Central Contra Costa Sanitation District,
A-Line Interceptor Project
Innovative Construction Techniques Deliver Critical Pipeline in Concord, California
Essential Sewer Line Replacement
The Central Contra Costa Sanitary District’s (CCCSD) 2000 Collection System Master Plan identified the need to parallel the existing A-Line Relief Interceptor to provide wet weather capacity. Carollo Engineers provided planning and design for the overall project, which included installation of approximately 36,500 feet of 66-inch through 102-inch concrete pipe. A joint project undertaken by CCCSD and the City of Concord, the A-Line Phase 2A extends the interceptor from the Buchanan Fields Golf Course to the intersection of Meridian Park Boulevard and Galaxy Way in Concord, California. The project replaces an existing 39-inch-diameter force main from the Concord Pump Station. The 30-year-old force main cannot be taken out of service for inspection or maintenance and offers no options for wet weather overflow. The new 4,400-foot, larger capacity gravity interceptor will allow CCCSD to decommission the Concord Pump Station, which is one of the largest sewer pump stations in Contra Costa County. This will save CCCSD over $300,000 in electricity, staff power, chemicals, and maintenance each year.
Minimizing Project Impacts
Because the project's path runs directly through a highly developed, high-traffic urban area, Carollo implemented an action plan that integrated innovative construction techniques with a solid knowledge of local conditions and stakeholder concerns to tackle a variety of challenges. According to CCCSD senior engineer, Alex Rozul, "This project was complicated not only by various terrain issues and soil conditions, but also by the large number of stakeholders impacted by construction." Affected stakeholders included the City of Concord, area businesses including bigbox home stores and government offices, and local residents. Further complicating the project was the presence of existing interceptors, storm drains and fiber optic cables, as well as challenges posed by major intersections, sequencing and tie-ins, and a stream crossing. To address these issues, Carollo evaluated a variety of construction methods. Due to the depth and magnitude of shoring required for open-cut construction, as well as the disruptive nature of this technique in heavily-traveled areas, Carollo chose trenchless construction strategies, such as tunneling and microtunneling as the lowest-cost, most effective option.
The EPBM excavates the soil at the cutting face and transports the soil via ribbon auger to muck carts and haul units.
Carollo engineered custom solutions for each site-specific challenge, especially where trenchless construction methods were warranted, including:
- Implementing a monitoring plan to address settlement in areas of shallow earth cover.
- Deploying an earth-pressure balanced machine (EPBM) to address ground subsidence and tunneling under groundwater pressure.
- Obtaining regulatory approval for the complicated sequencing of an open-cut sewer line crossing beneath portions of Walnut Creek, California.
- Devising underground structures, including a flow metering structure and junction structures along the length of the pipe, to advance the overall tunneling approach.
Longest EPBM Push in California History
The project features the longest EPBM push in California history. The tunneling contractor used an Akkerman Inc. manufactured 116-inch EPBM and intermediate jacking stations to reduce jacking force by distributing thrust on long drives. The EPBM excavates the soil at the cutting face and transports the spoils via ribbon auger to muck cars and haul units.
As the EPBM advances, a 20-foot length 96-inch-diameter RCP section is placed in the jacking frame and pushed forward. The tunneling contractor's scope of work entailed pipe jacking 3,000 feet of 96-inch pipe from Buchanan Fields Golf Course to the intersection of Meridian Park Boulevard and Galaxy Way in two drives just west of downtown Concord. The first drive was 1,300 feet and will bore under the Buchanan Field Golf Course greens, fairways, and driving range. This 9-hole, par 31 executive golf course experiences steady traffic year-round. Maintaining expert tunneling line and grade is necessary as to not disrupt the green just 25 feet above. The second drive is 1,700 feet and runs under Meridian Park Boulevard, a congested commercial area. The EPBM used the same 25-foot-deep, octagonal launch shaft to complete both drives. Upon completion of the first drive, the EPBM launched again 135 degrees from the first entry point for the second drive. Work also included connecting the 96-inch RCP to 72-inch RCP in two drives completed by the microtunneling contractor. This included installing approximately 870 feet with an Akkerman MTBM SL60 with an increase kit and MT890 jacking frame along side a Home Depot store. The 72-inch pipeline connects to 540 feet of 48-inch twin pipelines which was open cut 540 feet between the banks of the Walnut Creek channel. CCCSD will test it effectiveness in a wet season cycle to ensure its productivity.