|Location Map ( geo)|
|Hertfordshire • Middlesex|
| 1984 |
The Holmesdale Tunnel carries the M25 London Orbital Motorway between the Hertfordshire town of Waltham Cross and the northern fringe of Enfield.
A corridor had been reserved to take the proposed Ringway D through this area, but this corridor was not of sufficient width to take a Dual three-laned motorway. It was therefore decided that a section of the route should be constructed in tunnel with the land on top reinstated as open space. The tunnel was built to carry two three-lane carriageways with hardhoulder in two separate bores.
Construction of the tunnel began in 1981 with this section of the M25 opening to traffic in January 1984, creating continuous motorway between the A1(M) and the Dartford Crossing. A similar tunnel was built to take the M25 below Bell Common in Epping Forest, which opened at the same time. Two years later, the Hatfield Tunnel was opened on the A1(M), also of similar construction.
The tunnel is of cut-and-cover construction, comprising three reinforced concrete walls with a reinforced concrete roof slab spanning the three walls. The tunnel was fitted with mechanical ventilation systems. Raised walkways and emergency doors into the adjoining tunnel were provided for emergency purposes.
The tunnel is situated adjacent to M25 junction 25 (A10). When the motorway was originally built, the main carriageways of the motorway reduced from three to two lanes through the junction, and the tunnel was built to serve this arrangement. By the end of the 1990s this junction was running well over capacity and there was a compelling case for the provision of a third lane on each carriageway. This was achieved relatively easily on the anti-clockwise carriageway as the diverge for the slip road was situated after leaving the tunnel. This work was completed with minimal effect on the tunnel in around 2002.
The clockwise carriageway was more complicated as there was insufficient width within the tunnel to provide a full standard merge to a 3 lane motorway. At the same time, the ventilation and traffic control systems were reaching the end of their useful working lives and were in need of replacement. Furthermore, the raised walkways did not comply with the Disability Discrimination Act in that they were not accessible by mobility impaired users.
As a result a comprehensive refurbishment of the tunnel was carried out, which included the removal of the raised walkways in both tunnels to provide additional width for the new merge; provision of new emergency doors between the two bores at carriageway level; replacement of the ventilation system with a new nozzle type system situated outside the tunnel which could blow fresh air in at roof level; and the replacement of lighting and other control systems. As a result of the new ventilation system, each bore was lengthened slightly.
Following this work, the widening of the M25 through junction 25 was completed, with three lanes running through both junction and tunnel on each carriageway.