Soil Classification MCQ with answer and solution

Concept

A-Line Equation in Soil Classification

Formula

PI = 0.73(LL - 20)

Step-by-step solution

1. Identify Liquid Limit (LL) = 50. 2. Subtract 20 from LL: 50 - 20 = 30. 3. Multiply result by 0.73: 0.73 \* 30 = 21.9.

Explanation

In soil classification, the A-line equation separates clay from silt and is given by Plasticity Index (PI) = 0.73 × (Liquid Limit - 20). Substituting a Liquid Limit (LL) of 50 gives PI = 0.73 × (50 - 20) = 21.9. Soil is classified as silt if its PI falls below the A-line, which means its PI must be less than 21.9.

Shortcut

Remember the A-line equation PI = 0.73(LL - 20): soil below the line is silt and above is clay. For LL = 50, quickly calculate 0.73 × 30 = 21.9; a PI below 21.9 classifies the soil as silt.

Common mistake

Students often make a mistake by using the wrong constant in the A-line equation, such as subtracting 30 instead of 20 from the Liquid Limit [i.e., 0.73 × (50 - 30) = 14.6], which leads them to incorrectly select Option A instead of Option B.