Excavation is underway on what will be California’s longest highway tunnel. The Devil’s Slide Tunnel will bypass a section of Hwy 1 south of San Francisco plagued by landslides and grade subsistence that have closed the road for months at a time.
In September, Caltrans officials, local politicians and the public gathered at the construction site to celebrate this major milestone that has been more than 40 years in the making.
Prior to the celebration and under the supervision of the URS/Dr Sauer Corporation construction management Joint Venture, construction crews prepared San Pedro Mountain for the South Portals,excavating a series of cuts, followed by wire mesh, shotcrete, and rock dowels above and around the tunnel openings.
Temporary canopies roughly 8m in length extend from the prepared face at the portals. These will protect crews from potential rock falls during construction. Above the prepared face, natural rock formations were pinned to the mountain with rock bolts to preserve and protect the natural setting during excavation Meanwhile, Highway 1 has been reconfigured at the site to create space for the mobilized tunneling equipment and construction traffic as well as setup of the shotcrete batch plant.
Services Provided by URS/DSC
The URS / Dr. G. Sauer Corporation JV was awarded the Caltrans on-call contract for providing Construction Inspection Services for the Devil’s Slide Tunnel project in Pacifica, California. The URS/DSC JV will provide SEM/NATM engineers and field inspectors, engineering geologists, civil, mechanical and electrical inspection personnel and other technical support services as needed. URS/DSC will be working as an integrated Team with the Caltrans Construction Management Team in their field office, which is located in Pacifica, just South of San Francisco.
DSC is responsible for:
Project Background
Devil’s Slide is located south of the City of Pacifica in the county of San Mateo, California along the coastal road of Route 1. Landslides and grade subsidence in the Devil’s Slide area along Route 1 have lead to the road being closed for extended periods of time causing significant economic loss to the surrounding coastal communities. The solution: Construct a tunnel through the mountain to bypass the dangers and reoccurring problems associated with Devil’s Slide.
Geology
The ground conditions for the tunnel alignment were divided into three block units, South, Central, and North Block. The South Block consists of crystalline igneous rock, namely quartz diorite. The Central Block consists of three major lithology types: sandstone, siltstone/claystone, and conglomerate. The North Block consists of three lithology types: sandstone, siltstone/claystone, and interbedded layers of sandstone, siltstone, and claystone. The tunnel alignment intersects four inactive faults, which act as the boundaries for the three rock blocks. During construction, total inflows for both tubes may reach up to 50 liters per second.
Tunnel Design
The twin tunnels are 1250m in length, 9m wide and 6.8m high, and approximately 18m apart. They will be constructed using the New Austrian Tunneling Method (NATM), also known as the Sequential Excavation Method (SEM). Drill and blast methods and road headers excavation will be used depending on the rock encountered. Five support classes have been developed for the project and support elements range from rockbolts to steel pipe arch.
