Holmesdale Tunnel

Introduction

The UK Highways Agency has become increasingly pro-active in recent years, recognising that many of the hold-ups facing motorists can be reduced by means of improved communication between the control centres and the people on the ground – both the public and the emergency services. Work to refurbish the Holmesdale tunnel on London’s orbital motorway, the M25, and the extremely busy Junction 25 nearby, to improve traffic safety and reduce congestion, began on 6 May 2006 with an 18 month programme of works.

The refurbishment works include the introduction of a Public Address system to enable officials to communicate with drivers and passengers in the event of an incident or emergency. This may be used to warn people to stay in their vehicle in the event of a breakdown because of the danger from fast-moving traffic, or to order evacuation because of a threat of fire or explosion.

Involvement

SJA was engaged as part of the team as a sub-consultancy to this project by Capita Symonds who were in a design-build partnership with Costain Construction. .

One of SJA’s areas of expertise is the design of life safety sound systems to meet the exacting guidelines of both the British and European Standards. In the case of Holmesdale Tunnel, it was argued that a Public Address system to be used in the event of an emergency should meet the Standards already in place for buildings. Until recently the UK’s highways were not considered as buildings but today, the amount of mechanical and electrical equipment installed in and around a highway is easily equivalent to that of a building – while the tunnel’s 800-metre length means that its internal volume is actually greater than many public buildings.

The measured background noise in the tunnel was very high and, because intelligibility is everything when a system is designed to communicate with the public, a public address system had to be designed to overcome that noise and provide intelligibility. Special loudspeakers, custom-designed for use in tunnels, were commissioned, while amplification equipment was fitted into M&E plant holes in the tunnel walls and connected by fibre optic cables back to a central control point in the tunnel control room.