Two months after the official groundbreaking on the USD272 million three-year twin-tunnel project to redirect the road straight through Montara Mountain in California, workers finally started the long-anticipated excavation on 26th November, 2007. Click here.
About 224,000 cubic tonnes of soil and rock will be pulled out of the mountain to make way for the tunnels, offering a permanent solution to the slides that have closed this stretch of Highway 1 eight times since 1938. The first tunnel doesn't look like much now - just a 2 m recess into which machines issue their front ends.
Toiling 24 hours a day, seven days a week, workers will cut the two 9.1 m-wide tunnels through 1,280 linear metres of mountain using a road header and a drill-and-blast technique with explosives for the harder rock. Progress on the tunnel will be relatively slow, advancing at a daily rate of 3.2 metres. Part of the reason is that the work takes place in two 12-hour cyclical shifts per day, and the cycle cannot be rushed.
The process begins by excavating the rock and dirt with the road header. The interior of the tunnel is then lined with a layer of concrete, then a semicircular steel beam to keep the shape, then another layer of concrete. Once the concrete has cured, workers insert a series of steel dowels to consolidate the rock above and around the tunnel. A few hours later, the process begins again. The tunnel is expected to be open to traffic in winter 2010. Visit www.dot.ca.gov/dist4/dslide
Ventilation
The Division of Occupational Safety and Health, aka Cal-OSHA, has spent a lot of time at the Devil's Slide tunnel site, ensuring workers are properly trained in safety and emergency protocol. The first tunnel is still too short to need much ventilation, and work on the second tunnel won't begin until late December. But an elaborate ventilation system is already in place, which includes several accordion tubes and an air-discharge instrument the size of an airplane engine. The system pulls air out of the tunnel as a fan is activated, pulling an equal amount of fresh air back in.
The agency will inspect the site to ensure that workers are receiving the proper amount of air flow. Even that number is regulated at a minimum of 61 cubic metres of air per minute, per person. The idea is to provide enough air to breathe plus enough to dilute all airborne contaminants, including silica, which is found in rock dust. Although naturally occurring, silica can be hazardous because it lodges in the lungs and makes it difficult to breathe, similar to the effects of asbestos and coal dust in a mine. California was made from quartz and granite, and those rocks contain silica dust. Visit www.dir.ca.gov/dosh and www.osha.gov
Fire prevention
Kiewit Pacific, the company digging the tunnels, knows well how unpredictable tunnel work can be. More often than not, equipment fires are the greatest risk. In 2008, Kiewit will work with local fire departments in holding fire drills to simulate what would happen if workers became trapped in a fiery tunnel. It is a very plausible risk because there is diesel equipment with flammable lines. Visit www.kiewit.com
Gas
Methane is another problem in tunnels, prone to causing an explosion once it surpasses a certain atmospheric concentration. Methane is less of a concern in the case of Devil's Slide, however. Workers detected some methane when they first cleared the way for the tunnel's twin openings on the south side of Montara Mountain, but Cal-OSHA attributed it to decomposing plants and other organic matter growing along the rock face.
To make sure the tunnel's gas content is tested every day, as required by Cal-OSHA, the road header machine holds a gas sensor, which takes readings as it probes the wall. If the sensors detect too much diesel gas, methane or anything else, an alarm goes off and workers must pull out. They aren't allowed back in until Cal-OSHA inspects the site and gives them written permission to return.
Explosives
Once granite gives way to sandstone toward the centre of the mountain, workers will use a specialised drill and explosives to get through the hard material. The construction team employs PETN (pentaerythritol tetranitrate), a modern explosive that Cal-OSHA considers very stable. Instead of using a spark or an electric pulse to detonate it, workers use a long-distance blasting cap. Although the explosives are safer than most, the gas that comes from them, nitrogen dioxide, is not. It must be vented out of the tunnel before the crew re-enters. 52/07-01/08.
