Devil's Slide Tunnel in Early Stages of Excavation
Two 120-ton roadheaders are grinding through favorable ground conditions in the early stages of construction on the Devil’s Slide Tunnel.
Excavation on the Hwy 1 project south of San Francisco began November 2007 from the south portals.
Four months into excavation, contractor Kiewit Pacific is roughly 220m into the top heading in the north bound tube and 160meters in the south bound tube. The bench will be excavated concurrently at a minimum 40m behind.
The contract program for excavation is based on 4m per day. Three 8-hour shifts of 15 workers are currently averaging 3m a day.
According to Jürgen Laubbichler, VP of the Dr Sauer Corporation, Kiewit is still in a learning curve. “They have all the right components,” said Laubbichler. “They have good personnel, they have good equipment, although they are encountering some difficulties in terms of reacting to changing ground conditions and that slows them down a little bit.”
Support classes range from 1, 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. So far crews have mostly encountered rock class 1 and 2 with small sections of 3.
Systematic probing is 50 meters long with a 10-meter overlap. The most challenging ground is expected at the northern end of the tubes where a major fault zone will require special heavy pre-support including a 50m long grouted pipe umbrella in addition with spieling and rock bolting.
An elaborate ventilation system is designed for a potentially gassy tunnel with special conditions that requires outside jet fans be reversible to push or pull airflow through the tunnel.
3D optical targets continuously measure ground movement for daily interpretation, while two trucks dump muck in a dedicated disposal area on the construction site.
The final lining consists of a drained waterproofing system and a 350mm cast in-place concrete lining with two layers of rebar reinforcement.
“The project is going very well on a technical side of things,” said Laubbichler. “The learning curve is still in progress but in the last two or three weeks we have seen a big improvement and we anticipate the contract will eventually meet his schedule.”
When complete in 2010 the $273 million tunneling project will bypass a section of HWY 1 plagued by landslides and grade subsistence since it open in 1937.
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.
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.
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.
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.