2007 Volume 47 Issue 2 Pages 303-318
Estimating increase in undrained shear strength su of clay is an important purpose of consolidation analysis as well as settlement prediction when multi stage loading for construction of earth structures on soft clay is concerned. The present paper investigates the increase in su with regard to rate of consolidation. Since values of su are usually evaluated as a function of effective vertical stress σv′, rate effect on shear strength increment ratio in normally consolidated state sun/σv′ is first discussed considering the fundamental concept expressed by the equation of sun/σv′=suf/σy′, in which suf is in-situ undrained shear strength and σy′ is consolidation yielding stress. The paper also describes two case histories where actual increases in su were observed in soft clay deposits. The first site is located offshore Osaka-bay where a large-scale seawall was constructed, and soft clay in the site was improved by sand drains. The second is a reclaimed land in Yanai City where a sand fill and a subsequent test embankment were conducted on a soft clay deposit without improvement by vertical drains. It is found from the study that: (1) rate effect on sun/σv′ cancels rate effect on σy′, and the equation mentioned above is valid regardless of the rate of consolidation, and (2) the in-situ values of sun/σv′ observed in the two sites vary from 0.27 to 0.37 as consolidation progresses, and they are well related to rate of consolidation. According to the experience at the two construction sites, values of sun/σv′ with regard to rate of consolidation are proposed for design use at field/construction sites.