SOIL PLASTICITY AND STRENGTH--A NEW APPROACH USING EXTRUSION

The author uses experimental results to show that the consistency of soils can be represented by the water contents of the saturated remoulded clays, defining critical states for the soil. Critical states are related to unique shear strengths, proposed conditions for strength at liquid limit is 1.6 kn/sq M and for the strength at plastic limit 110 kn/sq M. The strength at the liquid is shown to be more reliably determined by the cone penetration test BS 1377 compared with the casagrande type percussion cap machine. The atterberg rolling bead test is shown to be unsuitable for the determination of the strength at the plastic limit. The mechanical principles of extrusion testing, a well established technique in metal-forming and brick-making, are discussed. Preliminary reverse extrusion tests on a low plasticity clay produced favourable results with other plastic materials and enabled shear strengths to be determined with a high reliability. It is suggested that extrusion could be a rapid, simple economical and reliable method for determining soil plasticity. (TRRL)