TUNNELLING UNDER THE LINE

This article reports on the construction of a road tunnel in Essex, under a busy commuter rail line to London, which links a new housing estate to a large shopping centre. When complete, the tunnel will pierce a British Rail embankment just east of the M25 motorway near the River Thames. The technique of thrust boring was the obvious choice. A 5000 tonnes precast section was pushed through the embankment by a battery of hydraulic rams; it is 16.5m wide, 9.5m high, 30m long, and its walls are over 1m thick. All the jacks were placed at the rear, and the load was reduced by lubricating the box, by running hundreds of small-diameter steel ropes along the top and bottom surfaces separating the earth from the concrete. Construction was started by building a large concrete ground slab next to the embankment; it acted as the casting base for the tunnel box and an anchor for the jacks. For the jacking operation, five strong steel columns were placed against the tunnel box's rear end. Because of the elaborate preparatory construction, the contractor took less time than the planned fortnight to move the box 32m without stopping the trains running on the railway above. (TRRL)