The Mt. Lebanon Tunnel is a light rail tunnel in

Allegheny County, Pennsylvania Allegheny County () is a county in the Commonwealth of Pennsylvania. It is located in Southwestern Pennsylvania. As of the 2020 census, the population was 1,250,578, making it the state's second-most populous county, following Philadelphia Co ...

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

Pittsburgh Light Rail The Pittsburgh Light Rail (commonly known as The T) is a light rail system in Pittsburgh, Pennsylvania, and surrounding suburbs. It operates as a deep-level subway in Downtown Pittsburgh, but runs mostly at-grade in the suburbs south of the ci ...

system. The long tunnel connects the and stations beneath Washington Road & West Liberty Ave. from Shady Drive East to McFarland Road at a maximum depth of , 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 The New Austrian tunneling method (NATM), also known as the sequential excavation method (SEM) or sprayed concrete lining method (SCL), is a method of modern tunnel design and construction employing sophisticated monitoring to optimize various wa ...

, a tunneling process developed in the early 1960s 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, gunite (), or sprayed concrete is concrete or mortar conveyed through a hose and pneumatically projected at high velocity onto a surface, as a construction technique, first used in 1907 invented by Carl Akeley. It is typically r ...

, concrete that is blown on under air pressure. The tunnel has continuous television surveillance.

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External links

* * - Southern portal * - Northern portal Tunnels in Allegheny County, Pennsylvania 5 ft 2½ in gauge railways in the United States Railroad tunnels in Pennsylvania {{US-tunnel-stub