Question to the Department for Transport:

To ask His Majesty's Government what assessment they have made of the risk of ventilation shaft collapse at HQU/3D, the Queensbury Tunnel.

National Highways undertakes annual examinations of the Queensbury Tunnel and its associated ventilation shafts due to their age and condition. These examinations monitor any changes to the structure and inform ongoing safety assessments. Evidence from these examinations shows that the tunnel lining has already collapsed in two locations between Shafts 3 and 4, and further deterioration of the lining is likely. However, the current assessed risk of Shaft 4 collapsing remains low. Given that the consequences of any failure would be significant, the priority for National Highways is to prevent any decline in the asset’s condition.

The most likely collapse mechanism previously identified related to failure of the tunnel lining close to the base of Shaft 4 which is why National Highways has strengthened the tunnel lining in this area to mitigate the risk. Failure of the shaft cap lining due to age & wet conditions, or failure of the shaft cap are less likely and these are things National Highways checks for in tunnel and shaft examinations.

Shaft 4 itself has not been permanently strengthened. The interventions undertaken to date have focused on reinforcing the tunnel lining at its base. Shaft 4 is located within the town of Queensbury, in a residential area close to an electricity substation and although the risk of shaft 4 collapsing is currently low, the consequence of failure is high, therefore National Highways wants to make sure that it does not allow the condition to deteriorate.