Hard rock

Barrel Vault Method

British Gypsum's operation at Leicestershire, a plaster production plant, improved infrastructure by establishing 2 new tunnels on site. Ground treatment for the control of water ingress during construction, drift excavation, final lining and a water ingress guarantee had to be taken into consideration. By applying NATM, the tunnel was excavated by roadheader and supported with lattice girders followed by shotcrete. The construction met the final water ingress guarantee by installing a PVC waterproofing membrane between the primary shotcrete lining and the final unreinforced in-situ concrete lining which is of a watertight quality in the invert.

The project was part of the extension of Glenmont Route north into Maryland and included Wheaton Station and two approx. 2500 m long running tunnels as well as ventilation shafts and a cross-adit. Ilbau America, advised by Dr. Sauer & Partners, recommended changes to the existing conventional design and introduced the New Austrian Tunnelling Method (NATM), which was then used to construct the station tunnels, running tunnels and ancillary structures. The shaft at Hildarose Drive was constructed in conjunction with a large double cross-over.

The original tunnel was built in 1865 as a 12 ft wide (4 m) single-track rail tunnel. It was widened in 1873 to 25 ft (8 m) double-track with a 26 in-thick (66 cm), brick-lined horseshoe design. After several structural repairs, mainly for spalling bricks, it was finally closed in the early 1960s. More than 30 years later the Port Authority of Allegheny County saw benefits in adapting the former railroad alignment and right of way to its new bus way connection between downtown Pittsburgh and the new Pittsburgh International Airport. A NATM value engineered proposal in the tunnel refurbishment contract provided US $ 2 mil cost savings compared to the original bid. The redesign was based on elevating the tunnel alignment so that the future tunnel crown could be excavated in better ground conditions and on changing the proposed horseshoe to a "rounded" cross section. This combined with the utilization of the NATM allowed a 40 % reduction in shotcrete thickness, a 22 % reduction in concrete lining thickness and the elimination of the reinforcement in the final lining. Furthermore, the NATM proposal eliminated pre-excavation grouting (200,000 bags of cement) and over 10,000 rock bolts while reducing lining thickness. During tunneling up to three headings were excavated simultaneously, one from each portal and one started within the tunnel, averaging a total excavation progress of 15.4 ft (4.7 m) /24h day. Following waterproofing, the concrete lining was completed 16 month after beginning of the reconstruction.