---
title: "Drained vs undrained shear strength"
subject: "Soil Mechanics"
chapter: 10
key: "semester_4/soil_mechanics:drained-vs-undrained-shear-strength"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/drained-vs-undrained-shear-strength
site: BCT Engineering
---

# Drained vs undrained shear strength

Soil Mechanics · Chapter 10: Shear Strength of Soil

- Asked in: 2075 Bhadra, 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/drained-vs-undrained-shear-strength
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-10

## Question

What are the differences between drained and undrained shear strength?

## Short answer

Drained shear strength is measured under slow loading allowing drainage, with zero excess pore pressure, using effective stress parameters c' and φ'. Undrained strength is measured under rapid loading with no drainage, causing excess pore pressure; for saturated clay, φ_u = 0 and τ_f = c_u. Drained governs long-term stability, undrained governs short-term.

## Answer

**Drained shear strength** is the strength mobilised when the soil is sheared slowly enough that water can drain freely, so no excess pore pressure develops. It is expressed in effective stress. **Undrained shear strength** is the strength when the soil is sheared quickly with no drainage, so volume cannot change and excess pore pressure develops.

| Point | Drained strength | Undrained strength |
|---|---|---|
| Drainage | Allowed during shearing | Not allowed during shearing |
| Excess pore pressure | Zero | Develops and changes the effective stress |
| Stress used | Effective: $\tau_f = c' + \sigma'\tan\phi'$ | Total: $\tau_f = c_u + \sigma\tan\phi_u$ |
| Saturated clay | $c'$ and $\phi'$ both present | $\phi_u = 0$, so $\tau_f = c_u$ |
| Test | CD test, slow direct shear | UU test, UCS test, vane shear |
| Loading rate | Slow | Rapid |
| Governs | Long-term stability (after construction) | Short-term stability (end of construction) |
| Soil usually | Sand, gravel, clay after long time | Saturated clay under quick loading |

## Check yourself

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

1. What happens to excess pore pressure in each case?
   A. Drained: develops; undrained: zero
   B. Drained: zero; undrained: develops
   C. Both: zero
   D. Both: develop equally
   Answer: B. Drained shearing is slow so water escapes; undrained shearing is fast so pore pressure builds up. (Key point: Pore pressure)

2. What parameters describe saturated clay in undrained strength?
   A. c_u = 0, so tau_f = sigma tan(phi_u)
   B. phi_u = 0, so tau_f = c_u
   C. phi_u = 90 degrees
   D. c' and phi' only
   Answer: B. Undrained strength of saturated clay does not vary with total normal stress. (Key point: Saturated clay)

3. Which strength governs short-term stability?
   A. Undrained strength
   B. Drained strength
   C. Residual strength
   D. Neither
   Answer: A. End-of-construction conditions are rapid, so undrained strength governs; long-term stability uses drained strength. (Key point: Which stability)

## Related in Soil Mechanics

- [Drained and CU tests on two samples](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Adrained-and-cu-two-samples-parameters) — 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
- [Shear strength on horizontal plane, WT 3.5 m](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Ashear-strength-cohesionless-water-table-3m) — Soil Mechanics, chapter 10
- [Shear strength of sand, WT rises](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Asand-shear-strength-water-rise) — Soil Mechanics, chapter 10
- [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

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