The engineering behaviour of fill materials: the use, misuse and disuse of case histories

A number of developments in our modern world, including increasing urbanisation, major land reclamation schemes and the disposal of vast quantities of solid waste generated by mining and industrial activities, should ensure that, for the foreseeable future, fills will be of increasing significance in geotechnical engineering. An engineered fill, which has been heavily compacted in thin layers under closely controlled conditions, should be a relatively uniform material and have behaviour that is easily predicted on the basis of average properties. In contrast, poorly compacted fill dumped with little control in deep lifts is likely to be in a loose state and exhibit great diversity in its geotechnical properties: the behaviour of such heterogeneous fill will bear little relation to average properties, and will be controlled largely by zones of fill in a metastable state with unpredictable behaviour. Case histories that include field measurements—that is, quantitative data—are of particular value in gaining an understanding of the performance of fill materials. Case histories of fill behaviour are examined in four areas of practical interest to the geotechnical engineer: (a) the geotechnical behaviour of opencast mining backfills; (b) the performance of rockfill dams; (c) the effectiveness of ground treatment; and (d) the condition assessment of embankment dams. In each of these areas the lecture focuses on the results of long-term field monitoring at a number of sites, from which some general conclusions are drawn. Discernment is required in the study of case histories, but despite shortcomings, they provide a much needed counterweight to excessive theorisation.