---
title: "Causes of earthen dam failure; safe design"
subject: "Hydropower Engineering"
chapter: 4
key: "semester_7/hydropower_engineering:earthen-dam-failure-causes-safe-design"
source: https://www.bctengineer.com/q/semester_7/hydropower_engineering/earthen-dam-failure-causes-safe-design
site: BCT Engineering
---

# Causes of earthen dam failure; safe design

Hydropower Engineering · Chapter 4: Headworks of Storage Plants

- Asked in: 2070 Ashad, 2+4 marks
- Video: [WRE-ll | unit 4 | Earth dams](https://www.youtube.com/watch?v=CMH5ZdGL9PM) — Yuvaraju sai kumar
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/hydropower_engineering/earthen-dam-failure-causes-safe-design
- Whole chapter: https://www.bctengineer.com/md/semester_7/hydropower_engineering/chapter-4

## Question

Explain the causes of failure of an earthen dam. What criteria do you adopt for safe design of an earthen dam?

## Short answer

Failure causes: overtopping, wave erosion, piping, sloughing of downstream slope, slope instability during drawdown or earthquake. Safe design criteria: spillway sized for design flood, freeboard, filters and drains to keep phreatic line inside, stable slopes (FS ≥1.5 steady seepage, ≥1.3 rapid drawdown), cut-off for pervious foundations, and proper compaction.

## Answer

#### Causes of failure of an earthen dam
An earthen dam fails when water, or the soil itself, is not controlled. About 90% of failures come from three groups of causes.

1. **Hydraulic failures**
   - Overtopping because the spillway is too small, gates fail or free board is insufficient. Water flowing over the crest erodes the d/s face quickly.
   - Erosion of the u/s face by wave action, and of the d/s face by rain and surface runoff (gullies).
   - Scour of the toe by tail water or by spillway discharge.
2. **Seepage failures**
   - **Piping**: a thin channel forms along a weak path (conduit, cracks, animal burrows, a pervious foundation layer), carries soil particles and grows backwards towards the reservoir.
   - Sloughing of the d/s face when the phreatic line emerges on the slope, softening the soil.
   - Heaving or boiling at the toe when the exit gradient exceeds the critical gradient $i_c=\frac{G-1}{1+e}\approx 1$.
3. **Structural failures**
   - Slope slides: u/s slope during rapid drawdown, d/s slope under steady seepage, or both slopes at the end of construction (high pore pressure).
   - Foundation shear failure or excessive differential settlement causing cracks.
   - Earthquake: liquefaction or cracking of the embankment.

#### Criteria for a safe design
- **No overtopping**: spillway and outlet sized for the design flood; free board is the sum of wave height, wave run-up, wind set-up and a safety margin (normally 1.5-3 m).
- **Seepage line inside the section**: the phreatic line must stay well inside the d/s face. Use a core, a toe drain, a horizontal blanket or a chimney drain to bring it down.
- **Exit gradient less than the safe value** (about 1/4 to 1/6 of $i_c$); provide filters wherever seepage leaves a fine soil, and a cut-off for pervious foundations.
- **Stable slopes** under every condition, with the minimum factor of safety: about 1.5 for steady seepage, 1.3 for end of construction and rapid drawdown, 1.0-1.2 with earthquake load.
- **Foundation**: adequate bearing strength, small and uniform settlement, removal of organic or weak layers, and a key or cut-off trench.
- **Slope protection**: riprap or stone pitching on the u/s face; grass turf, berms and drains on the d/s face.
- **Proper crest**: width of at least $\frac{H}{5}+3$ m, with camber for settlement, and soil compacted in thin layers at optimum moisture content.

## Check yourself

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

1. Overtopping of an earthen dam is mainly caused by
   A. Piping
   B. Slope slides
   C. Inadequate spillway, gate failure or insufficient freeboard
   D. Foundation settlement only
   Answer: C. These hydraulic causes let water flow over the crest and erode the d/s face. (Key point: Overtopping)

2. How does piping develop?
   A. A thin channel forms along a weak path and grows backwards towards the reservoir as it carries soil
   B. The crest settles uniformly
   C. Waves erode the u/s slope
   D. Rain forms gullies on the d/s face
   Answer: A. Seepage carries particles along weak paths, enlarging a pipe from the d/s side back to the reservoir. (Key point: Piping mechanism)

3. The critical exit gradient is i_c = (G-1)/(1+e), which is about
   A. 0.1
   B. 1
   C. 5
   D. 10
   Answer: B. For typical soils i_c is approximately 1; boiling occurs when the exit gradient reaches it. (Key point: Critical gradient)

4. Which slope is most at risk during rapid drawdown?
   A. Downstream slope
   B. Crest only
   C. Upstream slope
   D. The foundation only
   Answer: C. On sudden drawdown the u/s slope retains high pore pressure and may slide; d/s slope is critical under steady seepage. (Key point: Rapid drawdown)

## Related in Hydropower Engineering

- [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
- [Filter material in earthen dam](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aearthen-dam-filter-purpose-design) — 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
- [Toe-drain earthen dam: discharge and phreatic line](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aearthen-dam-toe-drain-2069) — Hydropower Engineering, chapter 4
- [Dam site evaluation factors; gravity dam failure modes](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Agravity-dam-site-evaluation-failure-modes) — Hydropower Engineering, chapter 4
- [Design elementary profile of masonry dam](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aelementary-profile-stone-masonry-design) — Hydropower Engineering, chapter 4

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