Contractor Implementation of SEM Tunneling and Comparison Between Design and Observed Ground Conditions, Caldecott Tunnel Fourth Bore, San Francisco Bay Area, California

The Caldecott Tunnel Fourth Bore was mined using Sequential Excavation Method (SEM) tunneling. The SEM design identified 18 Rock Mass Types (RMT) grouped into seven Ground Classes that correlate directly with seven Support Categories. Selection of Support Category was implemented by a cooperative and interactive decision-making process between the Contractor’s and Owner’s geologists. Initial agreement established at the tunnel heading was typically adopted during daily SEM meetings. This paper gives an overview of the design RMT’s, Ground Classes and Support Categories, followed by a discussion from the Contractor’s perspective that compares the design basis and actual observed geologic conditions, ground behavior and initial support during construction. Tunnel construction resulted in a different distribution of Ground Classes than the design.