This is the final installment of a series about the 5 finalists for ASCE’s Outstanding Civil Engineering Achievement (OCEA) award. Established in 1960, the OCEA Award recognizes a project that makes a significant contribution to both the civil engineering profession and society as a whole. The winner of this year’s OCEA award will be announced at ASCE’s Outstanding Projects And Leaders (OPAL) Gala, March 20, at the Renaissance Arlington Capital View Hotel in Arlington, Virginia. Today, read about the Tom Lantos Tunnels at Devil’s Slide. The project also was featured in Civil Engineering magazine.
Located in a region that is steep and in an unstable geological formation, the Tom Lantos Tunnels at Devil’s Slide project was designed to replace the infamous coastal stretch of California’s State Highway 1. Hugging the coastline between Pacifica and Half Moon Bay, this roadway had a long history of closure due to recurring rockslides.
After years of studies, Caltrans [California Department of Transportation] decided to bypass what is known as the Devil’s Slide with 2 inland tunnels, providing a safe, dependable highway access between the 2 cities. However, the tunnels would need to be constructed beneath the San Pedro Mountain, considered one of the most complex geotechnical settings in California.
When completed and opened for traffic on March 26, 2013, the $430 million Tom Lantos Tunnels at Devil’s Slide marked the first highway tunnels constructed in California since 1964. Among the project’s legacies is that it established a safe, permanent connection between Pacifica and Half Moon Bay; it met tough California environmental laws; it advanced the tunneling practice in California; and it helped set the standard in tunnel seismic safety.
ASCE News Associate Editor Doug Scott interviewed Yung-Nien Wang, P.E., M.ASCE, project manager of the Tom Lantos Tunnels at Devil’s Slide and associate vice president at the HNTB Corporation.
1. What is the most innovative or creative aspect of your project?
The Tom Lantos Tunnels project employed 4 innovations: Number one, it was the State [of California’s] first use of NATM [New Austrian Tunneling Method]. The length of the tunnels and the challenging ground conditions did not lend themselves to a tunnel boring machine technique. Instead, Caltrans chose to use the NATM, a sequential excavation and support tunnel-mining technique. Second, the flexible, adaptable NATM relies on observation of ground behavior during excavation, which facilitated unprecedented in-field team collaboration. Third, the shotcrete lining with structural synthetic fiber is unique in the U.S. NATM incorporates the strength of the surrounding rock during construction and uses 2 types of support: an initial lining and a final lining. And, finally, in addition to 2-dimensional numerical models of tunnel cross-sections, the project team prepared 3D structural analysis models for a typical cross-passage and the transition of the tunnel to the north portal.
2. What was the biggest challenge?
The Tom Lantos Tunnels were constructed under one of the most complex settings in California and the location presented 4 major technical challenges. The first was variable rock formations. As part of a geotechnical baseline report, the project team collected rock data during the early stages of the design process by drilling several borings along the proposed alignment and extracting rock cores. Second was [a] high seismic zone. Because the tunnels lie along the active San Andreas Fault and 4 inactive fault lines, the subsurface structures comply with some of the toughest seismic criteria in the world. They had to withstand a 7.5 to 8.0 magnitude earthquake, the largest geologists estimated for this arm of the San Andreas Fault. Third was mixed ground conditions. The groundwater table lies above the tunnel alignment, and original projections showed that excessive amounts of groundwater were anticipated during construction. Fourth, the tunnels were in an environmentally sensitive area. As if the rock formations, seismic activity, and ground conditions weren’t challenge enough, federally protected peregrine falcons nested near the construction site and California’s endangered red-legged frogs were found living in the Shamrock Valley wetland area. The alignment of the tunnels was carefully chosen to respect these environmental concerns.
3. What time and budget challenges did your project have and what did you do to overcome them?
In the early 1980s a major landslide and road closure prompted a study to replace the coastal road. However, a tunnel alternative was rejected for reasons of cost. In 1995 lawsuits and injunctions brought about by environmental and community groups opposed to a proposed bypass of 4 to 6 lanes led to a federal mandate: Caltrans would have to reconsider a tunnel alternative. After a report determined that a tunnel would be a reasonable and feasible alternative, San Mateo County voters approved by 74% a ballot measure in November 1996 for adopting the tunnel alternative. Tunnels are rare in California, where real estate is plentiful and relatively easy to obtain, making roads and bridges the more affordable option. However, after further environmental studies were conducted from 1996 to 2001, Caltrans opted for a tunnel. In 2001 Caltrans retained HNTB to prepare plans, specifications, and estimates for tunnel construction. On September 13, 2002, the Federal Highway Administration issued a record of decision authorizing Caltrans to proceed with final design of the tunnel alternative. Construction of the tunnels through San Pedro Mountain took about 2 years longer than expected, largely because the NATM tunnel-drilling crews encountered unanticipated and unusual soil conditions that slowed the work. The cost, paid entirely from federal funds, increased accordingly.
4. Sustainability is one of the 3 strategic initiatives here at ASCE. Describe how your project adheres to being sustainable.
The Tom Lantos Tunnels project addressed major environmental, social, and economic sustainability issues: In terms of the environment, federally protected peregrine falcons nested near the construction site and endangered red-legged frogs were found living in the Shamrock Valley wetland area. To protect the endangered frog habitat, no equipment was allowed to enter the wetland area. Crews cordoned the area with frog fences and conducted extensive wetland monitoring to ensure the frogs’ safety. To minimize the project’s impact and to connect the north portal to State Route 1 without disrupting the wetland area, twin balanced, cantilevered, cast-in-place segmental concrete box girder bridges were constructed. Next were social and economic issues. After a heavy winter rain, residents of Pacifica and Half Moon Bay, California, would wake up wondering if the highway they depended on daily still would be there, or if it had slid once again into the Pacific Ocean. The Devil’s Slide had been closed 9 times in the past 28 years, with the longest closure lasting 158 days. Commutes between the 2 cities, an easy 7.5 miles normally, became 45-minute slogs for the 20,000 motorists who traveled the route daily to visit family members or get to schools, jobs, healthcare facilities, and retail establishments. The economies of both cities suffered. Tunnel portals were designed to meet context-sensitive aesthetic and safety requirements, and were approved by both the California Coastal Commission and local commissions. Each end of the tunnels is different. The north portals’ architectural texture matches the north mountain terrain and the south portals blend with the south mountain terrain. HNTB worked with Caltrans’ chief architect and created 20 different design concepts before the Coastal Commission approved what exists today.