---
title: "Capillarity; water table effect on stress"
subject: "Soil Mechanics"
chapter: 6
key: "semester_4/soil_mechanics:capillarity-water-table-effective-stress"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/capillarity-water-table-effective-stress
site: BCT Engineering
---

# Capillarity; water table effect on stress

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

- Asked in: 2074 Bhadra, 2 marks
- Video: [Effect of Capillary Rise of Water on Effective Stress](https://www.youtube.com/watch?v=KTaGrX3Z4f4) — 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/capillarity-water-table-effective-stress
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-6

## Question

Define the meaning of capillarity in regard to normal soil ground. Also, explain the effect of water table variation on the effective stress.

## Short answer

Capillarity is the rise of water above the water table due to surface tension, with height larger in finer soils, and water is under negative pressure. A rising water table reduces effective stress because submerged unit weight decreases and pore pressure increases; a falling table increases effective stress and may cause settlement.

## Answer

**Capillarity** is the rise of water above the water table through the fine voids of the soil, caused by surface tension. The soil voids act like tiny tubes. The height of rise is $h_c = \dfrac{4T\cos\alpha}{\gamma_w d}$, larger for finer soil (silt and fine sand rise more than gravel). Water in the capillary zone is under negative pressure, $u = -\gamma_w z$.

#### Effect of water table variation

- **Rise of water table:** the part of the soil that becomes submerged has its unit weight reduced to $\gamma'$ and pore pressure increased, so effective stress at depth decreases. The soil loses strength and bearing capacity.
- **Fall of water table (drawdown):** pore pressure falls and the soil above becomes moist/dry, so effective stress at depth increases. This increases settlement (e.g. due to pumping).

For depth $z$ below the water table at depth $z_w$: $\sigma' = \gamma z_w + \gamma'(z - z_w)$. Raising the water table reduces $z_w$ and so lowers $\sigma'$.

## Check yourself

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

1. Capillarity is:
   A. Downward drainage of water through voids
   B. Rise of water above the water table through fine voids due to surface tension
   C. Flow of water under artesian pressure
   D. Swelling of clay
   Answer: B. Surface tension draws water up the tiny voids above the water table. (Key point: Capillarity)

2. Water in the capillary zone has pore pressure:
   A. Positive, gamma_w z
   B. Zero
   C. Negative, u = -gamma_w z
   D. Equal to total stress
   Answer: C. The water is in tension, so u = -gamma_w z. (Key point: Capillary pore pressure)

3. When the water table rises, effective stress at depth:
   A. Decreases because of buoyancy and higher pore pressure
   B. Increases
   C. Is unchanged
   D. Becomes infinite
   Answer: A. The soil becomes submerged (gamma') and u increases, lowering sigma'. (Key point: Rising water table)

4. A fall of the water table (e.g. pumping) causes:
   A. Heave of the ground
   B. Lower effective stress and strength
   C. Higher effective stress and more settlement
   D. No change in settlement
   Answer: C. Pore pressure falls so sigma' increases, increasing settlement. (Key point: Falling water table)

## Related in Soil Mechanics

- [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
- [Upward seepage stress variation diagram](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aupward-seepage-stress-diagram) — Soil Mechanics, chapter 7
- [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
- [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
- [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

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