Abstract
A flow rule proposed for virgin clays by Poorooshasb and Yong (1982) utilizes the concept of intersecting yield surfaces in conjunction with the Rendulic-Roscoe criterion. According to this criterion, during a loading process the successive states experienced by a clay element trace a unique surface in the state space. In the present paper the proposed flow rule is examined in the light of Drucker's work hardening postulate and a necessary condition for ‘stability’ is derived. Furthermore, a theorem which may be used to prove the uniqueness of the solution for a certain class of problems is stated and proved. Finally, the question of extremum is discussed and it is shown that a variational form can be obtained only under a special, yet important, circumstance, i.e., the rapid loading of the system.
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