Engineering geology

Mapping at a Large Scale – Tunnels

It has now become almost standard practice to map tunnels and chambers in engineering geological terms, as the tunnel advances. Such maps show geology, major discontinuities and may display the results of a rock mass classification system. They have a number of objectives, the most important of which are:

 to record the geology encountered, so that the site investigation data may be reinterpreted with this new data to give a better picture of the geology to be encountered in the rest of the tunnel drive;

 to check the success of any rock mass classification system used to predict support, rate of advance, cutter wear etc. against that achieved so that, if necessary, system modifications may be made.

Tunnel length, width, and height dimensions must be recorded together with roof and floor elevations. Usually little can be seen of the floor of the tunnel so that mapping is confined to sidewalls and roof. In tunnels that are lined or shotcreted, work has to be undertaken quickly between the advancing working face and the following support, giving little chance for lengthy consideration of geological details. A clear idea of the necessary data to be recorded must be established before beginning work. Orientation readings require an illuminated compass and clinometer. Data recorded need not to be restricted to a rock mass classification system and should always include the basic geology, discontinuity aperture and locations of water inflows.

Drawings presented are commonly in the form of foldout maps, prepared as shown in Fig 1. The same sorts of drawings may be used vertically for shaft descriptions.

Fig. 1. The tunnel in (a) shows two lithologies, one shaded, one blank. Two major discontinuities, A and B, are found. Strata and discontinuity traces are plotted on diagrams of walls and roof. The wall planes are, as it were, ‘folded out’ (b) to produce the report drawing (c)

These are usually large diameter holes and thus circular. The investigator is lowered down the shaft in a steel cage to examine the sides. This precludes the use of the geological compass to measure orientation so usually four tapes are stretched from top to bottom from the cardinal points of the compass on the surface. The depths of boundaries or discontinuities crossing the tapes are noted and dips and strikes can be calculated. This type of work is subject to the normal hazards of tunnelling.

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