Tom Lantos twin tunnels provide a sustainable solution to the Devil's slide problem

Recurring landslides in Devil's slide resulting in nine closures in the past 28 years, 45-minute commutes for 7.5 miles of distance for over 20,000 motorists, 995 lawsuits and injuctions from environmental and community groups, opposing to the lane expansion of the highway, composed the context in which Caltrans decided to bypass the existing highway with the construction of Tom Lantos twin tunnels. The $430 million project ensured a safe highway access between Pacifica and Half Moon Bay, it abided by the strict California environmental laws, promoted the tunneling design practice in the State and provided design standards in tunnel seismic safety.

The twin, 30ft wide and 4,200ft long tunnels spread beneath the San Pedro Mountain, having the most challenging and complex geological conditions in California. In addition to that, the high seismicity of the area, the complex ground conditions with large amounts of water anticipated during excavation as well as environmental restrictions in the area, compose the main challenges the designers had to overcome.

As a result of the length of the tunnels and the challenging ground conditions, Caltrans rejected the tunnel boring machine technique and applied the NATM [New Austrian Tunneling Method]. This staged excavation and support tunnel-mining technique relies on the observation of ground response during excavation and uses an initial and a final support system. It was applied on California ground for the first time and included, the unique in the U.S., use of a shotcrete lining with structural synthetic fiber. Numerical simulation of the tunnel included 2-dimensional analysis of the tunnel cross sections as well as 3-D structrural analysis for a typical cross-passage.

The Tom Lantos tunnels project is selected among the 5 finalists for ASCE's Outstanding Civil Engineering Achievement (OCEA) award. The award recipient is going to be announced at ASCE's Outstanding Projects And Leaders (OPAL) Gala, today, March 20, at the Renaissance Arlington Capital View Hotel in Arlington, Virginia.