Plastic limit:<\/strong> roll threads to 3 mm until they crumble; take the moisture content of the crumbled threads.<\/li>\n<\/ol>\nCalculation & Reporting<\/h2>\n
Report LL, PL and PI to the nearest whole number with the method used. Plot PI against LL on the plasticity chart: above the A-line (PI = 0.73(LL−20)) the soil is clay (C), below it silt (M); LL > 50 marks high plasticity.<\/p>\n
Acceptance Criteria<\/h2>\n
Typical values: low-plasticity clay LL 30–50, PI 10–25; high-plasticity clay LL > 50, PI > 25. Earthworks specifications commonly cap PI (e.g. sub-base PI ≤ 6, structural fill PI ≤ 20) \u2014 check the governing road authority spec.<\/p>\n
Frequently Asked Questions<\/h2>\nCone penetrometer or Casagrande \u2014 which is better?<\/h3>\n
The fall cone is the definitive BS\/Eurocode method: it is operator-independent and more repeatable. The Casagrande device remains the ASTM reference; results differ slightly, so state the method with every result.<\/p>\n
Why must the sample pass the 425 µm sieve?<\/h3>\n
Atterberg limits describe the fine fraction only. Coarser particles interfere with the groove and thread behaviour and would bias both LL and PL upward.<\/p>\n
Recommended Apparatus<\/h2>\n
NL Scientific manufactures the Motorized Liquid Limit Apparatus (ADV)<\/a> for this method. Browse the full Soil Testing Equipment<\/a> range or request a quotation<\/a> from our engineers.<\/p>\n
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