Soil strength - Notes on misunderstanding of ultimate strength and further fallacies in our practice
Abstract
Well assigned laboratory tests and their appropriate evaluation are a necessity for a successful site investigation or report in the field of engineering geology and geotechnics. Planning and evaluation of tests should respect the established soil mechanics theory, which unfortunately is not always the case.
Wrong types of soil strength are often used, such as the peak strength in landslide areas. Recently, the critical strength has often been determined from the last readings of the conventional shear box tests on undisturbed specimens, which is generally inadequate and wrong.
Another problem is the misinterpretation and misuse of unconsolidated undrained triaxial tests. Last but not least, strength is often overestimated due to excessively high shear rate and/or due to a wrong extrapolation of straight-line strength envelopes out of the applied stress interval.
In the paper we briefly summarize the theory of soil strength, analyze the above-mentioned mistakes and recapitulate the established methods of soil strength determination for geotechnical practice.