No abstract

A database of 641 fall cone tests on 101 soil samples from twelve countries has been analysed to determine the best mathematical relationship linking undrained shear strength with liquidity index. From the database, it is shown that the use of a linear relationship linking liquidity index and the logarithm of undrained shear strength that uses the commonly assumed 100-fold factor increase in strength from liquid to plastic limit over-predicts the measured data of soil strength. The use of a factor of about 35 for the ratio between the strength at liquid limit and that extrapolated to plastic limit is shown to be more realistic. Logarithmic liquidity index is examined and found to also correlate strongly with the logarithm of undrained shear strength, however it is shown that no great statistical improvement is present compared with the semilogarithmic formulation. When considering data of individual soils a power law fitting is shown statistically to be the preferred mathematical function.In this paper, it will be demonstrated, based on a large database, that the R MW value not only varies between soils but that an average value of around 35 is more appropriate, substantially less than the commonly assumed factor of 100. Fall cone strengths predicted from equation (2) will hence be un-conservative, 2013) The plastic limit of clays. Géotechnique, 63(6): 435-440. Hansbo, S. (1957) A new approach to the determination of the shear strength of clay by the fall cone test.

The plastic limit of soils was first described by Atterberg in 1911. The thread-rolling test was standardised at the US Public Roads Bureau in the 1920s and 1930s, and has subsequently become one of the standard tests of soil mechanics. This paper reviews the original definitions of plastic limit as proposed by Atterberg, and proposes that the brittle failure observed in the plastic limit test is caused by either air entry or cavitation in the clay. Critical state soil mechanics is used to show that the observed range of undrained shear strengths of soils at plastic limit is consistent with this hypothesis. The fallacy that strength at plastic limit is a constant is highlighted, and the implications for geotechnical practice are discussed.

The thermal properties of soils are important in a variety of applications, including the thermal performance of buried pipelines and geothermal heat pumps. A variety of methods exist for the prediction of thermal conductivity based on empirical fits to databases of soil thermal conductivities. In this paper an analytical model will be developed based on unidirectional heat flow through a three-phase soil element. The performance of this model will be validated against a database of 155 test measurements from the published literature. A variety of alternative prediction methods will be tested against this dataset, with the model derived here being shown to be at least as good as the best empirical model while retaining a physical origin.

The liquid limit of soils can be measured using either the cup apparatus developed by Casagrande or by the fall-cone method, which is now the standard in much of Europe. This paper will demonstrate, based on a Newmarkian sliding block analysis of the Casagrande percussion test, that these two methods measure different mechanical properties of the soil; the cup test being a measure of specific strength, i.e., strength divided by density, whereas the fall cone is a direct measure of strength. Sources of error in the operation of the percussion test are quantified and a modification to the fall-cone test is proposed to standardize results between the two test methods.