The San Marcos Interceptor Sewer is a major sanitary sewer transport line which combines with other lines to convey flows to the Encina Water Pollution Control Facility in Carlsbad for treatment. The replacement of the San Marcos Interceptor provides additional capacity for development projections expected in the areas. The project was initially designed in 1999, but was put on hold over environmental impact concerns. More recently, improvements along San Marcos Creek and the economic downturn caused additional delays. The project was designed in two phases – Phase 1 consisted of replacement of the existing 21-inch diameter sewer tunnel with two tunnel portions and cut and cover installation of new pipelines. Phase 2 will extend the work of Phase 1.
The scope of work for this project included installation of approximately 2,150 feet of sewer pipeline in the City of San Marcos, California using microtunneling and open cut methods. The first microtunnel drive under State Route 78, was 286 feet of 52-inch steel casing with 36-inch fiberglass carrier pipe. The second drive was 372 feet of 48-inch Hobas® centrifugally cast, glass fiber reinforced polymer mortar pipe. After completing both of the microtunneling drives, the annular space between the ground and the pipe was grouted. The open cut portion of the project included 1,485 feet of 36-inch Hobas centrifugally cast, glass fiber reinforced polymer mortar sewer pipe.
The remainder of the project included abandoning/removing manholes and sewer mains and patching and sealing parking lots impacted by the construction.
The soil conditions for the entire alignment included mixed face conditions with moist, dense clayey or silty sand with some weathered granitic rock.
The project was located adjacent to residential areas and required settlement monitoring during the sheet pile shaft construction and microtunneling to ensure the structures were not damaged by the construction activities.