Devil's Slide Tunnel breakthrough

Tuesday, October 5, 2010

After three years into excavation, crew members of contractor Kiewit Infrastructure West Co. and owner Caltrans holed through the Northbound Tunnel Top Heading at the North Portal of the Devil’s Slide Tunnels in Pacifica, CA on Friday October 1, 2010.They were joined by a cheering crowd of invited politicians, news crews and locals who followed the exciting event from the North Portal side during a big breakthrough ceremony.It was a gratifying moment when crews finally walked out through the hole and shook hands with the guests. The hole through is a major milestone for all parties involved in the project which broke ground on September 17, 2007 and is now one step closer to completion.

The last approximately 170 meters of the tunnels had to be excavated through the softest area of the project which was defined as the “North Block”.

As expected, the North Block geology consisted of intensely fractured, intensely weathered interbedded siltstone, claystone and sandstone which ranged from very soft to moderately hard. The material in this area was categorized as ground support categories III, IV and V. Within the North Block there were several fault and shear zones encountered with the biggest (Fault C) at the beginning of the North Block. Fault C was predicted to be the transition from hard sandstone/siltstone formations to the highly fractured material as well as the barrier between the dry material and the water bearing ground of the North Block. However, due to extensive dewatering of the North Block prior to excavation the tunnels were advanced in this softer area without any problems and deformations were kept to a minimum.

For excavation the contractor used a VOEST-ALPINE ATM-105 roadheader in harder areas and a TEREX TE210 excavator for the softer zones which showed soil-type behavior in some smaller fault zones.

The design for the “softer” North Block called for a Top Heading, Bench/Invert excavation sequence with canopy pipes (CAT V only), grouted spiling, rock dowels and shotcrete lining as initial support. In categories IV & V a temporary top heading invert was installed to limit deformations.

Additionally, the support system for the last 50 meters of the northbound tunnel had to be revised last minute in order to support the above mountain slope from within the tunnel as the North Portal Cut&Cover portion was not backfilled in time. The required support could be utilized by adding rock bolts and canopy pipe umbrellas in that area.

The Devil’s Slide tunnels are the first highway tunnels built in California since the 3rd bore of the Caldecott tunnel that was done in 1964. Once finished, the two new tubes will act as a bypass to the so call “Devil’s Slide” section on Highway 1 just south of Pacifica, CA. This section was subject to extensive weather damage and faced numerous rockslides and slip-outs that caused long-term road closures and heavy impacts on local communities. With the two new Devil’s Slide Tunnels, one for each direction and each housing a traffic lane, a shoulder and walkways, these issues will no longer be a burden for commuting traffic anymore.

On Monday after the breakthrough, crews resumed work and are now looking forward to hole through the second tunnel on Tuesday October 12, 2010. Also there is approximately 250 to 300 meters of bench and 150 meters of invert remaining in both tunnels in order to complete the entire initial lining. Prior to the start of this excavation work it will be necessary to remodel the tunnel ventilation system and install ventilation fans at the north portals of north- and southbound tunnel which will create a south to north airflow in the tubes, providing fresh air at the bench/invert headings.

Finishing bench and invert works is currently scheduled for end of 2010. The scheduled completion of the final lining and opening of the Devil’s Slide Tunnels to traffic is planned for spring 2012.

DSC Services

DSC in JV with URS is performing construction inspection and scheduling services. These are carried out for the construction of the portals, the tunnel excavation, installation of support elements, initial and final liner construction and the waterproofing system. DSC is responsible for analyzing monitoring data and documenting the project's progress, including daily site inspection reports, ground conditions encountered, monitoring data, and installation of support elements. In addition DSC is providing the owner with support in processing claims that are related to the tunnel excavation.