Tunnel Construction. Cross Passages.
Cross passages are a critical safety feature when tunnels are operational that allow people to evacuate the main tunnel to a place of safety in the event of an emergency. Usually these are only accessible to pedestrians, however some also need to be accessible by emergency service vehicles. Cross passages are spaced along the route of the tunnel to comply with regulatory requirements for evacuation and fire-fighting.
The construction of cross passages can be complicated, expensive and dangerous with the risk of destroying the stability of the surrounding soil layers and requiring costly ground reinforcement so as not to affect the main tunnels structural integrity. Geological reports are required to analyse the rock and soil types, their settlement behaviour, fault lines and water table levels which will all affect cross passage construction. This, along with safety regulations are used to determine the spacing between cross passages, which is typically every 250m along the length of the tunnel, however there are instances of cross passages being spaced between every 100m to 500m.
Tunnel Cross Passages. Excavation.
There are a range of excavation methods used to construct cross passages. The reduced diameter and short length, compared to the main tunnel mean they are commonly constructed without the use of a TBM and excavated by drill and blast or traditional excavation, using hand mining or mechanised excavation using small excavators, rock-hammers, or road-headers. Shotcreting and rock bolting processes are then used during the cross passage fit-out. Drill and Blast is still considered to be the most efficient method of excavating cross passages, however if this isn’t possible the more manual excavation approach may be required.