In 2017, DA Smith (formerly Agapito Associates, Inc. Field Services) conducted specialized around-the-clock core drilling for tunnel stability assessment inside a 9-mile section of a major northern California metropolitan water supply tunnel (confidential identity). DA Smith fabricated customized drill trailers and modified two existing underground drills to fit through the tunnel’s constricted penstock entrance and operate inside the confined space of the small-diameter tunnel. The drills were powered by portable diesel-electric power packs.
The rock mass was cored (HQ3 diameter) at multiple locations along the tunnel alignment to characterize surrounding rock mass properties. A second drill was used to sample multiple, shallow 4‑inch-diameter cores through the tunnel’s concrete liner. The deeper holes were pressure grouted upon completion to restore the hydraulic integrity of rock mass surrounding the tunnel. Special care was taken to preserve cores for later engineering analysis. Work was completed successfully within a tight operating schedule.