---
title: "Triaxial tests by drainage; UCS vs cu"
subject: "Soil Mechanics"
chapter: 10
key: "semester_4/soil_mechanics:triaxial-tests-by-drainage-ucs"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/triaxial-tests-by-drainage-ucs
site: BCT Engineering
---

# Triaxial tests by drainage; UCS vs cu

Soil Mechanics · Chapter 10: Shear Strength of Soil

- Asked in: 2076 Baisakh, 2 marks
- Video: [Understanding Triaxial Test | Difference b/w UU, CU & CD | Part 2 | Geotech with Naqeeb](https://www.youtube.com/watch?v=5OMO72vHQVQ) — Geotech with Naqeeb
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/triaxial-tests-by-drainage-ucs
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-10

## Question

Drainage condition plays an important role in the measurement of shear strength of the soil. Write down the names of triaxial shear strength tests depending upon the drainage condition. Differentiate unconfined compressive strength from undrained shear strength for the unconfined compression test.

## Short answer

Triaxial tests by drainage are UU (unconsolidated undrained), CU (consolidated undrained), and CD (consolidated drained). Unconfined compressive strength qu is the axial stress at failure with zero cell pressure, while undrained shear strength cu is the shear stress at failure; for saturated clay, cu = qu/2.

## Answer

#### Triaxial tests by drainage condition

1. **Unconsolidated undrained (UU)** test: no drainage during consolidation or shearing.
2. **Consolidated undrained (CU)** test: drainage during consolidation, none during shearing (pore pressure may be measured).
3. **Consolidated drained (CD)** test: drainage allowed during both consolidation and shearing.

#### Unconfined compressive strength vs undrained shear strength

| Point | Unconfined compressive strength $q_u$ | Undrained shear strength $c_u$ |
|---|---|---|
| Meaning | Maximum axial stress at failure with zero cell pressure | Shear stress at failure under undrained conditions |
| Type of stress | Normal (axial) stress, $q_u = \sigma_1$ | Shear stress (radius of the Mohr circle) |
| Relation | $q_u = 2c_u$ | $c_u = q_u/2$ |
| Confining pressure | $\sigma_3 = 0$ | Value of $\sigma_3$ does not matter ($\phi_u = 0$) |
| Used for | Quick strength index of clay | Total stress design: bearing capacity and stability of clays |

## Check yourself

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

1. Which names are the triaxial tests by drainage?
   A. UC, CC, CD
   B. UU, CU and CD
   C. DD, DU, UU
   D. UU, UD, CC
   Answer: B. Unconsolidated undrained, consolidated undrained and consolidated drained. (Key point: Triaxial tests)

2. In which stages is drainage allowed in the CU test?
   A. During shearing only
   B. During both stages
   C. During neither stage
   D. During consolidation only, not during shearing
   Answer: D. CU consolidates with drainage open then shears undrained. (Key point: UU drainage)

3. What is the relation between unconfined compressive strength and undrained shear strength?
   A. q_u = c_u
   B. q_u = c_u / 2
   C. q_u = 2 c_u
   D. q_u = c_u^2
   Answer: C. q_u is the diameter of the Mohr circle (sigma3 = 0), c_u is its radius. (Key point: q_u and c_u)

## Related in Soil Mechanics

- [Drained vs undrained shear strength](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Adrained-vs-undrained-shear-strength) — 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
- [UCS as special case of UU test](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aucs-special-uu-triaxial) — 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
- [Shear strength tests (lab and field)](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Ashear-strength-tests-names) — Soil Mechanics, chapter 10, asked 3×
- [Role of effective stress in shear strength](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Arole-effective-stress-shear-strength) — Soil Mechanics, chapter 6

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