Problems of embankment construction on soft clays

Abstract

The main problems encountered in an embankment design and construction on soft cohesive soils are discussed in this study and the concepts of the critical state model are used to interpret the stress-strain behaviour and the stability of an embankment. The theoretical and practical considerations on the behaviour of soft soils are discussed in detail. The load carrying capacity and the strength properties of soft clays can be improved by applying a stage-construction technique. The available methods of stage-construction are examined and a new design procedure for stage constructed embankments on soft clays is proposed. The existing design procedures are either relied on total stress analyses with which in the majority of cases, the values of safety factor are overestimated or the effective stress analysis which has the power of applyirig the fundamental factors governing the soil behaviour is used. The existing design methods based on effective stresses have the shortcoming of computing total stresses by the elastic methods and making use of these computed total stresses either for the prediction of excess pore water pressures or for the determination of the shapes of the effective stress paths, although it is a wellknown fact that the elastic conditions do not prevail in a yielding soil structure. This source of error is tried to be eliminated by the proposed design 'procedure which hypothesizes that, the at rest bonditions are recreated in a soil body by the end of the consolidation periods. This hypothesis is rationalized by showing theoretioally that anisotropic conditions are accompanied by a shearing stress and excess pore pressures in addition to that induced by the applied loading. This behaviour is reflected in the anisotropic, at rest character of the soils in the nature. The proposed design procedure has the power of simplicity and being derived from the fundamental factors governing the soil behaviour and also the parameters used ln the design procedure are a few and they can be determined by the conventional laboratory tests. According to the proposed design procedure an intermediate line is constructed and the effective stress states attained by the soil element during undrained loading are kept on the intermediate line and thus off the critical state line. The intermediate line is produced in such a way that, a sufficient value of the factor of safety is provided against, the failure shear stresses and also against the bearing capacity failure at each stage of construction. The increase ln the undrained shear strength during consolidation periods is also taken into account. The procedure is applied for both the isotropically normally consolidated and anisotropically normally consolidated soft clays and the results are correlated. The procedure is outlined step by step in chapter V and also the behavioupal aspects are discussed. The general conclutions are given In the last chapter.