TUNNELS The Holmesdale Tunnel east of A10 interchange (Junction 25), Waltham Cross has been constructed to reduce to a minimum the effect of the motorway through this urban area. The tunnel was designed to full motorway standards with a lid. The dip in the longitudinal profile resulting from putting the motorway in tunnel requires pumping of surface water. A fully automatic pumped system for ground and surface water is installed in the tunnel control complex near its east portal, along with the electrical supply plant and monitoring and control equipment. The tunnel itself was constructed by cut-and-cover methods. Construction had to allow for the busy A1010 Waltham Cross High Street to be kept open and finally carried on the tunnel roof, the majority of which has been covered with topsoil to form a park and recreation area. 
At Bell Common on the northern tip of Epping Forest the main problem was to minimise the effect on the forest itself which was vested in the Corporation of the City of London as conservators by the Epping Forest Act, 1878. Bell Common tunnel, like Holmesdale, is a full motorway with a lid. Unlike Holmesdale it is on a horizontal curve, is shorter, (some 470m as opposed to 650m), and is at a summit and not a trough. Above the tunnel sorne 600mm of drainage layer and topsoil restore Epping Forest and a cricket pitch as near as possible to their original condition. The tunnel complex is similar to that at Holmesdale but Bell Common tunnel does not require a surface water pump as it is drained by gravity. The operation of the tunnels required detailed consideration by the Department of Transport, the consulting engineers, the Agent Authority - Essex CC and the Essex and Metropolitan Police. From the control station near Chigwell the Metropolitan Police are able to monitor both the traffic conditions in the tunnels and at M11/M25 interchange by CCTV, as well as the status of all plant and warning devices at the tunnels. The information transmitted from the tunnels can be stored on floppy discs with the messages classified as 'record', 'alert', or 'danger'. |
