The φ = 0 Concept: Review of its Theoretical Basis and Pragmatic Issues with Implementation

The " = 0 Concept" is central to the specification of the undrained shear strength of saturated clays in engineering practice. While the theoretical basis for the Concept lies in Mohr's rupture hypothesis, its pragmatic value to geotechnical engineers lies in Terzaghi's effective stress principle. Specifically, the  = 0 Concept circumvents the onerous need for estimating (or measuring) excess pore water pressures in situ during construction, etc., which is required to perform an effective stress analysis. Despite the significance of the  = 0 Concept, a survey of geotechnical engineering textbooks published over the last 60+ years showed that most opt to just state the  = 0 Concept without putting it into proper theoretical or contextual perspective. The authors view this as a missed opportunity to put the various rules of thumb and sets of governing principles for the shear strength of soil into a consistent framework. Accordingly, the objectives of this paper are to review the theoretical basis of the  = 0 Concept in the context of Mohr's rupture hypothesis and Terzaghi's effective stress principle and to discuss some of the pertinent pragmatic issues with implementing the Concept. In relation to the latter objective, the procedure for multi-staged rapid drawdown analysis for earthen dams proposed by Duncan et al. and the procedure for determining the strength of liquefied soil proposed by Poulos et al. are reviewed. It is shown that while Duncan et al. fully address all the pragmatic issues in implementing their procedure, Poulos et al. do not.