Genuine and false pre-consolidation and yield pressures

Laurence D Wesley

doi:10.1051/e3sconf/20199205001

Abstract

“It remains a mystery why the international profession still uses the awkward e-log p plots, and the incomplete and useless coefficient Cc which is not even determined from the measured data, but from a constructed line outside the measurements”. These are the words of Nilmar Janbu (1998). This paper does not solve the mystery; what it does is highlight the issue behind the mystery in the hope that the profession will face up to the defects of the log scale. An examination is made of oedometer test results when plotted to both scales. Examples are given for sedimentary and residual clays, and sands. It is shown that the e-logp plot routinely produces graphs from which pre-consolidation pressures or yield pressures can be determined. However, when plotted using an arithmetic (linear) scale, this apparent pre-consolidation or yield pressure is found, in most cases, to disappear. It is a false value created by the way the data are plotted. The paper does not deny that pre-consolidation or yield pressures exist in many soils; it gives examples for each of the soil types listed above. It is recommended that the term “yield pressure” be used in preference to “pre-consolidation pressure”, and that only the use of a linear scale will reveal whether such a pressure exists.

Highlights

Soil mechanics is a recent addition to the field of civil engineering, having only become known worldwide in the 1930s
A brief history of the use of the log plot is given followed by examples of errors resulting from its use, and a preferred portrayal of soil compressibility is proposed
The earliest graphs of void ratio versus pressure for one dimensional compression appear to be those of Gilboy [2] shown in Figure 1, adapted and reproduced by Terzaghi and Peck [3] in their book “Soil Mechanics in Engineering Practice”

Summary

Introduction

Soil mechanics is a recent addition to the field of civil engineering, having only become known worldwide in the 1930s. It grew rapidly in the following decades and by about 1960 a firm theoretical framework had been established on which to base its further development. Along with this theoretical framework various empirical correlations and assumptions were adopted, many of which have proved valid and have survived to this day. An important example is the continued use of the log scale to portray soil compressibility. A brief history of the use of the log plot is given followed by examples of errors resulting from its use, and a preferred portrayal of soil compressibility is proposed

History of the log scale

Examples of log and linear plots

Over-consolidated clay

Residual clay

Further considerations

Conclusion