Changes from plastic to non-plastic behavior of clays and their relationship with specific gravity

Cambios del comportamiento plástico a no plástico de arcillas y su relación con la gravedad específica

Abstract

It is shown that the change from plastic to non-plastic behavior is complex and depends on the interaction of the grains of plastic material contained in the soil mixture with free water. That is, when the skeleton of non-plastic particles that define the structure of the mixture begins to transmit the forces between particles and the plastic material forms a kind of “paste” with the free water, similar to the cement paste in the mortar. This gives plastic properties to the movement of the grains but the forces are transmitted by the skeleton of the non-plastic material. When the high concentration of fine nonplastic material greatly increases the ratio of voids and free water, the clay-water paste reaches water concentrations well above the liquid limit. Consequently, the paste behaves as a viscous fluid within the pore spaces. These concepts apply to different types of soils: CL, CH, ML and MH. With these criteria, it was established for five clay samples from the Mesa Formation with a high content of non-plastic materials, that four of them are within the behavioral transition zone although they still present plasticity. However, they will have high friction angles compared to other clays. The other sample had parameters very close to the threshold for the change in behavior to non-plastic, so in undrained conditions it could, without a doubt, present non-plastic behavior. For these Mesa Formation clays, it is the fine-grained nonplastic silt (sizes in the clay range) that controls the change in plasticity and specific gravity.