---
title: "Purpose of soil classification"
subject: "Soil Mechanics"
chapter: 3
key: "semester_4/soil_mechanics:purpose-of-soil-classification"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/purpose-of-soil-classification
site: BCT Engineering
---

# Purpose of soil classification

Soil Mechanics · Chapter 3: Soil Identifications and Classification

- Asked in: 2079 Jestha, 2 marks; 2078 Chaitra, 3 marks; 2078 Poush, 2 marks (asked 3 times)
- Video: [Master USCS Soil Classification: Step-by-Step Examples for Engineering](https://www.youtube.com/watch?v=VQNxhNTslX0) — Soil Mechanics and Engineering Geology
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/purpose-of-soil-classification
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-3

## Question

What is the purpose (importance) of soil classification?

## Short answer

Soil classification groups soils with similar engineering behaviour using simple index tests. It provides a common language, allows estimation of strength and compressibility for preliminary design, aids material selection and site investigation, and enables correlation with past experience, reducing costly lab tests.

## Answer

**Soil classification** groups soils of similar engineering behaviour into the same class, using simple index properties (grain size and plasticity).

#### Purposes and importance
1. **Common language:** engineers share a standard name and symbol for a soil (e.g. SC, CH), so reports are understood by everyone.
2. **Prediction of behaviour:** from the group, the probable strength, compressibility, permeability, compaction and swelling can be estimated.
3. **Preliminary design:** gives approximate design values (bearing capacity, subgrade suitability, frost susceptibility) before costly tests are done.
4. **Selection of construction material:** helps to select borrow soil for embankments, dam cores, and road subgrades.
5. **Site investigation planning:** helps decide what tests are needed.
6. **Use of past experience:** data from similar soils elsewhere can be applied.
7. **Economy:** avoids costly lab tests for small or simple projects.
8. **Correlation:** index properties correlate with engineering properties (e.g. $w_L$ with compression index).

## Check yourself

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

1. Soil classification groups soils
   A. with similar engineering behaviour using simple index properties
   B. with identical chemical composition
   C. by colour and smell only
   D. by results of triaxial tests only
   Answer: A. It uses grain size and plasticity to group similar behaviour. (Key point: What classification does)

2. Why is classification useful for prediction?
   A. It gives exact settlement values
   B. From the group, probable strength, compressibility, permeability and swelling can be estimated
   C. It replaces all site investigation
   D. It gives the exact cohesion value
   Answer: B. It gives probable, not exact, behaviour. (Key point: Prediction)

3. One practical use of classification is
   A. finding the age of the deposit
   B. determining groundwater level
   C. selecting borrow soil for embankments, dam cores and road subgrades
   D. calculating the water table depth
   Answer: C. Classification helps in selecting construction materials. (Key point: Material selection)

## Related in Soil Mechanics

- [Soil classification systems](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Acommon-classification-systems) — Soil Mechanics, chapter 3, asked 3×
- [AASHTO classification of inorganic soil](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aaashto-classify-inorganic-soil) — Soil Mechanics, chapter 3
- [MIT classification system](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Amit-classification-grain-size) — Soil Mechanics, chapter 3, asked 2×
- [Basic requirements of classification](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Abasic-requirements-classification) — Soil Mechanics, chapter 3, asked 2×
- [Describe one classification system, limits](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Adescribe-classification-system-limits) — Soil Mechanics, chapter 3
- [Classification for subgrade suitability](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Asubgrade-suitability-classification) — Soil Mechanics, chapter 3

## Related in other subjects

- [Thermal comfort and its classification](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Athermal-comfort-classification) — Building Technology, chapter 1
- [Classification of 8085 instructions](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3A8085-instruction-classification) — Microprocessor, chapter 2