---
title: "Design criteria of earthen embankment dam"
subject: "Hydropower Engineering"
chapter: 4
key: "semester_7/hydropower_engineering:earthen-dam-design-criteria"
source: https://www.bctengineer.com/q/semester_7/hydropower_engineering/earthen-dam-design-criteria
site: BCT Engineering
---

# Design criteria of earthen embankment dam

Hydropower Engineering · Chapter 4: Headworks of Storage Plants

- Asked in: 2079 Bhadra, 6 marks
- Video: [Earth Dam - Introduction, types and calculation of seepage through it](https://www.youtube.com/watch?v=mIm13hrBoMs) — Elementary Engineering
- 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-design-criteria
- Whole chapter: https://www.bctengineer.com/md/semester_7/hydropower_engineering/chapter-4

## Question

Discuss the design criteria of an earthen embankment dam.

## Short answer

Design criteria: safety against overtopping (spillway capacity, freeboard), wave erosion (riprap), seepage failure (filtered drains, cut-off, safe exit gradient), and slope stability (FS ≥1.5 for d/s steady seepage, ≥1.3 for rapid drawdown). Use zoned section, compacted at optimum moisture, crest width H/5 + 3 m.

## Answer

An earthen embankment dam is a structure of compacted earth, and it is safe when it meets the following design criteria. The dam must be safe against all the failures: hydraulic, seepage and structural.

#### 1. Safety against hydraulic failure
- **Overtopping:** The spillway and outlet should discharge the design flood without allowing water to go over the embankment. Freeboard (normal and minimum) is provided above the MWL, taking account of wave height, wave run-up, wind set-up, settlement and earthquake effects.
- **Erosion of the u/s face by waves:** Provide rip-rap or slope protection (stone pitching) on the u/s face, and sod or gravel on the d/s face against rainfall.
- **Erosion at the toe:** Provide a toe drain/protection.

#### 2. Safety against seepage failure
- **Piping:** The exit gradient must be less than the critical gradient ($i_{exit}<i_c/FS$, $FS\approx3$-4). Use filters and cut-offs.
- **Phreatic line:** The seepage line should remain well inside the d/s slope. Provide a drainage filter (horizontal, toe or chimney drain) to keep the d/s face dry.
- **Seepage quantity** through the dam and the foundation should be small, and the foundation should have a cut-off (key trench, sheet piles or grout curtain) to reduce the flow.
- **Filter criteria** (Terzaghi): $D_{15}$(filter) $<4$-$5\,D_{85}$(base) and $D_{15}$(filter) $>4$-$5\,D_{15}$(base).

#### 3. Structural safety (slope stability)
- **Slopes** are chosen (about 2.5 : 1 to 3 : 1 u/s, 2 : 1 to 2.5 : 1 d/s) so that the factors of safety (FS) are adequate:
  - Downstream slope — steady seepage: $FS\ge1.5$
  - Upstream slope — sudden drawdown: $FS\ge1.2$-$1.3$ (end of construction $FS\ge1.25$)
  - With earthquake: $FS\ge1.0$-1.2
- Checked by the method of slices (Swedish, Bishop) for circular slip surfaces and the wedge method for weak layers.
- **Foundation:** Bearing capacity and settlement must be within the limit; the foundation must be strong enough to carry the embankment; consolidation settlement is allowed for by camber (about 1-2%).

#### 4. Other criteria
- **Crest width:** Sufficient for the road and construction (about $\tfrac{H}{5}+3$ m, or at least 5-6 m).
- **Material:** Compaction at the optimum moisture content; zoned section with an impervious core and a pervious shell wherever possible.
- **Cracking:** Avoid differential settlement and cracks in the core; provide filters to heal the cracks.
- **Instrumentation:** Piezometers, settlement gauges and inclinometers to monitor behaviour.
- **Economy:** Use local materials and keep the section minimum consistent with safety.

## Check yourself

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

1. What is the exit-gradient criterion against piping?
   A. Exit gradient must be greater than the critical gradient
   B. Exit gradient must be less than i_c/FS, with FS about 3-4
   C. Exit gradient is unimportant if there is a core
   D. Exit gradient must equal 1
   Answer: B. The exit gradient is limited to a fraction of the critical gradient, i.e. i_exit < i_c/FS. (Key point: Piping criterion)

2. What should be achieved for the phreatic line?
   A. It should emerge high on the d/s slope
   B. It should remain well inside the d/s slope, using drains such as toe, blanket or chimney drain
   C. It should be above the crest
   D. It is not controlled in design
   Answer: B. Drainage filters keep the seepage line inside the section and the d/s face dry. (Key point: Phreatic line)

3. Which condition requires the minimum factor of safety of about 1.5?
   A. End of construction
   B. Upstream slope in sudden drawdown
   C. Earthquake loading
   D. Downstream slope under steady seepage
   Answer: D. d/s slope under steady seepage FS >= 1.5; u/s sudden drawdown 1.2-1.3; earthquake 1.0-1.2. (Key point: Slope stability factors)

4. Freeboard above MWL is provided to allow for
   A. Wave height, run-up, wind set-up, settlement and earthquake effects
   B. Only the cost of the dam
   C. Only the spillway discharge
   D. Only the crest road
   Answer: A. Freeboard prevents overtopping from waves and other effects. (Key point: Freeboard)

## Related in Hydropower Engineering

- [Causes of earthen dam failure; safe design](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aearthen-dam-failure-causes-safe-design) — 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
- [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 under embankment dam on pervious foundation](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aseepage-pervious-foundation-consequences) — Hydropower Engineering, chapter 4

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