Refinement of the Reverse Extrusion Test to Determine the Two Consistency Limits

Abstract

Liquid limit (LL) and plastic limit (PL) are the two most commonly used index properties of fine-grained soils. They have been used in not only classification of soils but also in correlation with certain engineering properties. Therefore, they have been subjected to numerous researches since they were first introduced by Atterberg in 1911. While their mechanisms were well defined in many codes and they have been in use for decades, criticisms often arose pertinent to the uncertainties inherent to them. Incredible amount of effort has been exerted to invent more rational testing methods in place of both the Casagrande's cup and bead rolling methods. Part of those efforts has been on devicing a single tool to measure the two relative index properties together. Recently, the reverse extrusion test was brought into the use of geotechnical engineers. It was shown that this tool has a potential of measuring LL, PL, and even the shrinkage limit (SL). The aim of this investigation is to reassess the ability of the reverse extrusion test to determine LL and PL with further refinement. In this regard 70 fine-grained soils covering a large range of plasticity were employed. Fall-cone method and rolling-device method were employed to determine LL and PL, respectively. The reverse extrusion tests were carried out at least five different water contents per soil sample. Extrusion pressures were plotted against water content and a curve fitting was applied to data pairs, from which the y-intercept (the coefficient a) and the slope (the coefficieent b) of the curve were determined. Those reverse extrusion coefficients were utilized to determine the representative extrusion pressures corresponding to LL and PL, as was done by the earlier researchers; however, the degree of success for the prediction of LL and PL using the representative extrusion pressures was not encouraging. Different from the previously proposed approaches, the reverse extrusion coefficients (i.e., a and b) were subjected to a multiple regression analysis along with the results of the conventional testing methods of fall-cone and rolling-device to determine the LL and PL as functions of the reverse extrusion parameters. It was shown that LL and PL can be predicted with a great degree of success using the reverse extrusion coefficients. While a great majority of the liquid limits found by using the fall-cone method were predicted with a ±10% error, almost all of the plastic limits found by the rolling device were predicted with a ±10% error. This refined investigation on the reverse extrusion test confirmed and proved that the reverse extrusion test is a simple, robust and inexpensive method capable of predicting both of two fundamental consistency limits using a single device.