Introduction to Plastic limit test of soil

Most method for determination of plastic limit is by rolling a thread of soil (on glass plate) until it crumbles at a diameter of 3 mm.

Definition of plastic limit test

Plastic limit is defined as the minimum amount of moisture content, in percent, at which a cohesive soil change from a plastic state to a semisolid state.
OR

In the lab, the plastic limit is defined as the moisture content (%) at which a thread of soil will just crumble when rolled to a diameter of 1/8 in. (3.18 mm).

plastic limit test apparatus

• Evaporating dishes.
• Pulverizing apparatus – mortar and rubber covered pestle.
• No. 40 (0.425 mm) sieve.
• Spatula.
• Balance sensitive to 0.01 gm.
• Watering bottle, with distilled water demineralized or tap water.
• Drying tares with covers
• Surface for rolling: rolling surface, minimum area of 300 × 300 mm (12 × 12 in.), non-absorptive, non-corrosive, light surface texture (e.g ground glass plate, linoleum, or plastic-faced plywood).
• Oven – Capable of maintaining temperatures of (110±5 oC) for drying moisture samples.
• A ⅛ inch (3 mm) diameter rod may be used as a guide to help the operator estimate the thread size.

plastic limit test of soil procedure

• Take a sample of about 20 gm. of soil passing the #40 sieve. Thoroughly mix the sample with distilled water until the mass becomes plastic enough to be easily shaped into a ball. Take a portion of this ball (about 8 gm.)
• Deform 8 gm. test sample into an ellipsoidal-shaped mass.
• Roll the ellipsoidal shape soil between the fingers and palm or plan on the glass plate with just sufficient pressure to roll the mass into a thread of uniform diameter throughout its length.
• The rate of rolling shall be between 80 to 90 rolls per minute.

Note: When the diameter of the thread becomes less than 3 mm break the thread and squeeze the pieces into a uniform mass of ellipsoidal shape and re-roll.

• Continue this alternate rolling to a thread of 3-mm. in diameter, gathering together, kneading and re-rolling, until the thread crumbles under the pressure required for rolling and the soil can no longer be rolled into a thread.
• The crumbling may occur when the thread has a diameter greater than 3-mm. This shall be considered a satisfactory end point, provided the soil has been previously rolled into a thread 3 mm in diameter.
• Gather the portion of the crumbled soil together and place in a suitable container to determine the moisture content and record the result as the plastic limit.
• Report the plastic limit to the nearest whole number. Calculate the plasticity index of soil as the difference between its liquid limit as determined in experiment No.1 and its plastic limit as follows:
• Plasticity index = Liquid Limit – Plastic Limit
• Report the difference calculated as plasticity index except under following conditions:
• When the liquid limit or plastic limit cannot be determined, report the plasticity index as NP (non-plastic).
• When the plastic limit is equal to, or greater than, the liquid limit, report the plasticity index as NP.

Mathematical Formula to calculate plastic limit of soil

water content (%) = weight of water/weight of dry soil sample x 100

weight of water = (weight of container+wet sample) – weight of container + dry sample

weight of dry soil sample = (weight of container+dry sample) – (weight of the empty container)

plastic limit test calculation table

Plastic limit is calculated from the average moisture content from the calculation table so in this case Plastic limit is 20.33%.

plastic limit test video