For the longest times, tunnels were built using Sprayed Concrete Lining (a.k.a SCL). This technique involved an application of several layers starting with a primary lining of sprayed concrete over a membrane and was topped with a permanent layer of concrete lining over reinforced steel bars. This worked particularly well for soft ground. Because of its layered approach, this process can be costly in terms of time, material and money. It also generates a lot of waste material adding to the cost.
To address these challenges, new designs have been tested. They range from using a permanent sprayed concrete on various types of waterproofing membranes to using permanent waterproof concrete, to applying two coats of sprayed concrete on projects where water seepage could be an issue.
Design options have also expanded to work with these new techniques and accommodate different geological and hydrological situations. There are three main designs: Double Shell Lining (DSL), Single Shell Lining (SSL) and Composite Shell Lining (CSL).
Double Shell Lining process as its name implies includes a primary and a secondary linings that handle both temporary and permanent loads. Most primary linings usually contains structural fiber to reinforce them against post-crack resistance and increase the concrete ductability. This is considered a heavy duty design that will do well with ground loads and hydrostatic.
With the Single Shell linings a portion of the permanent load is handled through action with the secondary lining sometimes accomplished with multiple passes and with the use of a waterproof membrane. This technique is ideal for dry or mostly dry ground. The use of a single layer makes the construction process one of the fastest one. Many examples of Single Shell Lining can be seen in Norway, in areas with little or no hydrostatic load, with a watertight concrete design that accommodates for local seepage.
Composite Shell Linings consists of 3 steps, a sprayed permanent first lining, a sprayed waterproof membrane and a secondary sprayed lining. The first lining is sprayed directly on the ground and is not included in the long-term load capacity calculations. The secondary lining will handle the long-term water pressure, internal loads, shrinkage due to temperature variations etc. There are several advantages to this technique, one of them is not having to use lattice girders. They are known for being hard to spray, for leaking and causing corrosion over time.
Another advantage is making the site safer, since placement of lattice girders can be dangerous to the crew, as it takes place at the face of the tunnel when it isn’t yet supported.
In terms of equipment to do the job, we recommend Schwing and Mayco machines.
More information can be found on this topic at: www.tunnellingjournal.com