Lessons Learned from NATM Design and Construction of the Caldecott Fourth Bore

The Caldecott Fourth Bore is a 15.2 m wide (50 ft), 1,036 m long (3,399 ft) highway tunnel with seven cross passages located on State Route 24 in the San Francisco Bay Area, excavated through weak, highly fractured, and sheared sedimentary rock forma- tions. This paper describes key observations and lessons learned from the design and construction of this NATM (New Austrian Tunneling Method) tunnel, including:

•  Predicted versus observed ground behaviors and support performance based on direct observation and measured convergences
• Variations in tunnel production rate by support category
• Contractual considerations regarding support selection criteria
• Installation and performance of a 52 m long (170 ft) pipe canopy
• Effects of tunnel construction on slope stability