---
title: "UCS as special case of UU test"
subject: "Soil Mechanics"
chapter: 10
key: "semester_4/soil_mechanics:ucs-special-uu-triaxial"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/ucs-special-uu-triaxial
site: BCT Engineering
---

# UCS as special case of UU test

Soil Mechanics · Chapter 10: Shear Strength of Soil

- Asked in: 2074 Bhadra, 1 marks
- Video: [Unconsolidated Undrained Shear Strength in Triaxial Compression Test | Triaxial UU Test in English](https://www.youtube.com/watch?v=LUNg4dN2EB0) — incredible moments
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/ucs-special-uu-triaxial
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-10

## Question

Unconfined compression test is a special type of unconsolidated undrained triaxial test. Why?

## Short answer

The unconfined compression test is a UU triaxial test with zero cell pressure. For saturated clay, undrained strength is independent of confining pressure (φu=0), so the Mohr circle diameter is the same as in a UU test, giving cu = qu/2.

## Answer

In a UU triaxial test, a specimen is sheared without drainage and without prior consolidation under a cell pressure $\sigma_3$. The unconfined compression test is the same test with **cell pressure equal to zero** ($\sigma_3 = 0$).

For saturated clay, the undrained strength is independent of the confining pressure ($\phi_u = 0$). So a sample tested without any cell pressure gives a Mohr circle of the same diameter as one tested under a cell pressure; the circle is only shifted along the $\sigma$-axis:

$$
q_u = \sigma_1 - 0 = \sigma_1 - \sigma_3 = 2c_u
$$

Both tests also shear the sample quickly with no drainage (no change in water content). Hence the UCS test is a special case of the UU test with $\sigma_3 = 0$.

## Check yourself

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

1. What makes the UCS test a special case of the UU triaxial test?
   A. It is a CU test with high cell pressure
   B. It is a CD test with sigma_3 = 0
   C. It is a UU test with cell pressure sigma_3 = 0
   D. It is a UU test where sigma_1 = 0
   Answer: C. Both are unconsolidated, undrained shear tests; UCS simply has zero cell pressure. (Key point: UCS as UU case)

2. Why does a sample with no cell pressure give the same circle diameter as one with cell pressure in UU?
   A. Pore pressure is always zero
   B. For saturated clay phi_u = 0, so undrained strength does not depend on confining pressure
   C. Drainage is allowed in both tests
   D. The sample consolidates fully before shearing
   Answer: B. With phi_u = 0 the diameter sigma_1 - sigma_3 = 2 c_u is constant; the circle just shifts along the sigma axis. (Key point: Independence of confinement)

3. Which condition is common to UCS and UU tests?
   A. Quick shearing with no drainage and no change in water content
   B. Full consolidation before shearing
   C. Slow shearing with drainage
   D. Pore pressure is measured and controlled
   Answer: A. Both are undrained, so water content does not change during shear. (Key point: Common conditions)

## Related in Soil Mechanics

- [Triaxial tests by drainage; UCS vs cu](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Atriaxial-tests-by-drainage-ucs) — Soil Mechanics, chapter 10
- [Triaxial test pros, cons, procedure](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Atriaxial-advantages-procedure-parameters) — Soil Mechanics, chapter 10
- [UCS test soil type and parameters](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aunconfined-compression-soil-type-mohr) — Soil Mechanics, chapter 10
- [Application of UU, CU, CD tests](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Auu-cu-cd-practical-application) — Soil Mechanics, chapter 10
- [Draw UCS and direct shear results](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aucs-direct-shear-results-plot) — Soil Mechanics, chapter 10
- [Triaxial advantage over direct shear](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Atriaxial-advantages-over-direct-shear) — Soil Mechanics, chapter 10

## Related in other subjects

- [Cube versus cylinder compression test](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Acube-vs-cylinder-test) — Concrete Technology and Masonry Structure, chapter 5