Geology is an important determinant affecting the planning, design and construction of a road tunnel. A road tunnel has to satisfy certain geometrical criteria and the achievement of the best economic route for a tunnel may be a complex operation, where the related geology offers any considerable choice. In this paper examples are given of the principal characteristics of three recent road tunnels in Britain, explaining the degree to which the wide variations in tunnelling cost depends upon the nature of the ground.
It has generally been assumed that, where topography so allows, a road may be set underground by cut-and-cover construction at less cost than in a bored tunnel. In ground particularly favourable for economic tunnelling this is no longer necessarily true on direct engineering costs and, in an urban setting, other indirect costs tend to favour the case for a bored tunnel. Reference is made to the pattern of variations of cost with depth below surface for each of these types of tunnel.
The site investigations for a tunnel should be designed to determine the properties of the ground relevant to the selection of tunnelling method and the estimation of constructional difficulties. A highly mechanised system of tunnelling is particularly sensitive to geological variation and calls for an appropriate effort to eliminate, within reasonable limits, the risk of encountering an insuperable or disproportionately costly difficulty.
The problem of tunnel design relates directly to the competence of the ground. The concept of a competence factor is introduced in this respect. In soft ground, a principal problem is one of the adequacy of immediate support. In somewhat more competent ground, the extent of the need for immediate support depends on the knowledge of the stress/strain characteristics of the ground. For competent rock, the main problem concerns the means for determining the behaviour of the ground en masse. The method of excavation is also a factor affecting the behaviour of the rock around a tunnel.
Over the last 15 years or so there has been considerable experience in the use of rock bolts and shotcrete in providing support to the rock as an alternative to more traditional methods. The behaviour of such expedients is discussed in relation to the nature of the rock and of its discontinuities.
In soft ground, surface settlement may be important in determining the scheme for an urban tunnel. Settlement is not only largely dependent on the system of tunnelling but also upon the use of special expedients in relation to tunnelling in poor ground.