Soil Classification MCQ with answer and solution

Concept

Plasticity Chart Soil Classification

Formula

PI = LL - PL

Step-by-step solution

1. Calculate Plasticity Index: PI = LL - PL = 20 - 15 = 5. 2. Since LL \< 30 and PI is between 4 and 7, the soil is classified as CL-ML.

Explanation

The Plasticity Index (PI) is calculated as Liquid Limit minus Plastic Limit (20 - 15 = 5). On the plasticity chart, soils with a Liquid Limit under 30 and a Plastic Index between 4 and 7 fall into the dual-symbol region, classifying the soil as CL-ML (silty clay).

Shortcut

For soil classification questions on the plasticity chart:1. Calculate Plasticity Index (PI): = LL- PL 2. Shortcut Rule: If PI is between 4 and 7, it falls directly into the dual-symbol border region of the plasticity chart, making the classification automatically CL-ML (Silty Clay).

Common mistake

Students often calculate the Plasticity Index (PI = 20 - 15 = 5) and use the A-line formula (0.73 × (LL - 20) = 0), concluding that since PI > 0, it must be CL, while forgetting that any soil with a Plasticity Index between 4 and 7 falls into the dual-symbol border region (CL-ML).