Conventional bored tunneling (but not cut-and-cover tunneling) is a “trenchless” technology. The differences in trenchless and tunnelling methods used are primarily influenced by the scale of the installations – with conventional tunneling often dealing with large diameter tunneling and large urban or national infrastructure projects. A wide variety of tunneling techniques exist but are not fully described here. For further information on tunneling technologies, see www.ita-aites.org.
Soil sampling is an important part of the site/ground investigation process. It is typically carried out by drilling boreholes spaced at intervals along (but not directly within) the alignment of a project with extra boreholes located where additional information is needed for project design. The boreholes usally are sealed after the sampling is complete to prevent them from affecting the subsequent construction process. In some instances (e.g. to investigate the presence of cobbles and boulders for microtunneling), larger trial pits may be used to allow the identification and quantification of the potential obstructions.
A large proportion of tunnel projects are excavated by a Tunnel Boring Machine or TBM. These machines provide a system of equipment for excavating and supporting the ground at the face of the tunnel, removing and transporting the exavated ground to the surface, and installing any necessary ground support to keep the tunnel in a stable condition. At one time, the term TBM was used for rock tunnels and the term Shield Machine for soft ground tunnels but the term TBM is broadly used today. There is a huge variety of different types of specific equipment to deal with different ground conditions and the ground support installed may only be intended for temporary support prior to final tunnel lining or may be installed as a final lining in one pass. A common form of tunnel lining is a segmental lining in which lining segments are erected within the tail of the TBM to form supporting rings. A TBM pushes itself forward through the ground by gripping the surrounding ground (in rock) or by thrusting against the previously completed tunnel lining.
Drill and blast tunnelling is used for tunnelling in rock. For such tunnelling, the rock is generally able to support itself for a period of time during the excavation process. A sequence of drilling boreholes in the tunnel face, loading the boreholes with explosive, firing the round and then removing the fragmented rock advances the tunnel in discrete lengths. Lateral support to the tunnel walls is added as necessary during the excavation sequence.
For further information about the wide range of tunnelling technologies and their application, please consult the many books available on tunnel technology and/or visit the website of the International Tunnelling Association www.ita-aites.org.