Devil's Slide breaks through
Devil's Slide breaks through Oct 2010
Paula Wallis, TunnelTalk
Dozens of cheering supporters greeted the 120-ton roadheader as it broke through the north tube of the Devil’s Slide Tunnel in northern California. The men and women involved with the excavation then poured from the tunnel to shake hands with invited guests and dignitaries waiting to congratulate them on the milestone.

Roadheader bites through to complete the first road tunnel tube around the coastal slip zone

Keiwit Pacific broke through last Friday (1 October, 2010), four weeks ahead of schedule and three years after construction began. The hole though completes the top heading of the 4,200ft (1,250m) north tube. The south tube is expected to break through next week.
Ed Der, Project Manager for Caltrans said is has been a very interesting and challenging project for Caltrans, which has not done a tunnel project in 40 years.
"The hole through of the north bound tunnel is certainly a major milestone that helped people see the fruits of our labor, and it has re-energized the project team that have been involved with the excavation progress for the last three years. The quality of the work is very good. Kiewit has been very responsive to quality concerns and input from Caltrans."

Workers emerge into daylight

NATM excavation progressed through three geological units or blocks divided by three major faults. The South Block was mostly granite diorite, followed by sandstone and conglomerates in the Central Blockand siltstone, sandstone and claystone for the last 493ft (150m) North Block.
"The biggest challenge was that the ground has varied more than we thought it would," said Ivan Ramirez, Senior Engineer with Caltrans. "We expected larger stretches of consistent ground, but have had to adapt more than we thought we would and that is always a challenge, because you have to be constantly observing the ground and you can never really rest or be complacent."
According to Sebastian Kumpfmueller, Tunnel Engineer with the Dr Sauer Group, which holds the construction inspection and scheduling contract in joint venture with URS, Kiewit achieved average advance of 32ft/week (9.8m) in the north bound tunnel with a best of 19ft/day (6m/day) and 75ft/week (23m/week). Advance rates in the south bound tunnel saw a best of 22ft/day, 75ft/week and an average of 31.5ft/week (7m, 22m, 9.6m respectively).

Supporters celebrate

"As expected, the North Block consisted of intensely fractured, intensely weathered interbedded siltstone, claystone and sandstone which ranged from very soft to moderately hard," said Kumpfmueller. "There were also 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 to the highly fractured material and the barrier between the dry material and the water bearing ground of the North Block. However, due to dewatering of the North Block prior to excavation the tunnels were advanced in this softer area without any problems."
Excavation progressed ahead of finishing works that include a drained waterproofing system and a 350mm cast-in-place concrete lining with two layers of rebar reinforcement, which are roughly 50% complete in the north bound tunnel and 40% complete in the south bound tunnel.
Kewit Pacific won the construction contract in November 2006 with a bid of $273 million. ILF designed the NATM tunnel for the California Department of Transportation or Caltrans.
At the northern end, a 1,000ft (305m) twin-bridge connects the tunnel to Highway 1 after its bypass of the cliff side. The original road around the bluff on the edge of the cliff has been the site of numerous landslides and grade slips since the highway opened in the 1930s.

North portal bridge connection

When complete in early 2012 Devil's Slide will be the longest road tunnel in California.
"We haven't done this in forty years so we're learning as we go as an organization," said Ramierz.
Caltrans will take what it has learned from this project to the fourth bore of the Caldecott Road Tunnel east of Oakland, California, that is currently in the early stages of excavation. For the moment, the transportation authority is celebrating a milestone at Devil's Slide.
Pushing progress at Devil's Slide (video report) - TunnelTalk, Feb 2010
Digging begins at Caldecott - TunnelTalk, August 2010
Celebrating Caldecott groundbreaking (video report) - TunnelTalk, Jan 2010

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