---
title: "Grouting and drainage galleries in gravity dam"
subject: "Hydropower Engineering"
chapter: 4
key: "semester_7/hydropower_engineering:grouting-drainage-galleries-necessity"
source: https://www.bctengineer.com/q/semester_7/hydropower_engineering/grouting-drainage-galleries-necessity
site: BCT Engineering
---

# Grouting and drainage galleries in gravity dam

Hydropower Engineering · Chapter 4: Headworks of Storage Plants

- Asked in: 2076 Ashwin, 2+4+4 marks
- Video: [various components(shear key&water stops),types of gallery-Gravity dam:Design of Hydraulic Structure](https://www.youtube.com/watch?v=yI6G6e0SZkQ) — CIVIL Enlightener
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/hydropower_engineering/grouting-drainage-galleries-necessity
- Whole chapter: https://www.bctengineer.com/md/semester_7/hydropower_engineering/chapter-4

## Question

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.

## 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.

## Answer

#### 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
```text
 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
```text
        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.

## Check yourself

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

1. Curtain grouting along the heel is done mainly to
   A. Reduce seepage and uplift by sealing rock cracks and cutting off flow under the dam
   B. Increase dam weight
   C. Provide inspection access
   D. Fill contraction joints
   Answer: A. The grout curtain seals joints and forms a barrier to flow beneath the dam. (Key point: Curtain grouting purpose)

2. Contact grouting fills
   A. Contraction joints in the dam
   B. Drain holes
   C. The gap between concrete and rock
   D. Cracks deep in the rock mass
   Answer: C. Contact grouting fills the gap between the concrete and the rock; joint grouting fills contraction joints. (Key point: Contact vs joint grouting)

3. How do drains in the gallery reduce uplift?
   A. They block the water at the heel
   B. They collect water passing the grout curtain and release the pressure, so uplift falls to about one-third of the head difference at the gallery
   C. They increase the seepage path
   D. They raise the tail water
   Answer: B. Drain holes relieve pressure behind the curtain, lowering uplift. (Key point: Role of drain holes)

4. Besides drainage, galleries are also used for
   A. Power generation
   B. Spillway discharge
   C. Fish passage
   D. Inspection, instruments and as a working place for grouting
   Answer: D. Galleries give access for inspection, monitoring instruments and drilling grout holes. (Key point: Other gallery functions)

## Related in Hydropower Engineering

- [Formed drain, trap drain; grout hole criteria](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Adrain-arrangement-grout-hole-criteria) — Hydropower Engineering, chapter 4

## Related in other subjects

- [Stresses in gravity dam with drain holes](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Agravity-dam-stresses-figure) — Hydro Power, chapter 4