Miramar 2A and Extension Pipeline and Tunnel

GEI provided geotechnical engineering services to the City of San Diego (City), under subcontract to Camp Dresser & McKee (CDM), for design of the Miramar 2A and Miramar Extension pipeline. The pipelines are about 5,400 feet in length with diameters ranging from 36 to 84 inches.

One key feature of Miramar 2A pipeline is that it crosses beneath numerous utilities including two 72-inch-diameter pre-stressed concrete cylinder pipelines (PCCP) owned by the San Diego County Water Authority (SDCWA), a 72-inch-diameter steel pipeline owned by the SDCWA, and a 66-inch-diameter PCCP pipeline owned by the City.

The Miramar 2A pipeline was installed in a 1,600-foot-long tunnel below these utilities. The tunnel was excavated through soft ground, hard rock, and mixed face conditions.

GEI provided design geotechnical engineering services that included developing a subsurface investigation program, observing the performance of borings and installation of groundwater observation wells, a seismic refraction survey, and field permeability tests. The results of the subsurface exploration program indicated the geologic conditions along the proposed tunnel were extremely variable and ranged from completely weathered (similar to a very dense soil) conglomerate to moderately fractured, but very hard volcanic rock.

GEI evaluated the geologic conditions and their impacts on tunnel construction. GEI also provided geotechnical design and construction recommendations pertaining to tunneling beneath the SDCWA pipelines, excavation support, geotechnical instrumentation, and dewatering. GEI recommended performance criteria to limit utility settlement and ground vibrations during construction. GEI consulted with the City, SDCWA, and CDM to develop warning and maximum values for existing pipeline settlement to ensure that tunneling was performed without damaging the utilities.

GEI’s construction phase services for the project included submittal review, field observation to document tunneling operations, detailed geologic mapping during construction, and review and evaluation of geotechnical instrumentation data to monitor ground movements and their effects on existing pipelines above the tunnel. The tunneling was performed 20 hours a day, 5 days a week. GEI performed field observations without adversely affecting the construction schedule. Tunnel excavation, pipe installation, and tunnel grouting were performed without damaging the existing pipelines.