A leading environmental engineering expert says greater consideration should be given to the ventilation system for Sydney’s nine-kilometre NorthConnex tunnel given the unique features of the project and its importance for Sydney’s transportation system.
Noel Child of engineering and environmental consultancy NG Child & Associates says that the longitudinal ventilation system slated for installation in the NorthConnex project may not be the best option available, and could result in severe air quality problems for the project.
“This is a very long, very busy and necessary tunnel, and as with all underground structures it’s important to give full consideration to a suitable ventilation system based on conditions, instead of just plucking one based on costs,” he said.
The longitudinal ventilation system currently planned employs the force produced by fans in tandem with the “piston” effect created by vehicle traffic to expel any polluted or contaminated air at the end of the tunnel.
According to Child, longitudinal ventilation is a “simple and effective” method for tunnels under certain circumstances – specifically those that are not too long and are subject to comparatively low levels of traffic.
“Longitudinal ventilation is used for the Lane Cove and Cross City Tunnels, and saves them from air problems because they are relatively short, low traffic thoroughfares,” he said.
He points out, however, that the method is not well suited to longer tunnels greater than two kilometres in distance or tunnels that are subject to higher traffic levels – particularly trucks.
The NorthConnex project, however, which will run a distance of approximately nine kilometres between the M1 Pacific Motorway and the M2 Hills Motorway and is intended to slash above-ground traffic along the Pennant Hills Road, amply satisfies both these conditions.
The inefficacy of longitudinal ventilation for the NorthConnex project has already been proven by its use with the M5 tunnel, at present Sydney’s only long tunnel and one that possesses much in common with NorthConnex.
“The M5 tunnel is very similar to NorthConnex – it has an uphill slope at one end and sees a considerable amount of truck traffic," said Child. “Unlike the Lane Cove and Cross City Tunnels, the use of longitudinal ventilation with the M5 tunnel has resulted in serious air quality issues.”
For the NorthConnex project, Child advocates the construction of a transverse ventilation system, which he says is far better suited to a long-distance, heavy traffic tunnel, as it involves the delivery of fresh air into the tunnel at multiple points along its distance.
"A transverse ventilation system involves the progressive introduction of fresh air along the full length of the tunnel," he said. "This means that fresh air within the tunnel is continuously refreshed, while contaminants and polluted air are subject to continuous removal, instead of only being discharged at one end."