Posted November 7, 201410 yr comment_221 Tunnel Sections Tunnels range in dimensions of cross-sections from those of small galleries driven by miners working with hand tools, to tunnels large enough to accommodate rail road trains, double lane of highway traffic, or to transport very large volume of water as in diversion structures in dams. A minimum size of tunnel is 9 ft high and 4 ft wide at the working face. Designed shapes or sizes of tunnels in x-section conform to a planned uses to tunnel and the some extent to the nature of the material that is anticipated will be encountered during excavation, x-sectional shapes vary from square or rectangular as for example in mining operations in strongly bedded sediment rocks, to circular. A common type of x-section is horse shoe shaped to provide maximum stability in the roof portion of the tunnel. Geological Exploration The geological conditions that are likely to met in any given work of construction must be predicted. The line of the tunnel and the neighboring ground is geologically surveyed and sub-surface data obtained by exploratory boring. Careful control of such trial boring operations is necessary in order to extract the maximum amount of information from the ground. The cost of tunneling in general is least where construction is carried out in sound rock, and in one kind of rock throughout. Straight forward geological conditions such as simply dipping strata allow cost to be estimate easily; more uncertainties arise in connection with folded and faulted beds. Geological structures such as faults and joints should be mapped along the line of a tunnel. Strongly developed joints systems are potential channels for underground circulation and should be recorded. Badly fractured ground is to be avoided if possible. If unavoidable it may require special timbering or other treatment, and a prediction of where faults are likely to met underground is therefore of greater importance. Hard rocks where excavated may stand with little support (some tunnels are unlined throughout) because they are strong enough to withstand the lateral pressure exerted by surrounding rocks but if soft bands are present there may be a tendency to slipping on these weaker layers and suitable support for the walls of the excavation will be necessary. Inter- bedded hard and soft rocks, such as sand-stones and shales may give rise to many difficulties. Ground-water percolating through the sand-stones soaks into the shales and softens them and hence the slipping is promoted.
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