Contractor's View on Unlined Tunnels

The most important single aspect in unlined tunnel design is the geology of the tunnel alinement. Competent rock is the basic necessity in considering the construction of an unlined tunnel. Given this, the size and shape of the tunnel, the attitude of bedding or jointing the depth below the surface, the intended use of the tunnel and hydraulic considerations can then be taken into account. These factors are considered together with specific examples. The most economical tunnel can be obtained only when design criteria is properly balanced by careful consideration of the many points that lead the contractor to view the job and proposed contract more favorably. The contractor's view on a number of design and contract problems is given. Inasmuch as the final decision as to whether or not a tunnel may remain unlined is a matter of judgment, there is no substitute for men with long experience in underground work.