Summer Brings Caldecott Fourth Bore Milestone: Breakthrough Part 2

Two years after the start of tunneling, the team overseeing the Caldecott Fourth Bore project is reporting that boring of the massive hole through the East Bay hills is essentially complete. Last week, on the evening of Wednesday, August 8, 2012, miners finished excavating the lower level of the Fourth Bore, known as the “bench” in engineering parlance, and the tunnel is now nearly 50 feet wide and 40 feet tall for its entire 3,248-foot length through the East Bay hills. The latest milestone could be called “Breakthrough Part 2,” since the first such meeting of eastbound and westbound drilling occurred some eight months ago when excavation of the top layer of the bore was completed in November 2011.

At the same time as completing excavation of the bottom layer of rock separating the two sides of the bore, crews have begun the process of finishing the tunnel’s walls. They are applying a waterproof membrane, rebar strengthening and a final smooth layer of concrete, working their way from the west side of the bore to the east. And while one could now technically walk from Orinda to Oakland through the fourth bore, there’s still more excavation work ahead to shave the floor in certain areas. Known as the invert and measuring about eight feet deep, this depression in the floor is an engineering strategy to improve rock stability at the west end, and to reduce groundswell from moist soil at the east end.

Overall, construction of the Caldecott Fourth Bore is now 70 percent complete, and the project is on target to open in late 2013. MTC is a major funder of the Fourth Bore project, which will bring congestion relief to the perennially clogged Caldecott gateway between Alameda and Contra Costa counties. Also in the news this month is the selection of the student designers for the Caldecott Fourth Bore decorative medallions.

— Brenda Kahn

See also:

• Caldecott Tunnel Fourth Bore Website