Folder: S.F. Bridges-Bay-Construction-Yerba Buena Tunnel.
Printed on back: "Here is a novel method of excavating the world's largest bore tunnel conceived by Chief Engineer C.H. Purcell of the San Francisco-Oakland Bay Bridge and his staff, the novelty of which consists chiefly in that they first build the tunnel and then dig it. Three bores, each 14 by 14 feet were drilled through the mid-Yerba Buena Island for the double-deck vehicular tunnel for the San Francisco-Oakland Bay Bridge. The three bores, two at either lower side and one in the crown, are blocked out into a horseshoe-shaped excavation through the rocky island. This horseshoe-shaped excavation is then concrete and steel lined from two to four feet thick before the inside or core of the tunnel is dug out...".
