In reconnaissance and feasibility studies the geological information available to the civil engineer usually comprises the official geological map/maps showing, in an idealized manner, the distribution of the main ‘solid’ formations and superficial ‘drift’ deposits. Certain broad inferences regarding the three-dimensional structure of these formations can generally be drawn. However, the civil engineer is most often concerned with the geotechnical properties of the near-surface zone, and it is within this zone that the processes of weathering, gravitational movement, post-erosional stress relief, etc., operate most effectively. These processes often cause local departures from the situation depicted on the map. Failure to appreciate the possible range and extent of these changes underlies many of the past misapprehensions held by civil engineers of the value of geological information. From his knowledge of geological processes however, the engineering geologist should be able to make useful predictions concerning those departures from the mapped situation that might be expected in a particular area, and to devise investigational techniques best suited to disclose them, since most smaller superficial structures are not directly discernible from surface evidence in the course of routine geological mapping.
Effects due to the surface-operating agencies mentioned above may form only a transient stage in a continuously changing situation, leading from simple fracturing through increasing degrees of structural disturbance, to the complete redistribution of the original deposit and the creation of an independently recognizable superficial deposit, as for example in the process of solifluction. However, it is proposed to limit this short account