LIMIT STATE PRINCIPLES IN GEOTECHNICS

The author reviews the principles behind the limit state performance criteria and discusses some of the reservations expressed by British geotechnical specialists. Present problems in the application of the criteria stem from the fact that probability theory, statistics of variation and partial factors of safety have developed in parallel. For naturally occurring rocks and soils, statistics should only be used after the properties of the ground underlying foundations are typified by extreme values rather than a random distribution. It is argued that factors applied to rock or soil properties should be based on credible geotechnical limits rather than probabilistic factors which might define soils which could not exist. It is possible that the complexity of the statistical methods could divert the designer from an understanding of the geotechnics. It is also often felt that the majority of modern design procedures are based on overall engineering judgement and empirical data, and as such are unsuitable for applications of the partial factor approach. There are obvious difficulties in applying two different methods to the structure and the ground; these will be highlighted by the future development of Eurocodes. However, changes should only be made where it is safe to do so and the potential benefits justify the change. (TRRL)