SILVER STREET UNDERLINE

This article describes the construction of a new tunnel, to take a new alignment for the North Circular Road under Silver Street railway station in North London. As the railway line is a main commuter route, train services must be maintained through the station, though with a temporary speed restriction. A maximum settlement limit of 75mm under the tracks is in force. The station buildings are on top of an embankment, and their brickwork is not in very good condition. Due to lack of space, the preferred one-piece box-jack technique was not feasible. Instead, each of the 45m long twin tubes was segmented into 30 1.5m long units, and the available space was used for their prefabrication. A 27m x 17m jacking pit was constructed with 12m diameter concrete piles reinforced with steel sections. This box- jacking method allowed the tunnel to be completed well ahead of the advancing roadworks, including the cut-and-cover tunnels at either end. A 50m x 25m concrete casting bay was installed at the surface, across which wire ropes were strung longitudinally to form a low- friction floor. The ground conditions were difficult, with 2m of made ground over water-bearing Taplow gravels over London Clay intersected by a sand layer.