Devil’s Slide Tunnels

California Department of Transportation (CALTRANS) District 4

Twin highway tunnels were constructed through San Pedro Mountain, bypassing a stretch of US Highway 1 that is notorious for landslides. The project also included 11 cross-passages linking the tunnels for fire life safety, and three underground equipment rooms.

The tunnels were constructed using New Austrian Tunneling Method (NATM) with five different support categories for the main tunnels. Support categories were selected in the field in accordance with ground support requirements. The tunnels feature shotcrete linings and rock reinforcement for initial tunnel support, aided by a set of pre-support elements that include spilling and pipe arch canopies. An open (drained) PVC membrane based system was installed for waterproofing. Final tunnel support was accomplished by a reinforced cast-in-place concrete lining. Jet fans provided longitudinal ventilation.

Integrated On-Site Team (IOST) Support Services for NATM Engineering & Construction:

Gall Zeidler Consultants provided IOST support services including preparation of shop drawings, ground structure interaction analyses, evaluation of ground conditions in the field and selection of ground support categories, as well as the evaluation of NATM instrumentation and results monitoring and the provision of senior advisory field staff.


San Pedro Mountain on US Highway 1, Between Half Moon Bay and Pacifica, California


2006 – 2010


Highway Tunnels


8,000 feet (2,240 meters) Total Length (Two Bores)


37.4 feet (11.4 meters) at Springline


Highly Tectonized and Chemically Altered Granite, Granodiorite and Sandstone with Multiple Faults with Gauges; Groundwater Table Above Tunnel


Approximately $268 Million


Kiewit Construction Company


California Department of Transportation (CALTRANS) District 4