Tunnelling Begins at Devil's Slide in CaliforniaDevil's Slide, a winding and precarious stretch of California's Highway 1 which soars above the Pacific Ocean, was carved into the mountain. Almost since it opened in 1937, it has been the site of landslides, rockslides, mudslides and weather-related troubles that have caused significant road closures (47 days in 1938, 93 days in 1951, 25 days in 1952, 200 days in 1979, 84 days in 1983, 158 days in 1995 and 124 days in 2006), forcing drivers to take long, slow detours on Highway 92 to Interstate 280.The solution, discussed in detail since 1996 when voters overwhelmingly approved the project, was a twin-bore, nearly 1.3 km tunnel running through the mountain between Montara and Pacifica, allowing drivers to bypass the dangerous bit of highway. The Devil's Slide tunnel across San Pedro Mountain will provide drivers a safer, more reliable link between Montara and Pacifica and the rest of the San Mateo County coastal communities. The tunnel will be California's longest highway tunnel. The former route will become a trail overlooking the Pacific Ocean for hikers, cyclists and tourists. Environmentalists who feared building a freeway would ruin the area's natural beauty and lead to a haemorrhage of development in the sleepy coastal towns of Montara, Moss Beach and Half Moon Bay, first filed suit in 1972. Decades of legal delays ensued, followed by widespread support for an alternate plan to build two tunnels through San Pedro Mountain. The tunnel plan was heralded as a more environmentally sound choice.Preliminary work on the project began in 2005 but the official digging of the long-awaited tunnel began on 17th September, 2007 when a Terex TE210 earth mover started drilling a fraction of ground into the mountain. About 150 people attended the ceremony near the south portal. Tunnelling will proceed nearly non-stop until completion in December 2010. Excavation work had started earlier this year for the underground water tank just in front of the south portal. The 988,000-litre concrete tank is part of the rain water drainage system. Visit www.terex.com Workers will employ the NATM sequential excavation technique, which relies on the surrounding rock to support the tunnel. Excavation of each tunnel will take place in stages, working from top to bottom, then carving out the sides. Diggers will move at a 2% uphill grade, hitting groundwater along the way. The hardest rock must be fractured by small packs of explosives, slid into holes and ignited. Soft clay can be shoveled out with a backhoe. But the bulk of the work will be done by roadheaders, which will not begin the real heavy tunnelling work until the week of 7th October. In the next few weeks, crews will be busy preparing for the dig. That work includes installing a canopy over the bull's-eye to protect workers and passing motorists from falling rock. The tunnel project is split into six contracts and includes mitigation for environmental concerns, construction of a maintenance centre and both northbound and southbound tunnels. Each bore will contain one lane of traffic, a cycling lane and will have emergency walkways and shoulders on either side. Environmentalists are not worried that its capacity will be expanded because the state Coastal Act requires Highway 1 to remain two lanes in rural areas. A jet fan ventilation system will expel exhaust and keep the air in the tunnels fresh.Kiewit Pacific will be using two 120-tonne ATM 105 roadheader machines, one in each tunnel, made in Austria by Sandvik Group's Voest Alpine. Each machine is almost 15 metres long and cost over USD3 million. Moving south to north, they will excavate a pair of 1,220 m-long tunnels, each 9.15 m wide and 6.70 m high. Visit www.miningandconstruction.sandvik.com or www.vab.sandvik.com/sandvik/ and www.kiewit.comThe debris will be carried by conveyor belts onto dump trucks. Because the properties of the rock vary, geologists will constantly monitor the site, alerting the crew about what lies ahead (granite, sandstone, shale or clay). The debris will be hauled away to nearby swales, then covered with topsoil and native plants. The portals - gently sloped, not vertical - will also be re-vegetated. The tunnel will be supported by a combination of steel rods, wire mesh, and thin, fibre-reinforced concrete, preventing collapse. Among the finishing touches will be a concrete lining for the ceiling and panels on the sides.Construction of the tunnel first estimated at USD240 million is now budgeted for USD272 million but the entire project, including bridges across a canyon at Shamrock Ranch on the north and a highway realignment and operations centre on the south, will cost USD330 million. Click us/93. Read E-News Weekly 40/2004, 37/2004 & 31/2004. Visit www.dot.ca.gov/dist4/dslide and view a video at http://video.nbc11.com/player/?id=157188 40/07.