The Twentieth Terzaghi Lecture

Abstract

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 and to technical conclusions useful in geotechnical practice. Collectively, they illustrate that soils are not inert materials, but that they can, and often do, change with time and changes in environmental conditions. Soil mechanics, in either conventional or advanced forms, cannot account for three of the four cases described. All conceivable physicochemical and biological changes must be considered in any case to explain the unusual behavior. It is essential to expect the unexpected and to deal with soils as they are, not as we might wish them to be.