---
title: "Neutral and effective pressure"
subject: "Soil Mechanics"
chapter: 6
key: "semester_4/soil_mechanics:neutral-effective-pressure-define"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/neutral-effective-pressure-define
site: BCT Engineering
---

# Neutral and effective pressure

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

- Asked in: 2077 Chaitra, 2 marks
- Video: [Effective Stress Principle](https://www.youtube.com/watch?v=rUV8KHNGP64) — Elementary Engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/neutral-effective-pressure-define
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-6

## Question

Define 'neutral' and 'effective' pressure in soils.

## Short answer

Neutral pressure (pore water pressure) acts in the water filling voids, equal in all directions, and contributes no shear strength. Effective stress is the stress carried by the soil skeleton at grain contacts, controlling shear strength and volume change; it equals total stress minus neutral pressure.

## Answer

**Neutral pressure (pore water pressure), $u$:** the pressure of the water filling the voids of a saturated soil. It acts equally in all directions, so it does not cause any shear strength or volume change in the soil skeleton. In a static water table, $u = \gamma_w h_w$.

**Effective pressure (stress), $\sigma'$:** the part of the total stress carried by the soil grains through their contact points. It controls compression and shear strength of the soil:

$$
\sigma' = \sigma - u
$$

(Terzaghi's principle of effective stress.)

## Check yourself

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

1. Neutral pressure (pore water pressure) is:
   A. Pressure carried by grain contacts
   B. Pressure of water in the voids, acting equally in all directions
   C. Total stress minus effective stress of dry soil
   D. Pressure causing shear strength
   Answer: B. u is the water pressure in the voids; it causes no shear strength or volume change of the skeleton. (Key point: Neutral pressure)

2. Effective pressure is:
   A. The part of total stress carried by the water
   B. The part of total stress carried by the grains through contact points
   C. Equal to the pore pressure
   D. Always equal to total stress
   Answer: B. sigma' is carried by grain contacts and controls compression and strength. (Key point: Effective pressure)

3. Terzaghi's principle states:
   A. sigma' = sigma + u
   B. sigma' = u - sigma
   C. sigma' = sigma / u
   D. sigma' = sigma - u
   Answer: D. Effective stress is total stress minus pore pressure. (Key point: Terzaghi's principle)

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