—Tunnels through quicksand are driven by one of the ordinary soft-ground methods after draining away the water, or else as submarine tunnels.
Open-Cut Tunnels.
—Open-cut tunnels are those driven at such a small depth under the surface that it is more convenient to excavate an open cut, build the tunnel masonry inside it, and then refill the open spaces, than it is to carry on the work entirely underground. In firm soils the usual mode of operation is to excavate first two parallel trenches for the side walls, then remove the core, and build the arch and the invert. In unstable soils, since the invert must be built first, it is usual to open up a single wide trench. In infrequent cases where a tunnel is desired in a place which is to be filled in, the masonry is built as a surface structure, which in due time is covered.
Submarine Tunnels.
—The mode of procedure followed in excavating submarine tunnels depends upon whether the material penetrated is pervious or impervious to water. In impervious material any of the ordinary methods of tunneling found suitable may be employed. In pervious material the excavation may be accomplished either by means of compressed air to keep the water out of the excavation, or by means of a shield closing the front of the excavation, or by a combination of these two methods. Tunnels on the river bed are built by means of coffer dams which inclose alternate portions of the work, by sinking a continuous series of pneumatic caissons and opening communication between them, and by sinking the tunnel in sections constructed on land.
| Methods of Excavating Tunnels. | - | In hard rock. | - | By drifts. | |||||
| By a heading. | |||||||||
| In loose soil. | - | By upper half: the arch is built before the side walls. | - | Belgian method. | |||||
| By the perimeter: excavated and lined before the central nucleus is removed. | - | German method. | |||||||
| By whole section: the lining begins after the whole section is excavated. | - | English method. | |||||||
| Austrian method. | |||||||||
| American method. | |||||||||
| By halves: the lower half is excavated and lined, followed by the work of the upper half. | - | Italian method. | |||||||
| In quicksand. | |||||||||
| Open-cut tunnels. | - | In resistant soils. | - | By two lateral narrow trenches. | |||||
| In loose soils. | - | By one very large trench. | |||||||
| Built up. | - | By slices. | |||||||
| Submarine tunnels. | - | At great depths under the river bed. | - | By any method. | |||||
| At small depths under the river bed. | - | By shield. | |||||||
| By compressed air. | |||||||||
| By shield and compressed air. | |||||||||
| On the river bed. | - | By coffer dams. | |||||||
| By pneumatic caissons. | |||||||||
| By built-up sections. | |||||||||
The above diagram gives in compact form the classification of tunnels according to materials penetrated and methods of excavation adopted, which have been described more fully in the succeeding paragraphs. It may be noted here again that this is a purely arbitrary classification, and serves mostly as a convenience in discussing the different classes of tunnels without confusion.