M25 (Holmesdale Tunnels)

  • Cubic metres of air per second 370
  • Tonnes of ventilation equipment 100
  • Annual passengers 5.8 million

The M25 London Orbital Motorway is one of the busiest road networks in the country. Stretching 117 miles, it is the de facto geographical boundary for Greater London. The M25 now carries over 200,000 vehicles a day. After the turn of the century, a £75 million refurbishment scheme was launched by Highways England. It aimed to bring shorter journeys for drivers through additional lanes at junction 25 and through the Holmesdale Tunnel.

Project Brief

The Holmesdale tunnels, at junction 25 of the M25 motorway, opened in 1984. By the mid-2000’s they become a major point of congestion due to historical increases in traffic. The £75 million Highways Agency scheme to refurbish the tunnels involved the removal of jet fans located next to the carriageway to create more space.

Following Exyte Hargreaves’ success with HS1 (Channel Tunnel Rail Link) we were engaged by the project. The main contractor created new platforms extending 40 metres over the carriageways at the entrances to each tunnel. Each platform was intended to carry around 100 tonnes of ventilation equipment.

Conserving Valuable Road Space

Hargreaves’ track record with HS1, caused the client to challenge our engineers to develop new ventilation methods that did not occupy valuable roadside space. The ventilation system would still need to be capable of introducing high volumes of air into the tunnel to purge smoke in an emergency situation.

The specialist ventilation system comprised a bank of louvres linked to five high power fans that ram air through wedge-shaped chambers to the Saccardo nozzles. The nozzles deliver a high velocity air stream into the tunnel. Each of the five fan, duct and Saccardo units can operate independently. Attenuators are installed on both sides of the fans and the main ductwork structure has noise absorbing panels to contain noise break-out.

Protecting road users

Under normal circumstances, the ventilation systems will be non-operational. However, should an accumulation of carbon monoxide be detected in the tunnels, two fans run to introduce fresh air and dissipate the gas and fumes. This is common when traffic is slow or stationary.

In the event of a fire or accident in either tunnel, four fans operate to introduce 370 cubic metres per second of air at high velocity. Additional air is also entrained from the tunnel entrance. This creates airflow away from the tunnel entrance allowing drivers and passengers to evacuate and giving safer access for emergency services. The additional fan provides safety back-up in the event of breakdown and permits planned maintenance without affecting tunnel operation or the available ventilation capacity.

DfMA and Modularisation

Completed: 2007 | Project Value: £1.9 million

The site programme required the completion of ventilation works by April 2007. To meet this stringent requirement, Hargreaves prefabricating major elements of ductwork at our Bury factory. This was then transported to site and craned into position. On completion of the installation of air injection equipment, the main contractor built enclosures to screen the Hargreaves plant, protect it from weather and further reduce noise emission.

The project approach was incredibly successful, and the client achieved their project completion date nearly 3 months ahead of schedule in September 2007.

"The M25 ventilation systems were required to meet a minimum 25 year service life and so structural framework on the giant Hargreaves ducts is galvanised to marine specification. Our ductwork panels are made in maintenance free stainless steel."

Ryan Gorman

Operations & Business Development Director


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