Ventilation Design Systems in Tunnelling
Three Key Ventilation Design Systems
The Importance of Ventilation and Dust Control
A critical aspect of any tunnel construction is ventilation design and dust control, so that workers exposure to fumes and dust are reduced to safe, acceptable levels.
Silica or silicon dioxide is a naturally occurring and widely abundant mineral that forms a major component of most rocks and soils. Crystalline silica dust is the biggest risk to construction workers after asbestos as dust particles can penetrate deep into the lungs. Heavy and prolonged exposure to RCS can cause lung cancer, silicosis, chronic obstructive pulmonary disease and other serious respiratory diseases. Silicosis usually follows exposure to RCS over many years, but extremely high exposures can cause acute silicosis more quicky.
New health and safety regulations specifically relating to the control of respirable crystalline silica (RCS), have been introduced to bring it into line with asbestos. Many countries are now halving the workplace exposure limit (WEL) for RCS for an 8-hour time weighted average from 0.1mg/m3 to 0.05mg/m3.
Due to the high production volume and enclosed workspace, workers in underground operations have a higher risk of dust exposure. Management of dust within tunnel construction usually requires a hierarchy of control measures to be implemented including the use of dust scrubber (dust collector) and providing ventilation at the face through brattice wings or ducting.
Ventilation Design Methodologies
There are many types of ventilation design and dust control systems and as each tunnel is unique so is each ventilation design and dust control solution. Each ventilation system has limitations in terms of dust control, with the selection of the system being dependent on geology, climate, tunnel dimensions, location / site access, construction method and sequence, targets to be achieved, cost of materials and ease of maintenance.
Calculating Air Volume Required
There are three main factors affecting the typical volume of air flow required within the tunnel during construction works:
These factors all combine to determine what capacity of the ventilation / dust extraction system is required.
Forced Ventilation Systems
Forced Ventilation Systems are the most common type of primary ventilation system used in a tunnel under construction. Air moves when there is a difference in pressure between two points until the pressures are balanced. Difference in pressure occurs naturally and can also be created artificially. The pressure inside the tunnel is always higher than the pressure compared to the outside environment. This difference in pressure causes the air from the tunnel to exhaust into the outside atmosphere. (Air flows from a higher pressure point to a lower pressure point.)
With forced ventilation, fresh air exits at the inbye end of the tunnel via the bag and collects the contaminants as it moves towards the opening of the tunnel. In this type of system, outbye areas normally have higher level of contaminants.
This type of system is cheap to install and is typically used during shaft development where there are space constraints prohibiting installation of a scrubber system. However, there are typically high levels of contamination and dust exposure within tunnel and in exhaust areas, especially where no scrubbers are present.
Overlap Ventilation Systems
In Overlap Ventilation Systems air is pushed into the tunnel using a fan and a dust scrubber is overlapped with the ventilation outlet. Negative pressure is used to pull air through the tunnel and two air flows are created within the tunnel. As air flows need to be balanced (to avoid dusting out of the tunnel) it is critical when using this system to have a ventilation design that works with the construction sequence. An Overlap Ventilation System is normally deployed in the extraction face of a roadheader tunnel and is best suited for long road / rail tunnels and is less effective in short drives / confined spaces.
Extract Ventilation Systems
An Extract Ventilation System is where air is drawn through the tunnel from the outside atmosphere and is exhausted via ducting to a scrubber to deliver clean air to the atmosphere. Air inflow needs to balance with air exhaust volume. Scrubbers can be located within station boxes, where louvres balance airflow into the tunnel or within the shaft.
Sometimes full extract systems are perceived as being an expensive ventilation system design as capital is outlaid at the start of the project. This is not the case in short tunnels where they can be installed as permanent solutions for the duration of the project. The ability for Grydale Dust Scrubbers to be turned up / down also reduces their operational cost significantly. Constantly renewing clean air into the tunnel at minimum air flow plus treating dirty air before it is introduced to the atmosphere through a single contained duct are additional benefits.
- Full extract ventilation systems provide the following advantages:
- Reduced risk of contact with contaminants at the tunnel face.
- Reduced risk of contaminated air mixing with the surrounding tunnel air.
- Leakage occurs into the ductwork only.
- Dust scrubbers can be co0located with other dust generation works.
- Reduced noise pollution into the tunnel and reduced at the surface by attenuation through silencer and ducting.
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