In spite of our present very advanced analytical capabilities, our present ability to predict actual field behavior is, in many cases, woefully inadequate. Four cases are described to illustrate one of the reasons why actual and predicted performances are sometimes so widely divergent—namely, failure to understand how a soil might respond over time to changed conditions. The four cases are: (1) The aging of quick clay after sampling, in which the remolded strength increases in samples maintained at constant water content; (2) time effects in freshly densified or deposited sand, in which natural sand deposits can lose strength if disturbed but regain strength over time periods of weeks to months; (3) apparently sound lime‐stabilized soil that swells and disintegrates starting a few years after construction; and (4) the failure of excess pore pressures to dissipate as predicted during the consolidation of soft clays. Study of each of these problems has led to an understanding of the responsible phenomena an...

Practical problems from surprising soil behavior

Constitutive equations for normally consolidated clay based on elasto-viscoplasticity

A new framework for the development of constitutive models based on certain aspects of physical theories of plasticity is proposed. The resulting multi-laminate model of clays has the same response as the critical state model provided no rotation of principal stress axes takes place during plastic flow. The influence of rotation of the principal stress axes on the volumetric and deviatoric plastic strains has been numerically evaluated. In undrained saturated, normally consolidated clays considerable excess pore pressure may build up due to rotation of the principal axes only. Numerical studies of the collapse of footings indicate that significant rotations do not take place in this class of problems and consequently the collapse load is only marginally reduced. (Author/TRRL)

Multi‐laminate model of clays—a numerical evaluation of the influence of rotation of the principal stress axes

The cyclic behavior of normally consolidated clays is investigated by conducting a series of cyclic simple shear tests on one-dimensionally and isotropically consolidated kaolinite samples. The response of two types of specimens are evaluated and compared based on tests performed with different cyclic shear stress amplitudes and frequencies. The cyclic stress-strain-pore pressure and cyclic shear strength properties of the clay samples are studied and a simple procedure is suggested to define cyclic yield strength. The results also indicate that there is a threshold cyclic shear stress level below which no excess pore pressure will develop. An attempt is made to develop an empirical procedure to estimate the response of a soil element subjected to cyclic shear stresses for a given number of cycles.

Undrained Behavior of Clay Under Cyclic Shear Stresses

The experimental techniques pertaining to accurate measurement of creep in sand are explained in detail. Triaxial compression and proportional loading tests were performed in a triaxial setup for which special procedures were developed to maintain constant temperature, constant confining pressure, and constant axial load, whereas the axial and volumetric deformations were measured using two types of mechanical measurement systems, both free of zero drift. Special attention was paid to the avoidance of air and water leakage through the membrane in the long-term tests. The experiments show that the nature of creep strains is similar to that of plastic strains. They may be predicted from the framework provided by the hardening plasticity theory. In particular, the potential surface determined for the prediction of plastic strains may also be used for the prediction of time-dependent creep strains. From the experiments it also appears that the yield surface and the plastic potential surface move out together, and the point at which to evaluate inelastic strain increment directions is at the current location of the yield surface and the accompanying plastic potential surface.

Experimental Study of Drained Creep Behavior of Sand

Synopsis Stress controlled laboratory test data on the creep behaviour of a coastal organic silty clay under undrained conditions at different isotropic consolidation pressures and stress levels are reported. During undrained creep the sustained deviator stress was maintained at 90, 70, 50 and 30% of the ultimate deviator stress determined in normal strength tests. The pore pressures developed during creep have been shown to be due both to the distortional stress and also to the secondary compression effect. It is considered that any theory developed for the prediction of stress paths for undrained strength and creep behaviour of sensitive soils should take into account the pore pressures developed during secondary compression under zero deviator stress. The magnitude of pore-water pressure build-up during secondary compression is shown to be time and structure dependent. On donne les resultats d'essais au laboratoire, sous contrainte controlee, sur la tendance au fluage d'ne argile limonique organique co...

C. K. Shen

Papers

Undrained Creep Behaviour of a Coastal Organic Silty Clay