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.
In 2010, DA Smith (formerly Agapito Associates, Inc. Field Services) completed core drilling in support of rehabilitation of Cross Passage 5 in the Colorado Department of Transportation’s Hanging Lake Tunnel near Glenwood Springs, Colorado. DA Smith mobilized core-drilling equipment to characterize the ground conditions behind the shotcrete lining and to provide a basis for design of the remediation of the cross passage.
A total of 15 continuous core holes were drilled horizontally into the rib, at +45° into the spring line, and vertically into the roof of the tunnel cross passage. The coring tools used were NW conventional, 2.06-inch-diameter core, and 2.985-inch-diameter borehole.
Drilling was completed with zero discharge of drilling fluids. A summary report was prepared which included full descriptions of the core, videologs, and recommendations for rehabilitation of the tunnel.
DA Smith later conducted additional core drilling to confirm the effectiveness of remediation grouting. DA Smith installed RMT Telltales in the confirmation drill holes to measure tunnel convergence and monitor the stability of the tunnel.
In 2018, DA Smith (formerly Agapito Associates, Inc.) conducted specialized around-the-clock core drilling inside the Caldecott heavy rail commuter train tunnel operated by the Bay Area Rapid Transit (BART) authority. Exploration drilling was conducted on behalf of Earth Mechanics, Inc. for the purpose of characterizing fault-related squeezing conditions affecting concrete liner stability. Careful coordination was required with BART and others to mobilize and operate inside the active tunnel. The drill and crews accessed the tunnel and drilled off the deck of a mobile BART rail car.
High quality HQ core samples were obtained through the reinforced concrete liner, fault gouge, and fractured rock mass. Texam pressuremeter tests were conducted in the boreholes to measure the in situ stiffness of the surrounding ground.