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Grouting and drainage galleries in gravity dam

Short answer

Grouting reduces seepage and uplift by sealing rock cracks, and strengthens the foundation. Drainage galleries relieve uplift pressure via drain holes, collect seepage, provide access for inspection, monitoring, and grouting operations.

Explain the necessity of grouting and drainage galleries in a concrete gravity dam. Draw an elevation view of a concrete gravity dam showing the alignment of drainage galleries and a series of grout holes. Draw a section of a concrete gravity dam showing the arrangement of vertical formed drain, trap drain and drainage hole.

Necessity of grouting

  • Reduce seepage and uplift: Cement grout injected in holes along the u/s heel (curtain grouting) seals the cracks and joints of the rock and forms a curtain that cuts off the flow under the dam.
  • Strengthen the foundation: Consolidation grouting fills the fissures, increasing the strength and the modulus of the rock and reducing settlement.
  • Contact grouting: fills the gap between the concrete and the rock; joint grouting fills the contraction joints in the dam.

Necessity of drainage galleries

  • Relieve uplift: Drain holes drilled from the gallery into the foundation collect the water that passes through the grout curtain and release the pressure, so that the uplift on the base is reduced (to about one-third of the head difference at the gallery).
  • Collect seepage through the dam body and foundation, and carry it to a sump or to the tailwater.
  • Inspection and monitoring: Give access to inspect the dam, install instruments (piezometers, joint meters, plumb lines) and carry out maintenance.
  • Working place for grouting (a grouting gallery from which holes are drilled).

Elevation of a dam showing the galleries and grout holes

 Crest ____________________________
      |      Dam body             |\
      |  ===== gallery ======     | \
      |  ===== gallery ======     |  \
      |  ==== foundation gallery =|   \
 Base +--|--|--|--|---------------+----\
         |  |  |  |  grout holes  (curtain)
         v  v  v  v
       (holes drilled to a depth of 30-50% of H)
   Left <--- galleries run along the dam ---> Right

Section showing the drains

        u/s face            gallery
           |   grout curtain  |
  Water -->|   |   |   drain  |  Vertical formed drain
           |   |   |   holes  |  (in the dam body,
           |   |   |   |  |   |   spaced 3 m)
  Dam body |   |   |   |  |   |  Trap drain (collects
           |   |   |   |  |   |  from the formed drains)
 ----------+---+---+---+--+---+--------- Foundation
               grout holes   drainage holes (downstream)
  • Vertical formed drains: pre-cast porous concrete pipes or formed holes, 15-30 cm in diameter, spaced about 3 m, in the dam body near the u/s face; they lead the seepage down to the gallery.
  • Trap drain: a longitudinal channel in the gallery floor that collects the water from the formed drains and carries it to the sump.
  • Drainage holes: holes 75-100 mm in diameter, drilled from the gallery floor into the rock, about 3 m apart and 20-30% of the water depth in length, behind the grout curtain.

✓ Answer checked by Claude Opus 5.5 (high effort)

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