Seepage control measures in embankment dam

Seepage through and under an embankment dam is controlled to reduce water loss, keep the phreatic line inside the section, and keep the exit gradient below the critical value. The usual measures are shown below.

1. Control of seepage through the embankment body

 (a) Central impervious core     (b) Inclined core
      |  core |                       \   core
   ___|_______|___                 ____\____________
  /   | clay  |   \               /     \           \
 / shell|     |shell\             / shell \  shell    \

The core of clay (or an asphalt/concrete membrane on the u/s face) reduces the flow, while the pervious shells give stability.

 (c) Toe drain          (d) Horizontal blanket drain
  ___________              __________________
 /           \            /                  \
/    phreatic \__        /____________________\
/_____________ ▒▒|      ==== sand-gravel blanket ====
              toe drain (rock toe)

 (e) Chimney drain + blanket
      ____
     /  | \     vertical (chimney) filter
    /   |▒ \    connected to a horizontal blanket
   /____|▒▒▒\_____ drain

Toe, blanket and chimney drains collect seepage and carry it safely out, lowering the phreatic line; they are protected by graded filters.

2. Control of seepage under the foundation

 (f) Cut-off trench          (g) Sheet pile / grout curtain
   _______________             _______________
  /               \           /               \
 /_________________\         /_________________\
 ==pervious===|||==          ==pervious== | | | ==
 =====clay====|||=======     ======= grout holes ==
  (to impervious layer)       (rock / alluvium)

 (h) Upstream impervious blanket     (i) Relief wells at toe
 ____blanket_____/\_______________      ______________
 clay laid on bed,  /  dam \             /   dam      \
 lengthens seepage path                 /______ O O O __\
 ====== pervious foundation ====         wells relieve uplift

• Cut-off trench filled with compacted clay: the best if it reaches the impervious layer (positive cut-off).
• Sheet piles / grout curtain / diaphragm wall when the pervious layer is deep (partial cut-off).
• Upstream blanket increases the length of the seepage path (as in Bligh's creep theory) and so reduces the exit gradient.
• Relief wells and a d/s berm (weighted filter) release the uplift pressure at the toe and prevent boiling.