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Under & Through and No Respect
Bridges get all the attention. They stand out in the open for all to marvel and celebrate. They get the books, the photographs, the tourists...
Tunnels are, forgive the obvious, hidden.
Digging up information (sorry) about tunnels is not so easy. Aside from being less glamorized -- if that's possible to do with infrastructure -- they are commonly in less accessible locations. Obscured, protected, and private, tunnels often don't accommodate foot traffic and there's typically a long hike involved to visit a site. There are far more books and websites written about bridges, dams, inclines and other public works. And whether in print or in the ground, most of the workings of tunnels are buried.
There are two simple ways to classify kinds of tunnels. The construction method: they are bored -- the surrounding rock and ground is left in place as the tunnel is excavated and lined -- or, they are cut and cover -- a trench is excavated, the structure is built, the tunnel is covered with fill. The geological conditions: tunnels through hard rock or tunnels through soft or loose material.
Techniques used for soil and rock stabilization have evolved, but the basic tools include rock bolts (similar to giant nails), grout (cement or other chemicals to consolidate loose material), framing (post and beam supports) and lining (visible inner surface made of bricks, stone, concrete, shotcrete, modular panels, etc.).
Once the tunnel lining is complete, the construction method and geological conditions are obscured -- as are most of the systems which keep longer tunnels in operation: ventilation, drainage, lighting and communications.
For the purposes of this Bridges and Tunnels website, a seemingly simple definition -- what is a tunnel versus what is a bridge -- was somewhat elusive. Most structures are obviously one or the other. True tunnels are bored. But some, like the Corliss Tunnel or numerous others, have much in common structurally with filled arch or rigid frame concrete bridges. Formulating a length to width scheme doesn't work either -- there are plenty of bridges which are wider than they are long. Ultimately, it was determined that structures which are tunnel-like and have a significant overburden of fill have been classified as tunnels. These structures, usually through earthen fill or embankment, were designed to carry more than the load of traffic above.
Western Pennsylvania can make its claims to tunnel history. The first railroad tunnel in North America was the Staple Bend Tunnel bored in 1831-32 along the Conemaugh River near Johnstown, PA. It was built as part of the Allegheny Portage Railroad, in turn, part of the Pennsylvania Canal, which connected to Pittsburgh. The Canal also featured a tunnel under downtown Pittsburgh; the tunnel still exists, but has long been sealed. The PA Canal Tunnel, begun in 1827 and completed in 1830, is probably Pittsburgh's oldest transportation tunnel. The competition for the title of the first modern automobile tunnel is disputed between New York City's Holland Tunnel and Pittsburgh's Liberty Tunnels. Both were constructed by Booth and Flinn of Pittsburgh. While the plans for the Holland Tunnel were developed first, excavation began on the Liberty Tubes a year earlier and completed three years earlier.
As of 2001, the newest tunnel in Allegheny County is a rebuilding of one of its oldest. The Pittsburgh & Steubenville RR (later the Panhandle Division of the Pennsylvania Railroad) bored the Cork Run Tunnel between Sheraden and Ingram in 1865; the Port Authority rebuilt it as the Berry Street Tunnel on the West Busway.
At least four locations in Allegheny County are noted as tunnel sites which were subsequently daylighted: the overburden was removed and the passage was completed as an open-air cut through the hill. Around 1870, the Chartiers Railway tunneled through a small hill between Carnegie and Heidelberg at Hope Hollow. By the time the "Panhandle" railroad acquired the property, the tunnel was removed; today Washington Road and a pair of intersecting streets cross the cut on a bridge. The Pittsburg-Wabash Terminal RR Bigham Tunnel collapsed after a 1903 fire in the one-year-old tunnel destroyed the timber framing; today the Bigham Cut remains above Saw Mill Run Blvd near the Parkway West. Two other likely daylighted tunnels are the former Pittsburg & Western tunnel near Glenshaw and the Bessemer & Lake Erie tunnel at Culmerville.
When they're not overlooked, tunnels typically get no respect. The Wabash Tunnel under Mt. Washington is infamous for its siren song of grandiose plans never completed. Around Pittsburgh, the heaviest traffic jams are attributed to the tunnels: Liberty, Fort Pitt, Squirrel Hill. The engineered structures, which were intended to offer a shortcut through the obstacles of topography, are cursed as constrictors.
view page - Tunnel Listing by Location (USGS topo map)
view page - Tunnel Terminology
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