cast density of 40 lbs and a 28-day compressive strength of 300 psi. The 72-in. RCP was retracted at a rate of 40 ft per day, allowing the grout to have sufficient cure time. Once the pipe was completely retracted, the contractor tunneled through the cellular concrete and installed the pipe successfully on the correct grade.
Owner: Central Contra Costa Sanitary District
Engineer: Carollo Engineers, DCM-GeoEngineers and Bennett Trenchless Engineers
Contractor: Mountain Cascade Inc.
Sub Contractors: Pacific Boring and Nada Pacific
Manufacturers: Akkerman Inc. and Ameron International