Water Resources Center

The Water Resources Center is affiliated with the College of Food, Agricultural and Natural Resource Sciences and University of Minnesota Extension.

Procedure for Determining Plastic Limit

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Plastic Limit (PL or wP) – The minimum water content at which the mixture acts a plastic solid. More information can be found in the Manual for SSTS Professionals in Minnesota, Section 3 - Sewage Treatment Utilizing Soil.

Step-by-Step Instructions

Procedure for Determining Plastic Limit in the Field:

  1. Select a handful of soil for testing (any non-soil material (rocks, roots, etc.) should be removed). Do not add moisture or let dry-out. Sample should be taken at the depth of excavation (absorption area).
  2. Roll the sample between the palms (in the shape of a pencil/worm shape).
  3. Continue rolling the thread until it reaches a uniform diameter of 1/8 inch if possible (See Figures 3.23 and 3.24).
  4. If the sample does not reach a diameter of 1/8 inch, the soil is above the plastic limit and construction can proceed.
  5. If the sample is rolled into a diameter equal to 1/8 inch before breaking, the soil is too wet and construction should not occur.

Figure 3.23 Roll Sample Between the Palms


Figure 3.24 Field Determination of Plastic Limit