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

Capita Symonds were appointed as consulting engineers to the Highways Agency in a design-build partnership with Costain Construction. Among the many disciplines within the CS East Grinstead office were Transport Communications and Acoustics, for which Steve Jones worked at the time. Following the formation of Steve Jones Associates, SJA has been engaged as part of the team as a sub-consultancy to this project.

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.

The sound system, together with multiple CCTV images and remote controls for a wide variety of mechanical plant associated with smoke control, were connected back to the main communications and control centre many miles away – also using fibre optic cables.

Alignment and commissioning of the sound systems at what is called “site acceptance tests”, or SATs, is due to take place shortly. In order to create a simulation of “normal” site occupancy conditions, many of the construction team’s cars and lorries will be parked in the tunnel whilst the alignments and measurements take place. Whilst this is far removed from the world of rock’n’roll or even evacuating a shopping mall, for an acoustic consultancy the principles for public safety remain the same.