---
title: "Usefulness of plasticity chart"
subject: "Soil Mechanics"
chapter: 3
key: "semester_4/soil_mechanics:plasticity-chart-usefulness"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/plasticity-chart-usefulness
site: BCT Engineering
---

# Usefulness of plasticity chart

Soil Mechanics · Chapter 3: Soil Identifications and Classification

- Asked in: 2075 Baisakh, 2 marks
- Video: [Plasticity chart | Soil Mechanics | Madhukant Goyal (IES) #AlphaConcepts](https://www.youtube.com/watch?v=TCPfPscAikg) — Alpha Learning
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/plasticity-chart-usefulness
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-3

## Question

How is the plasticity chart useful for classifying fine-grained soils?

## Short answer

Plots Ip against wL with A-line and 50% line. It classifies fine soils into clay, silt, or organic and low/high plasticity, gives CL-ML for hatched zone, classifies fines in coarse soils, and indicates compressibility and swelling.

## Answer

The plasticity chart plots plasticity index ($I_p$) against liquid limit ($w_L$) and has the **A-line** ($I_p = 0.73(w_L - 20)$) and a vertical line at $w_L = 50\%$.

It is useful because:
1. It **classifies fine-grained soils** (more than 50% passing 75 micron) into **clay (C), silt (M) or organic (O)** by position relative to the A-line (above = clay; below = silt/organic).
2. It divides them into **low ($w_L<50$) and high ($w_L>50$) plasticity** (L or H), giving CL, CH, ML, MH, OL, OH.
3. The hatched zone above A-line with $I_p = 4$ to 7 gives the borderline class **CL-ML**.
4. It is also used to classify the fines of coarse-grained soils with more than 12% fines (GM, GC, SM, SC).
5. It indicates relative engineering behaviour: compressibility, swelling and strength rise with position upward and right.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Position relative to the A-line shows
   A. clay (above) or silt/organic (below)
   B. gravel or sand
   C. coarse or fine
   D. wet or dry
   Answer: A. Above = C. (Key point: Clay or silt)

2. The vertical line at wL = 50% divides
   A. clay from silt
   B. low from high plasticity
   C. coarse from fine
   D. sand from gravel
   Answer: B. L if wL < 50; H if > 50. (Key point: L or H)

3. Chart is also used to classify
   A. fines below 5%
   B. boulders
   C. fines of coarse soils with > 12% fines
   D. peat only
   Answer: C. GM, GC, SM, SC. (Key point: Coarse soils)

## Related in Soil Mechanics

- [Field identification of fine-grained soils](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Afield-identification-clay-silt) — Soil Mechanics, chapter 3, asked 4×
- [Draw plasticity chart with symbols](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Adraw-plasticity-chart-symbols) — Soil Mechanics, chapter 3, asked 2×
- [Systems by size and plasticity; ISSCS chart](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aissc-plasticity-chart-systems) — Soil Mechanics, chapter 3