---
title: "Role of effective stress in shear strength"
subject: "Soil Mechanics"
chapter: 6
key: "semester_4/soil_mechanics:role-effective-stress-shear-strength"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/role-effective-stress-shear-strength
site: BCT Engineering
---

# Role of effective stress in shear strength

Soil Mechanics · Chapter 6: Principle of Effective Stress, Capillarity and Permeability

- Asked in: 2077 Chaitra, 1 marks
- Video: [Shear Strength of Soils](https://www.youtube.com/watch?v=s5BX0mJFAus) — Sam Hashemi
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/role-effective-stress-shear-strength
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-6

## Question

What is the role of effective stress in shear strength of soil?

## Short answer

Shear strength is controlled by effective stress because water has no shear strength; only grain contacts provide friction. The Mohr-Coulomb relation is τ_f = c' + σ' tanφ' = c' + (σ-u) tanφ', so increased pore pressure lowers effective stress and strength, while consolidation raises it.

## Answer

Shear strength of a soil is controlled by the effective stress, not by the total stress, because water has no shear strength and only the grain contacts carry friction. By Mohr–Coulomb in terms of effective stress:

$$
\tau_f = c' + \sigma' \tan\phi' = c' + (\sigma - u)\tan\phi'
$$

A rise in pore pressure lowers $\sigma'$ and so lowers strength (as in liquefaction, quick sand and slope failure after rain). Increase of $\sigma'$ by consolidation raises strength.

## Check yourself

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

1. Shear strength of soil is controlled by:
   A. Total stress
   B. Effective stress, since water has no shear strength
   C. Pore pressure alone
   D. Seepage velocity
   Answer: B. Only grain contacts carry friction, so strength depends on effective stress. (Key point: Controlling stress)

2. In effective stress terms, shear strength is:
   A. c' + sigma tan(phi')
   B. c' + (sigma - u) tan(phi')
   C. c + u tan(phi)
   D. sigma' / tan(phi')
   Answer: B. tau_f = c' + sigma' tan phi' with sigma' = sigma - u. (Key point: Mohr-Coulomb)

3. A rise in pore pressure causes strength to:
   A. Increase
   B. Stay constant
   C. Reduce, as in liquefaction or slope failure after rain
   D. Become independent of phi
   Answer: C. Higher u lowers sigma' and so lowers strength. (Key point: Effect of pore pressure)

## Related in Soil Mechanics

- [Sea level fluctuation, effective stress](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Asea-water-level-effective-stress) — Soil Mechanics, chapter 6
- [Effective stress with up/down seepage](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aeffective-stress-upward-downward-flow) — Soil Mechanics, chapter 6
- [Clay over sand, effective stress at depths](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aclay-sand-profile-effective-stress-depths) — Soil Mechanics, chapter 6
- [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×
- [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
- [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

## Related in other subjects

- [Boundary shear stress and compound channel flow](https://www.bctengineer.com/question.md?key=semester_4%2Fhydraulics%3Ashear-stress-boundary-compound-channel-flow) — Hydraulics, chapter 6
- [Bent-up bars shear, 550 mm effective depth](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Abent-up-bars-shear-550-mm) — Design of RCC Structure, chapter 5, asked 3×
- [Normal/shear stress on 25 degree plane, principal stresses](https://www.bctengineer.com/question.md?key=semester_3%2Fstrength_of_materials%3Ainclined-plane-30-25deg-mohr-verify) — Strength of Materials, chapter 4, asked 3×
- [Plane stress: principal, max shear, normal](https://www.bctengineer.com/question.md?key=semester_3%2Fstrength_of_materials%3Aplane-stress-80-60-50-mpa) — Strength of Materials, chapter 4, asked 2×