Geotechnical safety in relation to water pressures

Provision of adequate safety in geotechnical designs dominated by water pressure has al- ways been difficult and controversial. It is also of very great practical importance since a significant pro- portion of geotechnical failures is caused by the unforeseen effects of water pressure. To varying de- grees, modern codes have attempted to guide rational judgments and also to provide precise formats in which safety can be prescribed. A recurrent difficulty lies in the fact that the density of water is known quite accurately, and many designers are therefore reluctant to apply factors that increase the design value of its density. Furthermore, changing the design density of water has complicated effects on the mechan- ics used in calculation, which may lead to unintended increases or decreases in safety. The paper references case histories of failures caused by water pressure and reviews the safety provi- sions related to water pressure in some existing geotechnical codes. It discusses the provisions of Euro- code 7 and the way they are currently being interpreted and applied in individual countries. Seven exam- ples that were discussed in the workshop on Eurocode 7 in Pavia, 2010, are considered in more detail in order to illustrate alternative approaches. The authors attempt to identify the common features of ap- proaches to water pressure that provide a sound, rational basis for design in problems in which water pressures are a major concern.