Every tunnel is a one-of-a-kind construction that fulfills the needs of its particular surrounds, but they all share a common element: the need for structural support during the different stages of construction.
Concrete and mortar are used throughout: from pre-excavation ground treatment such as terrain stabilization with micropiles, to shotcrete as primary and secondary lining, as well as cement bolts, slab lining, formwork and formwork filling.
The type of tunnel, characteristics of the terrain and construction method will all play a part in determining the support and lining requirements. Broadly, all the different steps follow a basic sequence that we will look into now. Refer to the image below to follow the sequence in question.
This treatment is carried out to establish whether there is any water that could cause excessive erosion and destabilize the terrain. A series of inspection boreholes are drilled throughout the length of the tunnel section: significant water currents are injected with high-pressure mortar pumping equipment to divert the flow, or alternatively treated with low-pressure grouting equipment to stem the flow.
Micro-piles for terrain stabilization
Micropiles are small steel pipes that are sunk into the terrain as the borehole is drilled, then filled in with mortar injection machines. They are arranged in an overlapping sequence, and run alongside the tunnel’s theoretical cross-section to further secure the terrain before construction.
They’re particularly useful in tunnel entrances ahead of excavation, and are also used as a deep foundation element running the length of the tunnel before excavation. In poor quality terrain, they are often used to create a ‘micro-pile umbrella’ that will lend greater stability and avoid further fragmentation.
2. Primary and Secondary Linings
Cement bolts & shotcrete
Cement bolts are another key support element: once the excavation is complete, the site is secured with a primary lining that usually combines shotcrete, bolts and steel mesh.
Bolts – usually steel rods that are inserted into boreholes and ‘anchored’ with mortar – lend further stability to the terrain. They are typically used along the tunnel’s section, from the crown to the invert, and may also run the length of the tunnel.
Bolts are usually injected with mortar equipment, while shotcrete is generally applied with mechanized spraying equipment, a fast and efficient method that also generates less dust onsite.
The following steps will create the basis for the final support. The first step is the pouring of the concrete slab, the equivalent of the pavement once the tunnel is complete. Since a classic tunnel section is circular, the concrete invert or slab will help consolidate the base before lining the rest of the section.
The method behind the concrete pour for the slab lining will depend mostly on the size of the tunnel’s section: those large enough to allow the transit and maneuvering of mixer trucks will be able to use truck-mounted concrete pumps. Smaller-section tunnels will require additional equipment such as long-distance pumps.
Often, primary linings are used to secure the worksite during construction, while secondary linings will confer the tunnel with the appropriate lifespan.
Several flagship infrastructure projects , such as Crossrail and the Gotthard Base tunnel have used Sprayed Concrete Lining (SCL) as final linings in some sections of the tunnels, in terrains with varying geotechnical profiles such as London Clay or Alpine rockmass.
Shotcrete is also an integral part of the New Austrian Tunneling Method (NATM), which allows for a degree of deformation of the terrain before stabilization, reducing the amount of additional support materials required.
Formwork molds allow cast-in-place concrete to cure onsite, and are key in tunnels requiring very specific finishes such as aerodynamic high-speed train networks.
For a classic circular tunnel section, the formwork will typically contain two complementary halves that can be filled symmetrically and in sync through a complex system of hoses.
- “Support Systems”, International Tunnelling Association (ITA), Last accessed 07/09/2016