The plastic limit is the empirically established moisture content at which a soil becomes too dry to be plastic. It is used together with the liquid limit to determine the plasticity index which when plotted against the liquid limit on the plasticity chart (see BS 5930) provides a means of classifying cohesive soils. It is recognized that the results are subject to the judgement of the operator, and that some variability in results will occur.
- A flat, glass plate, smooth and free from scratches, on which threads are rolled. A convenient size of plate is about 10 mm thick and 300 mm square.
- Two palette knives or spatulas.
- Apparatus for the moisture content determination of fine-grained soils
- A length of rod, 3 mm in diameter and about 100 mm long.
- Take a sample of about 20 g from the soil paste prepared and place it on the glass mixing plate.
- Allow the soil to dry partially on the plate until it becomes plastic enough to be shaped into a ball.
- Mould the ball of soil between the fingers and roll it between the palms of the hands until the heat of the hands has dried the soil sufficiently for slight cracks to appear on its surface. Divide this sample into two subsamples of about 10 g each and carry out a separate determination on each portion. Divide each subsample into four more or less equal parts and treat each part as specified in Step No. 4 to Step No. 8.
- Mould the soil in the fingers to equalize the distribution of moisture, then form the soil into a thread about 6 mm diameter between the first finger and thumb of each hand.
- Roll the thread between the fingers, from finger-tip to the second joint, of one hand and the surface of the glass rolling plate. Use enough pressure to reduce the diameter of the thread to about 3 mm in five to 10 complete, forward and back, movements of the hand. Some heavy clays will require 10 to 15 movements when the soil is near the plastic limit because the soil hardens at this stage. It is important to maintain a uniform rolling pressure; do not reduce the pressure as the thread diameter approaches 3 mm.
- Pick up the soil, mould it between the fingers to dry it further, form it into a thread and roll it out again as specified in Step No. 5.
- NOTE Gradually drying of the soil is effected by alternately rolling and moulding, not by continual rolling, either as a ball or as threads, which produces a dried crust.
- Repeat Step No. 6 until the thread shears both longitudinally and transversely when it has been rolled to about 3 mm diameter, as gauged by the rod. Do not gather the pieces of soil together after they have crumbled, in order to reform a thread and to continue rolling; the first crumbling point is the plastic limit.
- NOTE With soils that are marginally plastic it is often difficult to obtain the correct crumbling condition.
- Gather together the portions of the crumbled soil thread transfer them to a suitable container and replace the lid immediately.
- Repeat Step No. 4 to Step No. 8 on the other three portions of soil, placing them all in the same container. Determine the moisture content of the soil in the container as specified in BS 1377:1990-1 clause 3.2.
- Repeat Step No. 3 to Step No. 9 on the duplicate sample formed in Step No. 3.