---
title: "Systems by size and plasticity; ISSCS chart"
subject: "Soil Mechanics"
chapter: 3
key: "semester_4/soil_mechanics:issc-plasticity-chart-systems"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/issc-plasticity-chart-systems
site: BCT Engineering
---

# Systems by size and plasticity; ISSCS chart

Soil Mechanics · Chapter 3: Soil Identifications and Classification

- Asked in: 2075 Bhadra, 3 marks
- Video: [34.Soil Classification | Plasticity Chart | IS Soil Classification | A Line & U Line Equation | USCS](https://www.youtube.com/watch?v=viOJMngvWWw) — Mk Plus Point
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/issc-plasticity-chart-systems
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-3

## Question

Write down the names of soil classification systems based on particle size and plasticity of soil. Define the plasticity chart of a soil based on ISSCS (Indian Standard Soil Classification System).

## Short answer

ISSCS plasticity chart plots Ip against wL with A-line; vertical lines at 35% and 50% give low, intermediate, high plasticity. Above A-line are clays, below are silts/organics; hatched zone gives CL-ML.

## Answer

#### Systems based on particle size and plasticity
- Unified Soil Classification System (USCS)
- Indian Standard Soil Classification System (ISSCS, IS 1498)
- AASHTO classification system
- British Soil Classification System (BSCS)

#### Plasticity chart of ISSCS
The plasticity chart is a graph of **plasticity index $I_p$ (y-axis)** against **liquid limit $w_L$ (x-axis)**, used for classifying the fine-grained soils (passing 75 micron).

- **A-line:** $I_p = 0.73(w_L - 20)$. Inorganic clays (C) lie above it; silts (M) and organic soils (O) lie below it.
- **Vertical lines at $w_L = 35\%$ and $50\%$** divide the soils into low (L, $w_L < 35$), intermediate (I, 35 - 50) and high plasticity (H, $w_L > 50$).
- The hatched area above the A-line with $I_p$ between 4 and 7 gives dual symbol **CL-ML**.

The resulting groups are CL, CI, CH (clays), ML, MI, MH (silts), and OL, OI, OH (organic soils).

## Check yourself

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

1. Which is a system based on size and plasticity?
   A. MIT
   B. USDA triangle
   C. AASHTO
   D. Residual/transported
   Answer: C. AASHTO uses both. (Key point: Systems list)

2. ISSCS divides plasticity using wL at
   A. 35% and 50%
   B. 20% and 40%
   C. 50% only
   D. 25% and 75%
   Answer: A. L < 35, I 35-50, H > 50. (Key point: ISSCS divisions)

3. Below the A-line on ISSCS chart are
   A. inorganic clays (C)
   B. silts (M) and organic soils (O)
   C. gravels (G)
   D. peat only
   Answer: B. C is above; M and O are below. (Key point: Chart letters)

## Related in Soil Mechanics

- [Systems using size and plasticity](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aclassification-size-plasticity-systems) — Soil Mechanics, chapter 3
- [Soil classification systems](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Acommon-classification-systems) — Soil Mechanics, chapter 3, asked 3×
- [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×
- [Usefulness of plasticity chart](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aplasticity-chart-usefulness) — Soil Mechanics, chapter 3
- [ISSCS classification, 55% passing 200](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aissc-classify-passing-55) — Soil Mechanics, chapter 3
- [USCS classification from PSD curves](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Auscs-classify-psd-curves-a-b) — Soil Mechanics, chapter 3

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