Section 4: Soil and Water Conservation Engineering
Soil Mechanics
Subtopic: Index properties of soils
Question Statement:
A fine-grained soil has a liquid limit $w_L = 45\%$ and a plastic limit $w_P = 22\%$. According to Casagrande's plasticity chart, the plasticity index of the A-line ($I_{P, A\text{-line}} = 0.73(w_L - 20)$) in percentage is ________ (round off to 1 decimal place).
Answer Key:
18.3
Step-by-Step Solution & Explanation
The equation of the A-line in the Indian Standard / Casagrande Plasticity Chart is:
$I_{P, A\text{-line}} = 0.73 (w_L - 20)$
Given $w_L = 45\%$:
$I_{P, A\text{-line}} = 0.73 (45 - 20) = 0.73 \times 25 = 18.25\% \approx 18.3\%$