"A" line charts and what they mean

Depending on the amount of water present, cohesive soil can exist in three states: as a liquid slurry, a plastic substance or a solid. The tests for Atterberg Limits were developed as a means of distinguishing them. The "liquid limit" is the relatively high water content at which the soil changes from a liquid to a plastic state, and the "plastic limit" designates the relatively low water content at which soil changes from a plastic to a solid state. The procedures for determining the liquid and plastic limits are well established and are described in detail in publications of the American Society for Testing and Materials and of the British Standards Institution.

The difference in water content between the liquid and plastic limits is defined as the "plasticity index" of the soil. It follows that the greater the plasticity index, the more plastic and compressible and the greater the volume change characteristics of the soil. The plasticity index has proven to be one of the most useful of all soil indices and is essential to the description of a cohesive soil.

As a convenience for comparing a variety of soils, Dr. A. Casagrande devised a plasticity chart (Figure 1), in which an empirical boundary known as the "A" line separates inorganic clays from silty and organic soils. Soils of the same geological origin usually plot on the plasticity chart as straight lines parallel to the A line. The larger the plasticity index the greater will be the volume change characteristics. "Fat" or plastic clays plot above the line. Organic soils, silts and clays containing a large portion of "rock flour" (finely ground non-clay minerals) plot below it.  Here are four separate charts that help to make very clear the use of Dr Casagrande's charts.

The above chart shows both the "A" line and the "B" line, staying within these ranges is safe but other ranges can work perfectly. The charts show the level of clay and silt in the fines tested.