URS/Dr. Sauer Corporation JV Awarded the Construction Inspection Services Contract for Devil’s Slide Tunnel

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: 
• Providing inspection services for the construction of the portals, the tunnel excavation, installation of support elements, initial and final liner construction, and the waterproofing system.  
• Analyzing monitoring data. 
• Documenting the project’s progress, indlucing daily site inspection reports, ground conditions encountered, monitoring data, and installation of support elements.
• Developing a training program for the above mentioned project elements.

 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.  The Devil’s Slide area along Route 1 has been plagued with landslides and grade subsidence that 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 DesignThe twin tunnels are 1250 m in length, 9 m wide and 6.8 m high, and approximately 18 m 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 rosk encountered.  Five support classes have been developed for the project and support elements range from rockbolts to steel pipe arch.

The project was advertised on June 26, 2006 and bids are due at the end of October; construction will start in early 2007