The Holmesdale tunnels at junction 25 became a major point of congestion. A £75 million scheme from the Highways Agency scheme led to the refurbishment of the tunnels which involved the removal of jet fans located next to the carriageway to create more space. This required the engineers to develop new ventilation methods that would not occupy valuable roadside space yet capable of introducing high volumes of air in an emergency to purge the tunnels of smoke.
After delivering a similar contract previously for the Thames Tunnels on the Channel Tunnel Rail Link (CTRL) Exyte Hargreaves were in a unique position to contribute their expertise on this scheme, manufacturing and installing the crucial ventilation equipment in the Holmesdale tunnels on the M25. This equipment included the supply of the saccardo nozzles.
The main contractor, Costain, created new platforms extending 40 metres over the carriageways at the entrances to each tunnel. Each platform carries around 100 tonnes of ventilation equipment. This comprises a bank of louvres linked to five high power fans that ram air through wedge-shaped chambers to the Saccardo nozzles that deliver a high velocity air stream into the tunnel.
The fans were supplied by specialists Fläkt Woods. Each of the five fan, duct and Seccardo units can operate independently. Attenuators were installed on both sides of the fans and the main ductwork structure has noise absorbing panels to contain noise break-out. Systems were required to meet a minimum 25 year service life and so structural framework on the large Exyte Hargreaves ducts were galvanised to marine specification. The Ductwork panels were made in maintenance free stainless steel.
Under normal circumstances, the ventilation systems are non-operational. However, should an accumulation of carbon monoxide be detected in the tunnels, as may happen when traffic is slow or static, two fans will run to introduce fresh air and dissipate the gas and fumes. In the event of a fire or accident in either tunnel, four fans will operate to introduce 370 cubic metres per second of air at high velocity. Additional air will also be entrained from the tunnel entrance. This will create 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.
On completion of the installation of air injection equipment, Costain built enclosures to screen the Exyte Hargreaves plant from view, protecting it from weather and further reduce noise emission.
To meet the stringent requirements of the site programme Exyte Hargreaves prefabricated major elements of the ductwork in the North Manchester factory so that they could be craned into position.