What is Tunnell Lliner and Why Do We Use Them?

Temporary, Primary and Permanent Tunnel Linings

Tunnel linings are the support system of a tunnel or shaft. Tunnel lining can be temporary, primary or permanent, and constructed from steel and concrete in many forms.

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Temporary tunnel linings are used before the permanent lining can be put in place.

Primary tunnel linings are used to stabilize the tunnel surface before the permanent lining can be put in place.

The permanent tunnel lining is designed to carry the long-term loads of the tunnel. The design may include the primary support system.

The tunnel lining construction method will depend on the ground conditions and geological strata. There are many different types of lining, including:

• Precast concrete segments
• In-situ concrete that is cast in position
• In-situ concrete that is sprayed in position

When the ground condition is soft, the lining system must support the surrounding ground and the loads from the jacking equipment.

Cast-iron and steel segmented linings are also used in soft ground. They are durable and have high compressive strength. Cast- iron and steel segments are then finished using a sprayed concrete to infill the segments and protect them from corrosion.

Precast concrete linings are more economical than cast-iron and steel, but they can be problematic to achieve water tightness.

When tunnelling through self-supporting ground, expanding tunnel linings can be used. The tunnelling machine shield pushes forward, leaving an unsupported space. The precast concrete lining is then put into place, expanded against the tunnels exposed ground, and pushed out under pressure.

In hard ground, shotcrete is used to line the tunnel. Shotcrete is a wet mixed concrete that is sprayed onto the tunnel surface using a compressed air technique.

NATM (New Austrian Tunneling Method)

Australia has been an innovative country in the mining sector. Not only did Australia invent rock reinforcement technology, but it also developed the NATM system of tunnelling in the s. The NATM process revolutionized the tunnelling industry.

The NATM process uses the inherent geological strength of the surrounding rock mass to give the tunnel stabilization. The method uses a design as you go approach and has sped up productivity and made tunnelling more economical.

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The initial support system is sprayed on concrete with fibre and or welded-wire attached to the rock.

Mining Shotcrete and Grout Mixers

Fibo Intercon mining products are second to none. When looking for reliability, long life and quality shotcrete and grouting plant, look no further.

Fibo has several tunnel and mining shotcrete mixing layouts to suit different mine and tunnel requirements.

The mixing plant is controlled by a state of the art software that ensures concrete quality and efficiency in delivering outstanding productivity.

Learn more about Fibo Intercon shotcrete and grout mixing plant.

Just like for ponds, there are many different types of materials that get used to make tunnels watertight. Everything from inexpensive dampproofing materials that are meant to keep MOST of the water out, all the way up to full waterproof membranes that are designed to keep ALL water out of the tunnel. Depending on what's inside the tunnel will determine how much waterproofing protection will be desired. For example, for a pedestrian walkway under a road, it would likely be best to prevent water from entering the tunnel and dripping on pedestrians. However, if a little water did get through, it's not a mission critical failure. In contrast, if the tunnel contained some expensive electronics or other equipment that MUST remain dry at all costs, then it becomes much more important to choose a tunnel liner that is 100% waterproof.

One of the biggest misconceptions we hear from people looking for a tunnel liner is they might not need a liner because they’re using high quality concrete that is supposed to be waterproof. While it’s true that PERFECT concrete should be fairly waterproof, in our experience, we’ve never seen concrete that doesn’t crack over time, even if it’s just a tiny bit. Water loves imperfections in concrete and will find and seep through even the smallest cracks. Another place for water transmission is concrete pour joints, which are the seams between an already hardened concrete section, and a new pour of concrete. An example of a pour joint would be the floor to wall transition. If you ever walk through an underground parking structure, you will often see moisture coming out of the floor to wall concrete pour joint for this very reason.

One of the main factors that will determine what kind of tunnel liner will be best is the location of the tunnel. Is the tunnel above ground or below ground? What kinds of water sources need to be protected against? For example, for an above ground tunnel that may go under a road, or another application where the tunnel is going ‘through’ something rather than ‘below’ something, you may only have to deal with rainwater that seeps through from above. However, if there is a ‘green roof’ or other types of irrigated vegetation above the tunnel, then perhaps a more robust waterproofing system might be required to prevent irrigation water from entering the tunnel. Similarly, if a tunnel is below ground, then groundwater may become an issue. Depending on the location, If the tunnel is deep enough, the structure may actually end up below the water table. This means that the tunnel will be subjected to very wet conditions most, if not all, of the time. If the structure is far enough down, hydrostatic conditions might become apparent, which means that water is being forced at the structure under pressure. In these situations, a very robust waterproofing system is required to prevent leaks. A tunnel under hydrostatic pressure could be likened to it being built through a lake or pond, where the pressure of the water is pushing down on the tunnel, trying to force it’s way through every crack or weakness in the structure.

Although there are a few options out there for liners, our favorite tunnel liner option is Premium RPE, which stands for reinforced polyethylene. It is strong, lightweight and can be MADE IN A SINGLE PIECE in any dimensions you like up to about 60,000 square feet. That means that if you have a tunnel that is 50 feet long and a 10 foot diameter, we could make a single piece of RPE liner to wrap the tunnel from end to end. All the seaming will done by us at our facility and as such , there is no welding or assembly on site required. This is in contrast to something like PVC, which is often sold in rolls, and then heat welded together on site. This requires full crews of skilled workers, which can increase the cost of the liner tremendously. Although RPE is commonly used as a lake or pond liner, it is also used in many different industrial applications to either contain water, or keep water out of a structure like a tunnel. We have millions of square feet of RPE in use in North America..

If you have a tunnel liner application you’d like to hear our recommendations on, please contact us. We provide free design assistance with no obligation to buy.

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