Dust Control for Tunnel Cross Passage Construction

Dust Control. Tunnel Construction.

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.

Excavation of Tunnel Cross Passage

Shotcreting during Cross Passage Construction

Tunnel Cross Passage Construction. Dust Control.

Controlling the dust generated during all stages of cross passage construction is critical but can be challenging, in particular in hard rock geological conditions, where there can be a high dust load generated in a confined space. There are regulations globally on the requirement for ventilation and dust control to safeguard the health and safety of workers from potentially hazardous respirable dust, for example Respirable Crystalline Silica (RCS) which can cause long term health issues. The use of dust collectors (dedusters) is widely recognised as the most effective method of dust control within these confined environments.

Main ventilation systems are usually not always able to manage the high dust loading associated with conventional methods of cross passage excavation and therefore additional dust control is required to mitigate the health and safety risks during this construction stage. There are additional benefits of controlling dust including increased operational efficiency (tunnel construction rates) and reducing wear on excavation equipment.

Most dust collection units are too large to fit within this limited area, however Grydale designed and developed units specifically for this application, originally for the Melbourne Metro project. Units were designed to be easily repositioned in areas with limited space, to reduce downtime and increase productivity. The use of optional castor-wheels allowed units to be easily mounted on utility platforms to further minimize space constraints.

The compact mobile units can be easily positioned at the end of the cross passage or in the adjacent tunnel with ducting erected to the dust source. This stops dust travelling along the length to the tunnel reducing the impact and pressure on the main tunnel ventilation system and other tunnel workers. The use of a Variable Speed Drive also makes the units energy efficient with units only using power required to meet the required air volume while still achieving a filtration efficiency of 99.99% at 0.067 micron meeting the most stringent dust control regulations worldwide.

Grydale Mobile Dust Collectors.  Designed for Cross Passage Construction.

The JMS-6-MES-XP (6m³/s, Mobile, Electric, Skid) and JMS-10-MES-XP (10m³/s, Mobile, Electric, Skid) dust collectors have a compact footprint that can be dragged or lifted using the four designated lifting lugs at each corner of the machine. Compressed rigid castor-wheels can be added to either unit to allow further ease of movement. Castor Wheels can be supplied with or without brakes and customised to fit utility platform rails if required.

We have also designed the JMS-10-MES-SL to provide dust control during the excavation of cross passages on the HS2 Project in the UK. These units feature a high-pressure fan to create 10m³/s at -4kPa. The units also have additional side air intakes to allow extraction from multiple dust sources simultaneously. These units are located outside of the cross passages and flexible ducting is used to extract dust from the cross passage excavation area.