NATM underway at Devil's Slide - TunnelTalk
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NATM underway at Devil's Slide Mar 2008
Paula Wallis for TunnelTalk

Two 120-ton roadheaders are grinding through favorable ground conditions in the early construction stages of the Devil's Slide Tunnel Project on Highway 1 south of San Francisco. By mid-March, four months into excavation from the south portals, contractor Kiewit Pacific was about 220m into the top heading in the north bound tube and 160m in the south bound tube. The bench will be excavated concurrently at a minimum 40m behind.

The twin tunnels are 1,250m long, 9m wide x 6.8m high, and approximately 18m apart, and are being excavated using NATM (the New Austrian Tunneling Method) or SME (the Sequential Excavation Method). According to Skip Sowko, Caltans project manager, NATM provides the flexibility to use more or less support along the alignment. "You have the option of adding or subtracting features as you more forward so you can basically deal with any ground conditions you encounter," said Sowko.

Ground conditions are divided into three geological units or blocks divided by four inactive faults. The South Block is crystalline igneous rock, namely quartz diorite; the Central Block consists of sandstone, siltstone/claystone, and conglomerate; while the North Block comprises three lithology types of sandstone, siltstone/claystone, and interbedded layers of sandstone, siltstone, and claystone.

Roadheaders and drill and blast are excavating the tunnels depending on conditions encountered, and support is installed according to five different support classes - Class 1 being the most favorable, with minimal shortcreting and rock bolting on 2m rounds, to Class 5 needing heavy pre-support, shorter 1m advance lengths, additional rock bolting, and a temporary invert of the top heading.

The excavation program is based on achieving 4m/day working three 8hr shifts of 15 workers each per day. According to J├╝rgen Laubbichler, Vice President of the Dr Sauer Corporation,which holds the constrcution management contract in JV with URS, Kiewit was still in a learning curve by March 2008 and was averaging about 3m/day. "Kiewit has all the right components," said Laubbichler. "They have good personnel and good equipment, although they are encountering some difficulties in terms of reacting to changing conditions and that slows them down a bit."

In November 2006, Kiewit Pacific, with its $273 million bid won the construction contract. At the groundbreaking ceremony Caltrans director Will Kempton expressed confidence in the contractor. "We've had great luck with Kiewit," said Kempton. "They've done a lot of other work around the state for us we thing they're up to this task and we're looking forward to working with them to finish it."

So far crews have mostly encountered rock Classes 1 and 2 with small sections of Class 3. The most challenging ground is expected at the north end of the tunnels where a major fault zone will require heavy pre-support of 50m long grouted pipe umbrellas in addition to spieling and rock bolting. Water inflows for both tubes may reach up to 50 liters per second and systematic probing is designed on 50m long drill holes with a 10m overlap.

An elaborate ventilation system is designed for a potentially gassy tunnel with special conditions that require outside jet fans to be reversible to push or pull airflow through the tunnel. 3D optical targets continuously measure ground movement for daily interpretation, and two dump trucks haul muck to a dedicated disposal area on the construction site. The tunnels will be finished with a drained waterproofing system and a 350mm cast-in-place concrete lining with two layers of rebar reinforcement.

When complete in 2010 the $273 million tunneling project will bypass a section of the California's Highway 1 coast road that has been plagued by landslides and grade subsistence since it opened in 1937.

     

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