Increase in Undrained Shear Strength Due to Secondary Compression

Abstract

Undrained shear strength of clay is generally increased due to secondary compression or delayed compression. Though this increased strength is regarded to be derived from densification of clay particles, metamorphosis of clay minerals and dissolution of cemented matters into free water, mechanism of increase in strength during long term consolidation has not completely been clarified. In the present paper, described is a method for estimating the amount of increase in undrained shear strength of clay due to secondary compression. The most important postulate in derivation of the basic equation is that the quasi-overconsolidation of clay induced by secondary compression should be equivalent to overconsolidation due to release of over-burden pressure. In this way, the simplified relation for estimating the secondary compression-induced increase in strength is expressed by means of comparison with the strength of normally-consolidated clay. The relation suggests the increase in strength to be a power function of the elapsed time during secondary compression, including three parameters which can be obtained from oedometer tests and triaxial compression tests. The proposed method is proved to be valid for estimating the increase in shear strength of a reconsolidated clay and an undisturbed clay in direct shear tests. Additionally, the strength increment ratio defined by means of the equivalent consolidation pressure of Hvorslev (1960) is shown to be independent of the elapsed time of secondary compression.