Tunnelling Through Water-bearing Strata
Tunnelling through subaqueous or water-bearing strata is quite a different job. Shield tunnelling is generally preferred in such cases. A shield is a movable frame that is used to support the face of a tunnel. The tunnel is excavated and lined under the protection of the shield.
A shield is a device meant for excavation that is to be carried out beneath water-bearing strata. It basically consists of a cutting edge, a skin plate in the form of a shell structure, and a hood of jacks, ring girders, stiffening steel plates, ports as well as port doors, and a tail. The various methods of shield tunnelling through different types of soils are enumerated in Table 30.4.
Table 30.4 Methods of shield tunnelling in different soils
Tunnels constructed using the shield method usually have a circular section because of the following considerations.
(a) The rotation of the shield is easy in a circular section.
(b) It grants protection to the primary lining.
(c) The circular section provides the maximum cross-sectional area with the smallest perimeter.
(d) The circular section is ideally suited to resist the semi-fluid pressure exerted by the soft ground.