A theoretical envelope for internal instability of cohesionless soil

The nature of seepage-induced internal stability is characterised with reference both to effective stress in the finer fraction of the grain size distribution curve, and also to hydraulic gradient within the soil. The concept of a hydromechanical envelope is proposed in stress-gradient space. The onset of instability is triggered by a critical value of hydraulic gradient that is found consistent with the observations of Terzaghi on 'piping' in uniform sand, and the observations of Skempton and Brogan on 'segregation piping' in widely graded sandy gravels. The envelope takes the form of a linear relation that, for a particular soil, is governed by the proportion of effective stress in the finer fraction of soil grains.