Bennett Trenchless Engineers provided specialty construction management services for BKF for installation of a 12-inch FPVC water main inside a 16-inch FPVC casing beneath the water access for Bethel Island using HDD. The project provided water services from Diablo Water District to a new waterfront residential development on Bethel Island. The trenchless crossing passed beneath the waterway access to Sand Mound Slough for the future Bethel Island residents. The waterway, which had levees on both sides, had a water level approximately 15 feet higher than the ground elevation outside the levees. The levees had been built on top of 50-foot deep slurry walls to reduce permeation seepage. The levees were further supported in the weak surficial soils with a grid of 3-foot diameter stone columns that extended approximately 20 feet below the natural ground level. The trenchless crossing was designed beneath the stone columns and penetrated the slurry walls on both sides of the crossing. Conductor casings were necessary on both sides of the crossing to maintain bore stability and ensure thorough annular space grouting to prevent seepage along the bore annulus. The southern side of the crossing was located in a residential community; maintaining traffic through the work zone resulted in highly constrained work areas.
Diablo Water District requested a specialty inspector for the project to ensure that the pipeline would be installed to their specifications. BTE provided specialty trenchless inspection services to the design engineer, BKF. BTE reviewed contractor’s submittals for the HDD components and responded to RFIs related to the trenchless work. BTE provided full-time on-site specialty inspection services for the conductor casing installation, pilot bore, reaming, pullback, testing, and annular space grouting operations. On-site inspectors monitored down-hole pressure data, pull loads and pipe stresses, and for inadvertent drilling fluid returns. The project was successfully completed in November 2016.