Assigning numbers to geology requires a delicate balance between the commonly held opinion that geology cannot be quantified and the over-optimistic view that every physical quantity can be described in precise mathematical terms. In reality, many geological characteristics cannot be quantified precisely and intelligent guesses based upon experience and logical arguments are the best that can be hoped for.
This paper explores the processes used to make some of these guesses and describes how the results are then applied to engineering design. It is shown that, with care, rational engineering decisions can be made in spite of the limitations of the input data. In recent years the development of computer hardware and software has made it much easier to investigate the influence of ranges of values for each of the input parameters. However, care has to be taken that the design is driven by sound geological reasoning and rigorous engineering logic rather than by the very attractive images that appear on the computer screen.