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Seepage through earth filled dams
Seeapge through the dams is the most damaging factor for dams
Flownet
Flowlines
Flow channel: Space between two consecutive flow lines is called flow path / flow channel
Equipotential lines
Location of phreatic surface / phreatic line / top flow line
Properties of flownet:
- Each flow path transmits the same quantity of seepage.
- Area bounded by two consecutive flow lines and equipotential lines is a square.
Seepage Control in Dams
Types of Rolled filled dams:
- Sheet Pile wall
- Core wall + Sheet pile wall or Cut off wall
- Core wall + Cut off trench
- Core wall + Impervious blanket at the upstream surface
- Flattening the downstream face
- Rockfill toe on the downstream
- Horizontal drainage / filter on the downstream side
- Providing coarse section on the downstream face of the dam
Typical cross section of earth filled dams
Ther are three possibilities
1. Only pervious soil is available:
- Pervious dam body and base is impervious
- Pervious dam body and base is pervious
- Pervious dam body and base is pervious to a limited extent and impervious onward
2. Only impervious soil is available:
- Impervious dam body and base is also impervious
- Impervious dam body and base is pervious
3. Pervious and impervious both types of soil are available:
- Pervious dam body, base is pervious upto some extent and impervious onwards
- Pervious dam body and base is pervious
- Pervious dam body and base is impervious
Design of dam filters
Terzaghi's rules for design of dam filters
Perform sieve analysis test on the material from dam body and filter base materials and draw gradation curve for both.
- D15 (f) / D85 (s) < 5
- D15 (f) / D15 (s) > 4 upto 20
- D50 (f) / D85 (s) > 25
- Filter material should be well graded
- Gradation curves for both materials should be approximately parallel
- All materials should pass sieve of 3" and not more than 5% passing sieve no. 200
