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.
Procedure for Determining Plastic Limit in the Field:
- 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).
- Roll the sample between the palms (in the shape of a pencil/worm shape).
- Continue rolling the thread until it reaches a uniform diameter of 1/8 inch if possible (See Figures 3.23 and 3.24).
- If the sample does not reach a diameter of 1/8 inch, the soil is above the plastic limit and construction can proceed.
- 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