The Mt. Lebanon Tunnel is a light rail

Light rail

Light rail or light rail transit is a form of urban rail public transportation that generally has a lower capacity and lower speed than heavy rail and metro systems, but higher capacity and higher speed than traditional street-running tram systems...

 tunnel in Allegheny County, Pennsylvania

Allegheny County, Pennsylvania

Allegheny County is a county in the southwestern part of the U.S. state of Pennsylvania. As of the 2010 census, the population was 1,223,348; making it the second most populous county in Pennsylvania, following Philadelphia County. The county seat is Pittsburgh...

, also known as the Dormont/Mt. Lebanon Transit Tunnel, part of the Pittsburgh Light Rail

Pittsburgh Light Rail

The Pittsburgh Light Rail is a light rail system in Pittsburgh, Pennsylvania; it functions as a subway in Downtown Pittsburgh and largely as an at-grade light rail service in the suburbs. The system is owned and operated by the Port Authority of Allegheny County...

 system.

The 3000 feet (914.4 m) tunnel connects the Dormont Junction

Dormont Junction (PAT station)

Dormont Junction is a station on the "Beechview" branch of the Port Authority of Allegheny County's light rail network. It is located in Dormont, Pennsylvania. The station is an important park and ride facility, featuring 132 spaces. West Liberty Avenue, Dormont's main artery, is located one block...

 and Mt. Lebanon

Mt. Lebanon (PAT station)

Mt. Lebanon is a station on the "Beechview" branch of the Port Authority of Allegheny County's light rail network. It is located in Mt. Lebanon, Pennsylvania and is served by the Red Line. The station features a small 24 space parking lot, making it a minor stop for park and ride commuters. It is...

 Stations beneath Washington Road & West Liberty Ave. from Shady Drive East to McFarland Road at a maximum depth of 100 feet (30.5 m), and runs beneath a cemetery.

The pre-tunnel trolley street line, which ended in 1984, ran from the long-removed Clearview Loop stop on Alfred St. (Mt. Lebanon) to the intersection of McFarland Rd. and Raleigh Ave. (Dormont).

The tunnel was constructed by a technique called the New Austrian Tunnelling method

New Austrian Tunnelling method

The New Austrian Tunnelling method was developed between 1957 and 1965 in Austria. It was given its name in Salzburg in 1962 to distinguish it from old Austrian tunnelling approach. The main contributors to the development of NATM were Ladislaus von Rabcewicz, Leopold Müller and Franz Pacher...

, a tunneling process developed thirty years ago that has become the primary tunneling practice in Europe. This is the first time that this money-saving technique was used for transit construction in the United States.

The difference with NATM lies in the means of support required to maintain the rock cavity and the configuration of the tunnel structure. With conventional tunneling, support of the rock cavity consists of a rib cage framework of steel beams and horizontal wooden planks A reinforced steel concrete lining is added to create a perfectly shaped tunnel. With NATM, the rock surrounding the hole becomes the support system. This is done by lining the tunnel cavity with several thin layers of shotcrete

Shotcrete

Shotcrete is concrete conveyed through a hose and pneumatically projected at high velocity onto a surface, as a construction technique....

, concrete that is blown on under air pressure.

The tunnel has continuous television surveillance.