Tunnel Construction. Excavation Methodology.
Tunnel construction is typically completed by either:
- Drill and Blast
- Tunnel Boring Machine (TBM)
- Mechanical Excavation – using equipment such as Roadheaders, Rockhammers, Surface Miners, Shovels etc.
Tunnel Construction. Excavation Stages.
There are generally four broad categories of tunnel construction depending on the material through which they pass:
- Soft ground, (soil and very weak rock);
- Hard rock;
- Soft rock, (shale, chalk, and friable sandstone); and
- Subaqueous (immersed tunnels).
While different ground conditions require very different methods of excavation and ground support, tunnelling operations usually involve certain broad stages:
- Early Works
- Acoustic Sheds (where construction is taking place in urban areas)
- Top / Main Heading
- Cross Passages
- Ventilation Shafts
- Caverns / Stations
Grydale Mobile Dust Collectors have been used to effectively manage dust and provide a safer working environment and enhance the operational efficiency of tunnel construction.
Tunnel Construction Methodologies. Dust Control.
Mechanisation in tunnelling has improved the speed of construction but increased the health and safety risks to tunnel workers from increased dust pollution (airborne contaminants) associated with the use of larger more efficient excavation equipment (e.g. Roadheaders, Surface Miners, Large Track Trenchers). In Australia where there are heavy concentrations of sandstone, dust control is paramount to the health and safety of workers.
Cut and Cover Tunnels
Within a Cut and Cover Tunnel the structure is cast-in-situ or precast in an excavation. After construction, the structure is back-filled with new or excavated soil. Cut and cover construction is adopted when the depth of tunnel is shallow and safe excavation is possible from the surface without walls collapsing from excavation and when it is economically acceptable. This methodology is usually used for the construction of underpasses, approach sections of other tunnels & tunnels in flat terrain or shallow depth.
Mobile dust collectors are an effective dust control method that can be used for the excavation of cut and cover tunnels. Grydale track mounted units can extract dust at the source to minimise the impact on workers and the surrounding environment.
Mined / Bored Tunnels
Mined tunnels are built without excavating the ground surface. In bored tunnelling, the excavation takes place at the portal or at a shaft. This minimises the impact on existing traffic, air and noise quality, and utilities. For depths 10m to 12m, cut-and-cover is usually more economical and practical than mined tunnelling.
Within mined or bored tunnels, mobile dust collectors can be located within the shafts or advance along the length of the tunnel with excavation equipment.
In this method the tunnel is constructed at greater depth from the ground surface. A shaft is built up to the depth where tunnel is required. The shaft is a permanent structure similar to a well with concrete walls. Tunnels are then excavated using TBM’s. Shafts are provided at both inlet and outlet of tunnels. Intermediate shafts are also provided if tunnel is too long. After the construction process, these shafts can also be used for ventilation purpose as well as emergency exits.
Grydale are able to provide onboard dust collection solutions for TBMs and also have units specifically designed for the excavation of the tunnel cross passages.
Tunnel Construction Methodology. Rock Tunnelling.
Rock tunnels are excavated either by drilling or by blasting or rock tunnel TBMs. The Sequential Excavation Method (SEM) is also used in some cases, in particular where there are soft rocks.
The behavior of rocks can change place to place and type to type so depending on this stabilization measures ranging from no support at all to anchor bolts to steel sets to even heavily reinforced concrete lining and combination of all these are used. It is therefore one of the most challenging tunnelling geologies.
In hard rock conditions, Grydale Mobile Dust Collectors have provided an effective solution for dust control at the source. They have also been used as part of the main ventilation system at the face by pairing a road header with a mobile dust collector either mounted on a drag skid, hydraulic stepping system or tracks. Mobile dust collectors are able to advance at the speed of tunnel construction rates and offer significant operational efficiencies.
Tunnel Construction Methodology. Sequential Excavation Methodology.
The Sequential Excavation method (SEM) is used for the construction of shallow mined tunnels that have ground conditions that are fully dry or have been effectively dewatered. The method is also known as the new Austrian tunneling method (NATM), as the idea for this method originated in Austria.
Grydale Mobile Dust Collectors again offer an effective solution for dust control.
For more information on how Mobile Dust Collectors can be used for effective dust control on Tunnel Construction projects Contact Us.
Tunnel 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.
Main Tunnel Ventilation. Overlap System.
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.
Within an overlap ventilation systems air is pushed into the tunnel using a fan and a dust collector 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.