Introduction

Determining reliable values for soil parameters, either for use in an elastic—plastic soil model such as Cam clay or to obtain values of strength and stiffness for some less elaborate analysis, usually requires that laboratory tests such as triaxial or oedometer tests be performed on undisturbed samples of soil. To obtain good quality undisturbed samples is usually expensive and frequently difficult. Performance of good-quality triaxial and oedometer tests requires time and skill and is also expensive. It is possible to characterise or classify soils with quicker, less-sophisticated tests which do not require undisturbed samples of the soils. This characterisation for cohesive, clayey soils is achieved by using index tests which determine the natural water content and the so-called liquid limit and plastic limit of the soil.*

Although the procedures which have been adopted for performing these index tests may appear quaint, it is possible, using the ideas of critical state strengths and models of soil behaviour such as Cam clay, to relate values of index properties to other properties of engineering importance. Empirical correlations between index properties and strengths and compressibilities have been used for many decades: critical state soil mechanics points the way to a rational basis for many of these correlations.

It would be extreme to suggest that the availability of these correlations makes it unnecessary to perform any tests more sophisticated than the index tests.