Tunnel Rehabilitation

Inspected tunnel connects two quarries at the Luck Stone Corporation Fairfax plant. The tunnel crosses beneath the Lee Highway (US Route 29), in a North-South direction. Built ca. 1975, the tunnel is partially unlined and in rock; approx. 190 feet long and approx. 22 feet wide and 22 feet high. As the Northern quarry side is already exhausted, the tunnel is rarely used with just one or two transits per day.
Dr. Sauer & Partners performed an in-depth inspection of a connector tunnel; assessed the tunnel condition and provided remedial measures.

The brick lined tunnel constructed by Mark Brunel between 1825 and 1841 was the first to be bored underwater using a shield and comprised a twin arch tunnel, 375m in length, with cross passages every 5.5m. The new lining has been designed to minimise the removal of existing structural brickwork and has a similar shape with cross passages to match the original tunnel. The reinforced concrete/shotcrete lining includes steel fibres to increase the strength and durability of the lining. A waterproof membrane has been installed around both tunnels and the cross passages between the structural brickwork and the new concrete lining.

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.

The Bergen Tunnel on the New Jersey Transit network is a double track, horseshoe shaped rail tunnel, built in the 1870s, passing through fractured diabase. Over time, water ingress has caused operational problems. In 2000 the owner decided to undertake major structural rehabilitation. The brick lining was stripped out and the original profile was enlarged to take a new reinforced in-situ concrete lining and drainage system. Old ventilation shafts were backfilled. Two large openings were retained for tunnel ventilation, emergency access and to house new power and electrical substations.