---
title: "Surcharge and capillary effect on stress"
subject: "Soil Mechanics"
chapter: 6
key: "semester_4/soil_mechanics:effective-stress-capillary-surcharge"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/effective-stress-capillary-surcharge
site: BCT Engineering
---

# Surcharge and capillary effect on stress

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

- Asked in: 2078 Poush, 4 marks
- Video: [Effect of Surcharge on Effective Stress](https://www.youtube.com/watch?v=8n7ANzhQhY0) — 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/effective-stress-capillary-surcharge
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-6

## Question

Describe the effect of surcharge and capillary action on the effective stress.

## Short answer

Surcharge adds to total stress; under drained conditions effective stress increases by the surcharge value, but in undrained clay the pore pressure initially absorbs it. Capillary action above the water table creates negative pore pressure, increasing effective stress by (unit weight of water × height) and providing temporary apparent cohesion.

## Answer

Effective stress is $\sigma' = \sigma - u$ (Terzaghi). Surcharge changes the total stress, while capillary action changes the pore water pressure.

#### Effect of surcharge

A surcharge $q$ (extra load on the ground surface, e.g. a stockpile or embankment) adds to the total stress at every depth by the same amount:

$$
\sigma = q + \gamma z
$$

If water is able to drain (long term, or in sand), the pore pressure $u$ is unchanged, so the effective stress increases by $q$:

$$
\sigma' = q + \gamma z - u
$$

In a saturated clay, immediately after loading the pore pressure takes up the extra stress ($\Delta u = q$), so $\sigma'$ does not change at first. It rises later as the water drains (consolidation).

#### Effect of capillary action

In the capillary zone above the water table, water is held in tension. The pore pressure is negative:

$$
u = -\gamma_w z_c
$$

where $z_c$ is the height above the water table. Hence

$$
\sigma' = \sigma - (-\gamma_w z_c) = \sigma + \gamma_w z_c
$$

so the effective stress increases. The increase is maximum at the top of the capillary rise ($\gamma_w h_c$) and is zero at the water table. This extra stress (apparent cohesion in sand) gives the soil added strength; it disappears if the soil dries or is flooded.

| Cause | $\sigma$ | $u$ | $\sigma'$ |
|---|---|---|---|
| Surcharge $q$ (drained) | increases by $q$ | no change | increases by $q$ |
| Capillary rise | slightly changes (saturated weight) | negative | increases by $\gamma_w z_c$ |

## Check yourself

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

1. For a drained soil (sand or long-term clay), a surcharge q on the surface causes:
   A. Effective stress to increase by q, pore pressure unchanged
   B. Pore pressure to increase by q
   C. No change in effective stress
   D. Effective stress to decrease by q
   Answer: A. Total stress rises by q and u is unchanged, so sigma' rises by q. (Key point: Surcharge, drained)

2. Immediately after loading a saturated clay by q:
   A. Effective stress rises by q
   B. Pore pressure rises by q and effective stress is initially unchanged
   C. Total stress is unchanged
   D. Pore pressure becomes negative
   Answer: B. The water takes up the stress (delta u = q); sigma' rises only as consolidation drains the water. (Key point: Surcharge, saturated clay)

3. In the capillary zone, the effective stress is:
   A. Reduced by gamma_w z_c
   B. Unchanged
   C. Zero
   D. Increased by gamma_w z_c because u is negative
   Answer: D. u = -gamma_w z_c, so sigma' = sigma + gamma_w z_c. (Key point: Capillary effect)

4. The capillary increase in effective stress is:
   A. Maximum at the water table
   B. Maximum at the top of the capillary rise and zero at the water table
   C. Constant through the zone
   D. Permanent even if the soil dries
   Answer: B. It equals gamma_w z_c, zero at the water table and maximum at the top; it vanishes on drying or flooding. (Key point: Variation of capillary increase)

## Related in Soil Mechanics

- [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
- [Uniform sand, water table 2 m, capillary](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Auniform-sand-effective-stress-capillary) — Soil Mechanics, chapter 6
- [Capillarity; water table effect on stress](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Acapillarity-water-table-effective-stress) — Soil Mechanics, chapter 6
- [Excavation effect on vertical stress](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aexcavation-effect-vertical-stress) — Soil Mechanics, chapter 8
- [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
- [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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