ABSTRACT
Geology is the most important factor that determines the nature, form and cost of a tunnel (Taylor and Conwell, 1981). For example, the route, design and construction of a tunnel are largely dependent upon geological considerations. Accordingly, tunnelling is an uncertain and sometimes hazardous undertaking because information on ground conditions along the alignment is never complete, no matter how good the site investigation. Estimating the cost of tunnel construction, particularly in areas of geological complexity, therefore is uncertain. Generally, geological investigations for tunnel sites are conducted in three stages. In the
initial stage, a desk study is undertaken using available maps and aerial photographs to obtain an overall impression of the geological conditions and to plan subsequent investigations. The second stage requires a more detailed investigation and is geared to the determination of the feasibility of a particular location. At this stage consideration is given to alternative tunnel alignments. Once a tunnel site is selected, then investigation enters the third phase when special additional work is conducted to assist the final design and estimation of tunnel costs. The investigation should produce a geological map of the area and a cross section along the centre line of the tunnel. Wherever possible, the position of the water table should be shown on the section. The complexity of the surface geology determines the accuracy with which it can be projected to tunnel level. The subsurface geology is explored by means of pits, drifts, drilling and pilot tunnels. Exploration drifts driven before tunnelling proper commences are not usually resorted to unless a particular section appears to be especially dangerous or a great deal of uncertainty exists. Core drilling aids the interpretation of geological features already identified at the surface. Geophysical investigations can give valuable assistance in determination of subsurface condi-
tions, especially in areas in which the solid geology is poorly exposed. Seismic refraction has been used in measuring depths of overburden in the portal areas of tunnels, in locating faults, weathered zones or buried channels, and in estimating rock quality. Seismic testing also can be used to investigate the topography of a river bed and the interface between the alluvium and bedrock, as well as in sub-seabed investigation. For example, Arthur et al. (1997) described a seismic survey carried out to help assess the geological conditions along the route of the Channel Tunnel. This survey also involved geophysical logging of deep drillholes. Seismic logging of drillholes can, under favourable circumstances, provide data relating to the engineering properties of rock (McCann et al., 1990). Resistivity techniques have proved useful in locating water tables and buried faults, particularly those that are saturated. Resistivity logs of boreholes are used in lateral correlation of layered materials of different resistivities and in the detection of permeable rocks. Ground probing radar offers the possibility of exploring large volumes of rock for anomalies in a short time and at low cost, in advance of major subsurface excavations.
