According to Cassagrande’s plasticity chart, the A-line separating clays from silts is given by the equation:
AIP = 0.73 * (LL - 20)
BIP = 0.50 * (LL - 10)
CIP = 0.90 * (LL - 20)
DIP = 0.73 * (LL - 10)
Explanation
Cassagrande’s A-line equation is IP = 0.73 * (LL – 20). Inorganic clays lie above the A-line, while inorganic silts and organic soils lie below it.

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