---
title: "Seepage under embankment dam on pervious foundation"
subject: "Hydropower Engineering"
chapter: 4
key: "semester_7/hydropower_engineering:seepage-pervious-foundation-consequences"
source: https://www.bctengineer.com/q/semester_7/hydropower_engineering/seepage-pervious-foundation-consequences
site: BCT Engineering
---

# Seepage under embankment dam on pervious foundation

Hydropower Engineering · Chapter 4: Headworks of Storage Plants

- Asked in: 2078 Bhadra, 4 marks
- Video: [Failure of Earthen Dams](https://www.youtube.com/watch?v=Am4_e4UC9rM) — Mission Mpsc
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/hydropower_engineering/seepage-pervious-foundation-consequences
- Whole chapter: https://www.bctengineer.com/md/semester_7/hydropower_engineering/chapter-4

## Question

For an embankment dam on a pervious foundation, soil seepage underneath the dam poses a serious problem. Briefly discuss the consequences of this problem and how it is reduced.

## Short answer

Seepage through a pervious foundation causes water loss, uplift that reduces downstream stability, piping (boiling) when the exit gradient approaches critical, internal erosion and settlement. It is reduced by positive cut-offs, upstream blankets, relief wells and downstream filter berms, keeping the exit gradient below about one‑fourth of the critical gradient.

## Answer

When an embankment dam rests on a pervious foundation such as sand, gravel or fractured rock, a large part of the reservoir water flows under the dam. This is a serious problem for both safety and economy.

#### Consequences
1. **Loss of stored water** and reduced power generation, especially when the flow is large.
2. **Uplift pressure** under the d/s part of the dam reduces effective stress and the shear strength of the foundation, so the d/s slope and toe become unstable.
3. **Piping and boiling**: if the exit gradient at the toe reaches the critical value
   $$
   i_c=\frac{G-1}{1+e}\approx 1,
   $$
   the soil is lifted (quick condition), and a pipe grows backwards beneath the dam, causing sudden failure.
4. **Internal erosion** of fine particles, settlement of the dam and cracking of the embankment.
5. Wet, swampy ground at the d/s side and damage to the toe.

#### How it is reduced
- **Positive cut-off**: compacted clay trench, concrete diaphragm wall or secant piles reaching the impervious layer.
- **Partial cut-off**: sheet piles or grout curtain when the pervious layer is deep.
- **Upstream impervious blanket** of clay, which lengthens the seepage path and reduces gradient.
- **Relief wells and toe drain** with filter to release uplift pressure safely.
- **Downstream loaded filter / berm** to resist heaving.
- Keep the exit gradient below the safe value, about $\frac{1}{4}$ to $\frac{1}{5}$ of $i_c$ for the design.

## Check yourself

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

1. Why is uplift under the d/s part of the dam harmful?
   A. It increases dam weight
   B. It reduces effective stress and shear strength, making the d/s slope and toe unstable
   C. It improves bearing capacity
   D. It only wets the ground
   Answer: B. Pore pressure reduces effective stress, hence strength. (Key point: Uplift effect)

2. When does boiling or quick condition occur?
   A. When exit gradient is zero
   B. When the foundation is clay
   C. When exit gradient reaches the critical value i_c = (G-1)/(1+e), about 1
   D. When the reservoir is empty
   Answer: C. At the critical gradient seepage lifts the soil, and a pipe grows backwards. (Key point: Boiling)

3. What is a positive cut-off?
   A. A trench or diaphragm wall reaching the impervious layer
   B. A short sheet pile
   C. A surface blanket
   D. A toe drain
   Answer: A. It fully intercepts the pervious layer, unlike a partial cut-off. (Key point: Positive cut-off)

4. The design exit gradient is kept at about
   A. Equal to i_c
   B. 1/4 to 1/5 of i_c
   C. Twice i_c
   D. Zero
   Answer: B. A safety factor of about 4-5 is applied to the critical gradient. (Key point: Safe exit gradient)

## Related in Hydropower Engineering

- [Seepage control measures in embankment dam](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aembankment-seepage-control-measures) — Hydropower Engineering, chapter 4
- [Seepage through dam with central core (45 m dam)](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aearthen-dam-impervious-core-seepage-2075c) — Hydropower Engineering, chapter 4
- [Design criteria of earthen embankment dam](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aearthen-dam-design-criteria) — Hydropower Engineering, chapter 4
- [Seepage line for 22 m high earthen dam](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aearthen-dam-seepage-line-22m-45deg) — Hydropower Engineering, chapter 4
- [Seepage discharge through dam body (k = 1×10⁻⁶ m/s)](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aearthen-dam-seepage-gradient-figure-2072k) — Hydropower Engineering, chapter 4
- [Seepage through dam with central core (33 m dam)](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aearthen-dam-impervious-core-seepage-2070c) — Hydropower Engineering, chapter 4

## Related in other subjects

- [Steady 2D model: seepage under a dam](https://www.bctengineer.com/question.md?key=semester_8%2Fcomputational_techniques_in_civil_engineering%3Aseepage-under-dam-2d-model) — Computational Techniques in Civil Engineering, chapter 7, asked 3×
- [Continuity equation; seepage under a dam algorithm](https://www.bctengineer.com/question.md?key=semester_8%2Fcomputational_techniques_in_civil_engineering%3Acontinuity-gw-seepage-algorithm) — Computational Techniques in Civil Engineering, chapter 7
- [Earth dam with filter, seepage quantity](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aearth-dam-horizontal-filter-seepage) — Soil Mechanics, chapter 7
- [Impervious dam seepage discharge](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aimpervious-dam-seepage-discharge-numerical) — Soil Mechanics, chapter 7