Effective stress response of clay to undrained cyclic loading

M. Zergoun, Y.P. Vaid
1994 Canadian geotechnical journal (Print)  
In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written
more » ... without my written permission. Department of Ci\hu GM cSti\)ee_R-M£- ABSTRACT No information exists in the current soil mechanics literature on the behaviour of saturated clays under slow cyclic reversal in stress and strain, that will allow an effective stress interpretation of their undrained response. This study fills such a gap in our knowledge of clay behaviour. The undrained response of normally consolidated Cloverdale clay to slow constant stress amplitude cyclic reversal in stress and strain follows two distinct patterns. Under low cyclic stress levels (less than half the undrained strength), residual strain and pore water pressure development with loading cycles tends toward equilibrium. Under high cyclic stress levels (higher than half the undrained strength), strains at peak of cyclic stress eventually accumulate at an accelerated rate per cycle. A single threshold principal effective stress ratio signals the onset of strain acceleration for all cyclic stress levels. A step increase in cyclic stress level may shift the clay response from equilibrium type into instability type within the same cyclic loading event. During cyclic loading the effective stress states for a given number of cycles form a collapsing stable state boundary surface common to all cyclic stress levels. The relationship between principal effective stress ratio and strain at peak of cyclic stress depends on both cyclic stress level and loading phase. Cyclic stress level also affects the correlation between cumulative hysteretic work and residual pore water pressure. However, damping ratio remains essentially ii ABSTRACT constant for all cyclic stress levels. Upon cyclic instability, the clay fails on the same effective stress strength failure envelope as under monotonic loading. Undrained cyclic loading clay response depends on the initial loading direction. Cyclic loading initiated in the direction opposite to natural clay deposition is more damaging than loading initially congruent with natural direction of material deposition. Constant stress ratio amplitude cyclic reversal in stress and strain results in limited strain and pore water pressure development with loading cycles. The collapsing stable state boundary surface found for constant stress amplitude cyclic loading is however also a bounding effective stress state for constant stress ratio cyclic loading. Postcyclic undrained monotonic loading results in strength and stiffness degradation directly related to the maximum strain developed during cyclic loading and to the effective stress stability threshold found for constant stress amplitude cyclic loading. A unique relationship exists between normalized undrained strength and overconsolidation ratio regardless of the type of overconsolidation history, provided that the overconsolidation ratio is defined with respect to the equivalent consolidation stress. iii TABLE OF CONTENTS ABSTRACT ii LIST OF TABLES viii LIST OF FIGURES ix INDEX OF NOTATIONS xvi 157 vii The process of writing this thesis has been graced with help from many people. I would like to thank everyone and especially the University of British Columbia Civil Engineering faculty and staff members. I am especially grateful to Professor Y.P. Vaid who inspired and continuously encouraged this study with his constructive criticism and his unswerving patience and to Fred Zurchirken who provided technical support and friendship that extended beyond the scope of this work. Financial support in form of awards, grants, bursaries, work-study programs and loans from the University of British Columbia, the National Engineering Sciences Research Council of Canada, the Worthington Memorial Bursary Fund, the Peter Demco Memorial Bursary Fund, the Carl J. Culter Bursary Fund, the British Columbia Ministry of Education and the Secretary of State of Canada is gratefully acknowledged. xviii during post-cyclic loading monotonic loading, M.A.Sc. dissertation,
doi:10.1139/t94-083 fatcat:o3nh6ksg65do5im53df3ksoo7e