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GATE 2026 CE (CE2) – Question 21

Geotechnical Engineering · Three-phase system, phase relationships, index properties and soil classification · 1 mark · Multiple choice

Based on the Casagrande's plasticity chart, plasticity index (in %) of inorganic clays having a liquid limit of 40 %, ranges between

  1. 18.0 and 28.8
  2. 14.6 and 28.8
  3. 14.6 and 23.4
  4. 18.0 and 43.8

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Correct answer: (B) 14.6 and 28.8

Explanation

Inorganic clays plot above the A-line and below the U-line on the plasticity chart. At $LL = 40$ the A-line gives $PI = 0.73(LL - 20) = 0.73 \times 20 = 14.6$ and the U-line gives $PI = 0.9(LL - 8) = 0.9 \times 32 = 28.8$. So the plasticity index ranges from 14.6 to 28.8.