---
title: "Why undisturbed samples are required"
subject: "Foundation Engineering"
chapter: 2
key: "semester_5/foundation_engineering:why-undisturbed-samples"
source: https://www.bctengineer.com/q/semester_5/foundation_engineering/why-undisturbed-samples
site: BCT Engineering
---

# Why undisturbed samples are required

Foundation Engineering · Chapter 2: Soil Exploration

- Asked in: 2075 Asoj, 2 marks
- Video: [Disturbed and Undisturbed Samples | Madhukant Goyal (IES) | #AlphaConcepts](https://www.youtube.com/watch?v=LkJM2d2RLr8) — Alpha Learning
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/foundation_engineering/why-undisturbed-samples
- Whole chapter: https://www.bctengineer.com/md/semester_5/foundation_engineering/chapter-2

## Question

Why are undisturbed samples required?

## Short answer

Undisturbed samples keep the soil's natural structure, density, water content, and stress history. They are required for shear strength tests (unconfined, triaxial) to obtain true c and φ, consolidation tests for settlement, and to determine permeability and sensitivity. Disturbed samples alter properties, making design unsafe or uneconomical.

## Answer

Undisturbed samples keep the in-situ structure, density, water content and stress history of the soil. They are required because:
- Shear strength tests (unconfined, triaxial, direct shear) must be done on soil with its natural structure to get true $c$ and $\phi$.
- Consolidation tests need them to estimate the settlement of clays.
- They are needed to determine permeability, unit weight and sensitivity.
- Disturbed samples change the soil properties, so the results would be wrong and the design either unsafe or uneconomical.

They are needed mainly for cohesive soils and for final design, while disturbed samples are enough for classification.

## Check yourself

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

1. Undisturbed samples are needed because they retain
   A. only the colour of soil
   B. only particle size
   C. in-situ structure, density, water content and stress history
   D. nothing of the field state
   Answer: C. Properties depend on natural structure. (Key point: Reason)

2. Shear strength tests on a disturbed sample would
   A. give exact values
   B. give wrong c and phi, making design unsafe or uneconomical
   C. not matter
   D. be better than on undisturbed
   Answer: B. Disturbance changes strength. (Key point: Shear test)

3. Undisturbed samples are mainly needed for
   A. classification of sand only
   B. rough identification
   C. cohesive soils and final design
   D. gravel grading
   Answer: C. Disturbed samples suffice for classification. (Key point: Which soils)

## Related in Foundation Engineering

- [Disturbed vs undisturbed samples](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Adisturbed-vs-undisturbed-sample) — Foundation Engineering, chapter 2, asked 2×