Tunnel Boring Machines (TBMs)
Modern Tunnel Boring Machines (TBMs) are highly sophisticated machines, that can measure more than 150 metres long and weigh hundreds or even thousands of tonnes.
The TBM cutterhead excavates the ground as the machine advances, while at the same time the permanent tunnel lining is installed. The lining is made up by a series of precast concrete segments, which are grouted and completed as the TBM proceeds.
TBMs can be used in soft or hard, cohesive or non-cohesive ground conditions. There are different types of TBM, with the most common version used in the UK providing active support to the tunnel face, through either conditioned spoil or an engineered slurry, (Earth Pressure Balance or Slurry/MixShieldTM TBMs respectively). In delivering the London Tunnels section of HS2, we are using a number of Earth Pressure Balance TBMs.
Sprayed Concrete Lining
Sprayed Concrete Lining (SCL) is a construction technique where concrete is sprayed at high velocity onto surfaces, typically in tunnels or underground excavations, to provide immediate structural support and stabilisation. SCL is deployed in predominantly cohesive ground conditions.
Tunnels using SCL can be of varying sizes, the larger diameters constructed using a staged approach, with two or more phases of excavation.
A versatile technique, SCL can be used to construct shafts and tunnels of varying profiles.
In ‘soft ground’, SCL is primarily used as initial ground support, with the permanent lining constructed as a follow-on activity. However, when used in rock tunnels, SCL can be used as the final lining.
We have used SCL extensively on our works for HS2, including in the construction of 20 cross passages which provide a means of access and egress from the main running tunnels.
Drill and blast
Drill and blast tunnelling is a traditional excavation method used to construct shafts and tunnels in rock. It is particularly effective for creating large tunnel profiles and complex underground structures.
The technique involves drilling holes into rock in a defined pattern, which are filled with explosives and detonated to break the rock into manageable pieces for removal. Following excavation, ground support is applied, as required, predominantly in the form of sprayed concrete lining and/or rock bolts.
Drill and blast offers flexibility to create large and irregular tunnel profiles and requires lower equipment investment compared to tunnel boring machines. With adaptable blasting patterns, it allows for precise control of rock breakage and tunnel shape. We use the technique extensively, particularly for tunnelling projects in Norway.