- Twin 2,600′ bundles under Mare Island Strait
- 12″ rec’d water, 22″ sewer, two 4″ conduits
- Two 18″ water, two 4″ conduits
- Young Bay Mud over Old Bay Mud
- Claystone bedrock
- Possible contamination from Naval Yard
- Soil to rock interface
- Hydrofracture risk in soft surficial soils
- Proximity of bridge piles for causeway
The City of Vallejo and the Vallejo Flood and Wastewater District collaborated on a project to install various utilities to from Vallejo to Mare Island under the Mare Island Strait. The utilities to be installed were a 22-inch sewer force main, a 12-inch reclaimed water main, two 18-inch water mains, and four 4-inch conduits for fiber optic cables. Ground conditions included 40 to 50 feet of very soft Young Bay Mud underlain by denser Old Bay Mud and claystone bedrock.
Preliminary design evaluated various trenchless methods to install the various utilities, including microtunneling and horizontal direction drilling. Due to the length and required separation of the sewer and water lines, parallel twin 2,600-foot by 48-inch diameter HDD bores were recommended to install the utility bundles. The design solution used the intersect HDD method with HDD rigs on each side. Approximately 150-feet of conductor casing was required on each end of the bores to maintain circulation and reduce hydrofracture and settlement risks in the very soft Young Bay Mud. In addition to the challenging ground conditions, BTE carefully selected alignments which did not endanger bridge piles and fender piers of the Mare Island Causeway. Soil and groundwater contamination were also significant concerns due to the Navy’s operations at the former Mare Island facility.
Bennett Trenchless Engineers helped to plan the geotechnical investigation, evaluated results and implications for construction, prepared technical specifications and drawings, conducted design calculations to evaluate and mitigate risks associated with hydrofracture, conducted settlement calculations, evaluated pipe stresses during pullback, and provided engineering services during construction, including on-site specialty construction inspection.