An apparatus for the determination of the workability and plastic limit of clays

An apparatus is described that replicates Atterbergʹs procedure of rolling a thread of clay for the determination of the plastic limit of soils with minimal operator interference or judgement of the crumbling condition required. From nominal stresses applied during rolling and changes in diameter measured for each rolling traverse the toughness of the clay is determined from the stress–strain curves, as work/unit volume for a specified amount of deformation. Threads are tested at water contents ranging from near the sticky limit through the workable plastic region to the brittle state. An abrupt ductile–brittle or plastic–brittle transition is found allowing the plastic limit to be defined as the transition water content. A less abrupt but distinct stiffness transition is frequently obtained that separates the workable plastic region into a soft-plastic and a stiff-plastic region. Good relationships between toughness and water content are achieved from which the maximum toughness, Tmax, at the plastic limit is derived together with other new and useful properties such as the water content at the stiffness transition, wS and at zero toughness, the toughness limit, wT. Also derived are the gradients of the relationships, the toughness coefficients, TC, the range of water contents over which the clay is plastic and tough, the toughness index, IT and the water content in relation to the toughness index, the workability index, Iw. Each clay is found to have a closely correlated toughness–water content relationship depending mainly on the clay content and clay mineralogy demonstrating that control of the plasticity of the clay and its associated water contents are achievable.