---
title: "Sea level fluctuation, effective stress"
subject: "Soil Mechanics"
chapter: 6
key: "semester_4/soil_mechanics:sea-water-level-effective-stress"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/sea-water-level-effective-stress
site: BCT Engineering
---

# Sea level fluctuation, effective stress

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

- Asked in: 2078 Baisakh, 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/sea-water-level-effective-stress
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-6

## Question

Fluctuation of water level in the sea affects the effective stress of the soil lying in the sea bed. Do you agree with this statement? Answer the question with proper explanation.

## Short answer

No, for a fully submerged sea-bed soil, sea-level change adds equal amounts to total stress and pore pressure, so effective stress remains (γ_sat-γ_w)z, independent of water depth. Strength is unaffected as long as the soil stays saturated.

## Answer

**No, the statement is not correct** (for a fully submerged sea-bed soil).

Consider soil below sea level at depth $z$ below the bed, with water depth $h_w$ above the bed.

- Total stress: $\sigma = \gamma_w h_w + \gamma_{sat} z$
- Pore pressure: $u = \gamma_w (h_w + z)$

$$
\sigma' = \sigma - u = (\gamma_{sat} - \gamma_w) z = \gamma' z
$$

The water-depth term $\gamma_w h_w$ appears in both $\sigma$ and $u$ and cancels. So the effective stress depends only on the submerged unit weight and the depth into the soil, not on the sea level. A rise or fall of the sea changes the total stress and the pore pressure by the same amount.

Remarks:
- Even if the sea bed is exposed at low tide, $\sigma'$ stays equal to $\gamma' z$ as long as the soil remains saturated with the water table at the bed surface.
- The effective stress changes only if the pore pressure does not change by the same amount as the total stress, for example if the water table inside the soil falls below the exposed bed surface (drainage), or if the soil is not fully saturated (gas in the pores).
- In normal situations, the strength of the sea-bed soil is not affected by sea-level fluctuation.

## Check yourself

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

1. Does sea level fluctuation change effective stress in a fully submerged sea-bed soil?
   A. Yes, it increases with water depth
   B. Yes, it decreases with water depth
   C. No, it is unaffected
   D. Only in sand
   Answer: C. The water depth term gamma_w h_w is in both sigma and u and cancels. (Key point: Verdict)

2. The effective stress at depth z below the sea bed is:
   A. gamma_sat z + gamma_w h_w
   B. gamma' z
   C. gamma_w (h_w + z)
   D. gamma_sat z - gamma_w h_w
   Answer: B. sigma' = (gamma_sat - gamma_w) z = gamma' z, independent of h_w. (Key point: Why it cancels)

3. When can sea level change effective stress?
   A. Whenever the tide rises
   B. When pore pressure cannot respond equally, e.g. rapid drawdown in low-permeability clay or bed exposed at low tide
   C. Only in gravel
   D. Never
   Answer: B. If u lags (clay) or the bed is exposed (buoyancy lost), the cancellation no longer holds. (Key point: Exception)

## Related in Soil Mechanics

- [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
- [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
- [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
- [Ring foundation water tank max stress](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aring-foundation-water-tank-max-stress) — Soil Mechanics, chapter 8
- [Two-layer profile stress diagram, 6 m sand](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Asand-clay-profile-stress-diagram-capillary) — Soil Mechanics, chapter 6

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