ABSTRACT
The geology at a site can range from apparently simple to apparently complex scales of metres, tens or hundreds of metres (Figures 3.1 and 3.2). Geological complexity does not, however, always equate with difficulty in engineering terms. Conversely, even where the soil or rockmass is apparently relatively uniform there may be a single feature or property that will cause problems (Figure 3.3). It is the task of the engineering geologist to interpret the geology at a site and to identify those characteristics and properties that might be important to the engineering project. Much of the detail will be insignificant; the skill is in recognising what is and what is not. At some stage during the design process the geology will need to be differentiated in some way into units that can be characterised with essentially uniform mechanical properties or where the properties change in some definable way, perhaps with depth. Sometimes the way to do this is obvious – for example, a layer of fill (man-made ground) overlying alluvium, which in turn overlies bedrock, which will define the way foundations are designed – but at other locations identification of the key attributes is more difficult. Thin layers that might be overlooked in logging a borehole could turn out to be the most important features at a site. For civil engineering, ground models need to be prepared that are
simplified representations of a site and that should incorporate all the important elements relevant to design and construction. The models are generally developed from a preliminary 3D interpretation of the geology based on desk study and surface mapping and then refined by further study of environmental factors such as earthquake hazard and hydrogeology. Models will be improved by ground investigation and testing and finally presented as a design model
specifically tuned to the project. The process is illustrated simply in Figure 3.4 and expanded upon later in this chapter. One of the key features of many ground models is differentiating between upper, soil-like materials, and underlying rock, with the separating boundary being called rockhead or, sometimes, ‘engineering rockhead’. Care must be taken in using this term because it has various definitions and connotations and is sometimes used in an over-simplistic way for what is a complex situation. The consequences of wrong perception can be severe if, for example, soil is encountered at depth and below the water table, unexpectedly in a hard rock tunnel. Definitions of rockhead are set out in Box 3-1.
