---
title: "Triaxial drainage conditions in the field"
subject: "Soil Mechanics"
chapter: 10
key: "semester_4/soil_mechanics:triaxial-drainage-field-conditions"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/triaxial-drainage-field-conditions
site: BCT Engineering
---

# Triaxial drainage conditions in the field

Soil Mechanics · Chapter 10: Shear Strength of Soil

- Asked in: 2073 Magh, 3 marks
- Video: [Drained and Undrained Soil Shear Strength](https://www.youtube.com/watch?v=ImHQLxuC_IU) — UL Civil Eng
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/triaxial-drainage-field-conditions
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-10

## Question

How are the drainage conditions adopted in a triaxial shear test realized in the field?

## Short answer

UU test represents rapid loading on saturated clay with no drainage, like end-of-construction stability. CU test models consolidation under existing load followed by sudden loading, such as rapid drawdown. CD test corresponds to slow loading or long-term conditions where excess pore pressure has dissipated, applicable to sands and long-term clay stability.

## Answer

The three triaxial tests represent three field situations, depending on how fast the load is applied compared with the rate at which water can drain from the soil.

| Triaxial test | Drainage in test | Field condition it represents |
|---|---|---|
| **UU** (unconsolidated undrained) | No consolidation, no drainage during shear | Rapid loading on saturated clay of low permeability, with no time for pore pressure to dissipate: end of construction of an embankment or foundation on soft clay, rapid excavation. Short-term stability. |
| **CU** (consolidated undrained) | Consolidated under $\sigma_3$, no drainage in shear | Soil first consolidated under the existing load, then loaded suddenly: rapid drawdown in an earth dam after long seepage; embankment raised in stages; sudden loading after consolidation. |
| **CD** (consolidated drained) | Consolidated and sheared with drainage open, so $\Delta u=0$ | Slow loading, or a long time after construction when the excess pore pressure has dissipated: long-term stability of cuts, slopes and embankments, and loading of sands and gravels. |

**Key idea:** clays under quick loading behave undrained (UU), clays under slow loading behave drained (CD), and sands drain quickly, so they are almost always treated as drained.

## Check yourself

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

1. Which field condition does the UU test represent?
   A. Slow loading of sand
   B. Rapid loading of saturated clay, such as the end of construction
   C. Long-term stability of slopes
   D. Dissipated excess pore pressure
   Answer: B. UU models short-term stability when no drainage occurs. (Key point: UU test)

2. Which field condition does the CU test represent?
   A. Sand loaded slowly
   B. Soil consolidated under existing load then suddenly loaded, like rapid drawdown in an earth dam
   C. Rapid loading on unconsolidated soft clay
   D. Long-term stability of cuts
   Answer: B. The soil consolidated first, then is loaded without time to drain. (Key point: CU test)

3. Which condition does the CD test represent?
   A. Sudden loading of clay
   B. Slow loading or long term, when excess pore pressure has dissipated
   C. Rapid drawdown
   D. End of construction on soft clay
   Answer: B. With drainage open delta u = 0, as in long-term stability and loading of sands. (Key point: CD test)

## 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 advantage over direct shear](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Atriaxial-advantages-over-direct-shear) — 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
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- [CU test NC clay 200/350/80, φ and φ'](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Acu-nc-clay-200-350-80) — Soil Mechanics, chapter 10

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