THE HOLMESDALE TUNNEL

Of the 190km long M25 motorway, which when completed will encircle London, the 3km section from the A10 to the west bank of the River Lea navigation is considered to be the most expensive section of road in Britain. The highway runs through an existing residential area where, for environmental reasons, the largest cut-and-cover tunnel under construction in the UK, the Holmesdale Tunnel, will be located. The 670M long tunnel is designed to carry the 3-lane dual carriageways and has an overall width between sidewalks of 39.2M. The site lies at the edge of the London basin comprising alluvial clay overlying terrace gravel. The outer walls of the tunnel are propped cantilevers, constructed of 1M reinforced concrete by the diaphragm wall technique to a depth of 19M below ground level. The tunnel roof comprises prestressed composite construction using precast M beams placed discretely in composite action with an in-situ top slab. Full details are given of techniques and equipment used in the diaphragm wall and tunnel roof construction. (Author/TRRL)