Overview

Tunnel engineering makes possible many vital underwater and underground facilities. Unique design and construction techniques are involved because of the necessity of protecting the constructors and users of these facilities from alien environments. These facilities must be built to exclude the materials through which they pass, including water. Often, they have to withstand high pressures. And when used for transportation or human occupancy, tunnels must provide adequate lighting and a safe atmosphere, with means for removing pollutants.

Tunnels are constructed using many methods, depending upon the kind of soil and/or rock through which they will pass, their size, how deep they need to be, and the obstructions that may be encountered along the route. These methods include cut-and-cover construction, drill and blast, tunnel boring machine (TBM), immersion of prefabricated tunnels, and sequential excavation methods (SEM). More specialized methods, such as ground freezing and tunnel jacking, are used less frequently and often under very difficult conditions. Compressed air working has become uneconomical because of working hour restrictions, time for decompression that results from high working pressures (over 40 psi is not unusual), union labor agreements for work under compressed air, and high workmen s compensation and health benefit rates. Occasional entry under compressed air may still be required, such as to clear obstructions ahead of a tunnel boring machine, or to perform essential maintenance on parts of such a machine.

The design approach to underground and underwater structures differs from...