Tunneling advances

Tunnels can now be built in the US more economically and to better dimensions following the belated collapse of federal resistance to European technologies. Papers at the Transp Research Bd (TRB) conference (Wash DC Jan 13-16) suggest that costs of tunnels can be reduced 10% while giving motorists full benefit of headroom excavated and hence a greater feeling of space.

Traditionally highway tunnels in the US have been built with huge air ducts above a false ceiling space above the roadway using large exterior ventilation buildings housing centrigugal fans, whereas the Europeans and Asians have done away with the separate air ducts and exterior buildings and move the air within the roadway tunnel by using jet fans, so called because they look like airplane jet engines being a propeller in a circular housing. They are simply hung in the top of the tunnel, usually in pairs and push the air through the tunnel. For 20 years the US Federal Highway Administration has refused to allow any federal funds to be used in tunnels with jet fans, effectively banning them in the US on the argument that in a fire they might be damaged by the heat and collapse just when strong ventilation is most needed to extract smoke and superheated air. The Pennyslvania Turnpike, not subject to FHWA controls, was the first agency to install jet fan ventilation in the US — in the Le High #2 tunnel on the Northeast Extension between Philadelphia and Scranton, opening in 1991. A TRB paper by pike engineer Bernard Bydlon shows that there was a $2.3m saving or 6% on a $40m job in competitive tendering of the two systems on the 1300m tunnel. Operating costs are also substantially reduced with jet fans because the separate ceiling ducts of so-called transverse ventilation involve greatly increased turbulence and air friction and higher electric bills — over five times the energy cost by one estimate. The 'false ceiling' of the duct-style tunnel collects dust and sometimes water infiltration and is an extra maintenance and replacement problem, adding substantially to lifetime tunnel costs. And that false ceiling adds to the sense of confinement in motorists that makes many of them dislike tunnels. Jet fans can be installed at intervals along a tunnel, anywhere from 100m to 200m apart. They are usually installed in higher ceilinged tunnels built with the New Austrian Tunneling Method that closely tailors the built tunnel to rock and soil conditions and doesn't require the unifromly heavy lining of older methods.

Mohammad Irshad a tunnel specialist at De Leuw Cather in Washington DC told us the saving with jet fan highway tunnels over false ceiling tunnels is "tremendous" and will increase as contractors and operators become more familiar with them. Bydlon says that in a one-way tunnel, and most highway tunnel tubes are one-way, the moving vehicles do most of the day by day ventilation by dragging the tunnel air along with them. The fans are only needed if traffic is very slow and dense. Carbon monoxide sensors trigger the fans. The Le High #2 tunnel has 10 fans, 4 of which would be sufficient to maintain air quality even in the worst traffic — idling vehicles in case of an incident. The other 6 fans are to provide the ventilating power to cope with the smoke of a serious fire.

Tunnel fire tests: Another TRB presentation by Sergiu Luchian of the Massachusetts Highway Dept detailed the results of probably the world's most extensive series of instrumented tunnel fire tests in history — 98 controlled fires with 2400 sensors and different ventilation systems and tunnel cross sections built specially for the tests in an 850m abandoned tunnel near Charleston WV. Some of the test fires had the heat of a gasoline tanker truck burning. They found the jet fans survived and indeed performed somewhat better than false ceiling ventilation ducts because the key to containing fires is a instant fan response to dissipate quickly forming heat and smoke. Luchian says deadly smoke buildups can occur within 20 second from tunnel fire ignition so prevention requires getting the fans pulling strongly within seconds of the fire start. Jet fans are reversible and individually controllable which allows different heat and smoke control arrangements to be computer stored in advance and triggered by smoke or heat sensors. Often jet fans will be used to suck smoke out both ends of the tunnel keeping it high to the roof.

Reuter Gall of the Sauer Corp, tunnel consultants of northern VA, says the first federally approved use of jet fans was the 1400m Cumberland Gap tunnel opened a few months ago by the National Park Service in the Appalachians near the VA/KY/TN border, a $52m project on which nearly $20m was saved by several 'value engineering' techniques, an important one being jet fans. Gall says the practical limit of fan jet ventilation is about 6 to 7km and beyond that shafts or separate ducts are advisable. The Boston Central Artery tunnel system, a very large collection of short tunnels was designed entirely around old ducted ventilation systems with 4-hour fire rating, one of the main reasons for its huge cost. Luchian says that the last section to be built was redesigned to use jet fans and 2-hour fire ratings as verified in the WV fire tests. "Anything beyond 2-hours is superfluous," Lucian says. As a result the system's 8th ventilation tower was eliminated savings $20m alone. (Contacts B Bydlon PA pike 610 292 3793, M Irshad 202 775 3377, S. Luchian 617 342 1174 sergiu@aol.com www.firetest.com, Reuter Gall 703 707 0700)

Tube freight: In another aspect of tunneling Joe Sinfield of MIT outlined modelling of modern microtunnel methods for developing a network of freight delivery by automated tube transport under congested urban areas. Midtown Manhattan could be given a delivery tunnel grid for around $1.2b, Sinfield estimates. A principal proponent of such systems William Vandersteel of Ampower Corp Alpine NJ envisages a network of pipe tunnels that could deliver encapsulated freight more economically than manned trucks and vans. He sees customers and carriers owning the unpowered vehicles and the underground tube system utility, which would provide propulsive power and switching, charging tolls for its use (To be detailed in future TR). The Fed Highway Admin is sponsoring research in the hope some such automated tube freight system might relieve the pressure of trucks on the highways and city streets. (Contacts Joe Sinfield MIT 617 263 9734, W Vandersteel Ampower NJ 201 768 6014 subtrans@aol.com)