Cut and cover tunnelling is a common, well-proven technique for constructing shallow tunnels. This technique consists of an in-situ cast concrete structure in an excavated trench, which is covered afterwards. This method accommodates changes in tunnel width and non-uniform shapes and is often adopted in the construction of underground transit stations. To minimize surface disruption, cut and cover tunnelling can be accomplished using the traditional bottom up method or as a top-down construction.
Tunnel Boring Machines
For deeper, longer tunnels in urban areas, or for a tunnel crossing major bodies of water, a pressurized-face Tunnel Boring Machine (TBM) is the best fit, because it is capable of handling the full range of expected ground conditions. A single pass, precast concrete segmental lining forms the tunnel behind the TBM. The selection and design of the precast segmental liner is critical for successful application. The segments are equipped with waterproofing gaskets and act as the structural support system and water barrier.
Compared to the cut and cover approach, TBMs significantly reduce the disturbance of traffic and the associated environmental impacts in urban areas. WSP has been a pioneer in the advancement of single pass liners, including its first application in the USA with the use of fibre reinforcement and double gaskets. We also introduced the use of special seismic joints in TBM driven segmental liner tunnel, outside of Japan.
There are two major TBMs used in soft ground tunnelling: Earth Pressure Balanced (EPB) and Slurry Type Shield Machines. An EPB TBM will perform better where the ground is silty and has a high percentage of fines. A slurry TBM is ideal in loose water-bearing granular materials. However, with the application of appropriate ground conditioning agents, the range of ground conditions for each machine can be extended. TBM technology has advanced significantly in the last 15 years, allowing for the construction of larger, deeper, and longer tunnels in more difficult ground conditions.
New Austrian Tunnelling Method
For shorter tunnel sections, non-circular tunnels, or tunnels with variable geometry, the New Austrian Tunnelling Method (NATM) – also known as the Sequential Excavation Method - provides another cost effective, flexible, and safe tunnelling option. The tunnel is sequentially excavated and support is provided by shotcrete, in combination with fibre or welded-wire fabric reinforcement, steel lattice girder arches. Ground improvement methods such as jet grouting, dewatering, ground freezing, and grouted pipe spilling are also available to stabilize the face.