Site investigation techniques may be divided into two categories, destructive and nondestructive.
Destructive techniques are those which, in their execution, in some way change the nature of the ground they are investigating. Thus, for example, plate bearing tests at the bottom of a shaft test ground influenced by the excavation of the shaft rather than undisturbed ground. Non-destructive techniques examine the ground without permanently altering its characteristics and are generally of either an observational or a geophysical nature.
Geophysical techniques fall into two main streams. In the first, naturally occurring phenomena, such as gravity, the earth’s magnetism or telluric currents are measured with great accuracy, for values anomalous within general trends will reflect local geological conditions. In the second stream fall methods in which some form of signal is passed into the ground and changes or responses to this signal, resulting from geological conditions, are observed. Seismic and geo-electric techniques are the best known of this group.
Geological structures or anomalies are detectible because different types of ground materials have different physical properties such as density, elasticity, conductivity etc.
Geophysical methods detect differences in these properties which may also coincide with other geological boundaries. The first and ‘traditional’ use of geophysics was to locate boundaries. These techniques are now quite well known and are continually improving. In the shallow depth geophysics utilised in the investigations for engineering projects the accuracy of location of boundaries is usually somewhat less than that desired for design purposes. However it is often rather more important to know the shape of a boundary than its exact depth. Figure 1 gives a simple example.
Two boreholes in Fig. 1a show soil over weathered rock over fresh rock. Rockhead would appear to be relatively level between the two boreholes. After a seismic refraction survey on a line joining the two boreholes together the rockhead is seen to be in the form of a buried valley. The depth of rockhead found by the geophysics is not that as found in the boreholes, probably because the velocity of the upper weathered rock is much the same as that of the overlying soil. Depths to rockhead are thus not accurate but the shape of the boundary between soil and rock is indicated.
If geophysical anomalies caused by material properties allow the definition of geological bodies then it would seem possible to establish material or mass properties by studying the geophysical characteristics of known and defined geological bodies. Thus, some 30 years ago work was begun relating the excavatability of rock to the velocity of shock waves through that rock. Attempts have been and are being made to relate ground mass deformability to shock wave parameters.
While the experience of particular engineers has lead them to enthusiasm for one of the techniques or to distrust them all, many engineers are beginning to realise that each geophysical method is but an investigation tool, which, if properly applied, will give data. The interpretation of this data may be difficult and the accuracy of the results obtained may be limited, depending on the complexity of the geological situation, the quality of the data, the skill of the interpreter and the sophistication of the interpretative techniques used. However, it must not be forgotten that the results, providing that they have been properly obtained, will always have meaning. Thus on one particular site a seismic refraction survey may give rather accurate depths to rockhead.
On the next site the field data may be difficult to interpret and the results of limited value because of the complexity of the geology. This then demonstrates that additional work, perhaps using other investigation techniques, is required in order to define site geology accurately.