---
title: "Infiltration gallery construction"
subject: "Water Supply Engineering"
chapter: 2
key: "semester_5/water_supply_engineering:infiltration-gallery"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/infiltration-gallery
site: BCT Engineering
---

# Infiltration gallery construction

Water Supply Engineering · Chapter 2: Sources of Water

- Asked in: 2073 Chaitra, 4 marks; 2068 Baisakh (old course), 4 marks; 2066 Jestha (old course); 2066 Bhadra (old course) (asked 4 times)
- Video: [Infiltration Gallery - How Does it Work?](https://www.youtube.com/watch?v=Ia1rFV0LXBs) — Civil Engineering Fanatics
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/infiltration-gallery
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-2

## Question

What is an infiltration gallery? With a neat sketch, describe its construction.

## Short answer

An infiltration gallery is a horizontal porous pipe or conduit laid in a water-bearing stratum along a river or lake to collect seepage. It is built by digging a trench below the water table, laying a perforated pipe surrounded by graded gravel and sand, sloping to a sump well, sealing the top with clay, and providing manholes for cleaning.

## Answer

An **infiltration gallery** is a horizontal, porous conduit (tunnel or pipe) laid in a water-bearing stratum, usually along the bank or bed of a river or lake or in a water-bearing sand layer, to collect ground water by seepage. It gives naturally filtered water and is also called a horizontal well.

#### Construction
- A trench is dug below the water table, parallel to the river, about 3 to 6 m deep.
- Perforated pipes (or porous masonry conduit with open joints) of diameter about 0.3 to 0.9 m are laid with a slight slope (about 1 in 500 to 1 in 1000) toward a collecting (sump) well.
- The pipe is surrounded by graded gravel and coarse sand layers of increasing fineness, to prevent entry of fine particles.
- The trench is back-filled with the excavated soil and sealed with clay at the top, to prevent surface pollution.
- Manholes are provided at intervals for inspection and cleaning.
- Water flows to the sump well from where it is pumped to the treatment or supply system.

```text
 River bank
 ~~~~~~~~~~~  river water level
     \  seepage
 =====\==== ground level
   [gravel+sand layers]
   o o o o o  perforated pipe --> sump well
   -------  impervious bed
```

#### Features
- Yield is about 1.5 to 5 litre/second per 10 m length depending on soil.
- Water is clear, cool and low in bacteria, needs little treatment.
- Suitable for rural and small town supplies; capacity may reduce with clogging, so periodic cleaning is required.

## Check yourself

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

1. An infiltration gallery is:
   A. A vertical tube well
   B. A tank on a hill
   C. A surface intake in a lake
   D. A horizontal porous conduit in a water-bearing stratum collecting water by seepage
   Answer: D. It is a horizontal well collecting seepage water. (Key point: Definition)

2. Why is the pipe surrounded by graded gravel and sand?
   A. To increase clogging
   B. To prevent entry of fine particles
   C. To heat the water
   D. To store water
   Answer: B. Graded layers of increasing fineness keep fine particles out. (Key point: Gravel layers)

3. Why is the top of the trench sealed with clay?
   A. To prevent surface pollution entering
   B. To raise yield
   C. To drain the river
   D. To cool the pipe
   Answer: A. Clay seal stops surface pollution entering the gallery. (Key point: Clay seal)

## Related in Water Supply Engineering

- [Spring formation; infiltration gallery](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Asprings-formation-infiltration-gallery) — Water Supply Engineering, chapter 2