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2015WinterShotcreteEMag

2014 Honorable Mention Liberty Tunnel Arch Restoration with a Shotcrete Alternative By Axel G. Nitschke and John Becker Tto downtown Pittsburgh, PA. The Liberty-he Liberty Tunnel provides a direct commuting route from the South Hills suburbs Tunnel is a horseshoe-shaped tunnel consisting of two northbound and southbound tubes and has an overall length of 5888 ft (1795 m). The tunnel consists of two vertical vent shafts to draw exhaust from the midpoint of each tunnel and force a supply of fresh air into the tunnel through the so-called “arch walls.” An arch wall is an arch structure which is offset from the structural lining Fig. 1: Tunnel ventilation arch wall section plan view of the tunnel to provide for air channels. The ventilation arch wall section acts like a macro- scopic air nozzle—fresh air is supplied from the ventilation shaft and pushed along the vent supply area on either side of the arch wall. The arch walls are open at the end of the nozzle, which allows the fresh air to enter into the tunnel away from the exhaust point (Fig. 1 through 4). Swank Construction Company was awarded the Liberty Tunnels rehabilitation project by the Pennsylvania Department of Transportation (PennDOT) in May 2013. The project included, Fig. 2: Tunnel ventilation arch wall section—longitudinal section among other scopes, the demolition and renewal of the ventilation arch walls inside the tunnels, close to the ventilation shaft. Gall Zeidler Con- sultants (GZ), in cooperation with Swank Con- struction and Coastal Gunite, provided an alternate design and construction concept for the Liberty Tunnels rehabilitation project. Structurally, the arch wall section can be divided into three sections from left to right in Fig. 1 and 2: 1) merging area from the shaft; 2) full-arch area, where the arch wall is closed at the bottom; and 3) suspended arch area, where the arch wall is open at the bottom to provide an outlet for the fresh air (see also Fig. 4). This article focuses on the full-arch area (center) and does not address the merging area from the shaft (left) or the suspended arch area (right). Arch Restoration Original Design The original arch wall used U-shaped steel profiles as structural members, which were tied Fig. 3: Alternative design self-bearing shotcrete arch section with radial hangers to the structural tunnel arch 44 Shotcrete • Winter 2015


2015WinterShotcreteEMag
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