Geotechnical engineering involves many different and complex materials and many different mechanisms of behaviour. The direct use of experience as a guide to prediction and design is effective, provided these are understood. Geotechnical engineering is a relatively new science. Its successful application to prediction requires realistic assumptions to be made, and predictions must be tested against reality. Methods of prediction need then to be refined. Examples of the importance of making realistic assumptions are examined in this Paper. They include the effects of stress and compression on permeability, the importance of realistic modelling of the surface boundary condition in seepage analysis, and the importance of modelling material properties realistically, according to their behaviour as measured in the laboratory and verified by field observation. The link between material and prototype behaviour is stressed. It is concluded that, in dealing with engineering problems, the understanding of real behaviour is more important than accurate calculation. Numerical analysis now provides great power in analysis as it predicts mechanisms of behaviour. These no longer need to be assumed in advance. Examples are given in which numerical analysis combined with realistic material modelling gives improved prediction of behaviour. Better analysis offers better prediction and better understanding. Both are only possible when reality is modelled. There are occasions when mechanisms are too complex for predictive analysis. Prediction must then be based directly on experience, applied with an understanding of the mechanisms involved. Moreover, methods of analysis may become too sophisticated for everyday use. However, pseudo-analysis, involving standardized methods based on oversimplified and unrealistic mechanisms of behaviour and material properties, is dangerous. The use of engineering experience as a guide to prediction and design may offer a more effective alternative, provided it is based on a realistic understanding of mechanisms and materials.
