Safety considerations for the HYD limit state

摘要

The HYD limit state, defined in Eurocode 7, relates to upward flow of water through soil to a free surface, such as may occur in front of a retaining wall in the base of an excavation. Terzaghi (1922) proposed that safety in this situation may be checked by studying the equilibrium of a rectangular block of soil extending a depth t from the free surface to the toe of the wall and of width b=t/2. He proposed a calculation for a factor of safety, but did not recommend a specific value for it. Many other publications have shown Terzaghi's calculation and have recommended values for the factor of safety. The range of recommended values is very large, generally with little obvious reason for such disparity. Orr (2005) discussed the application of partial factor methods to this problem, showing that the precise way they are applied may have a big effect on results, while Simpson (2012) argued that proper understanding of the concept of limit state "design values" is needed. In this paper, Terzaghi's calculation is reviewed, considering whether b=t/2 is a sensible prescription, what part is played by the stress state and strength of the ground and, most significantly, what is the influence of variations in permeability within the soil body. It is shown that it is far more important that designers think incisively about ground permeability than that they verify a particular factor of safety.