An infiltration gallery is a horizontal or nearly horizontal tunnel with permeable boundaries that are constructed near highly permeable aquifers to permit groundwater to infiltrate into the same.
|Infiltration chambers - LID SWM Planning and Design Guide (sustainabletechnologies.ca)|
These structures are rectangular in cross-section and are called horizontal wells.
|Infiltration Galleries - Subsurface Sources of Water for Water Supply|
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Fig. 2. below shows the cross-section of an infiltration gallery that is constructed near a river stream. It is generally constructed in highly permeable aquifers with a high water table so that an adequate head is available for the gravity flow of groundwater into the gallery.
|Fig.2. Plan of Infiltration Gallery|
Based on how infiltration galleries are laid, they can form different types as shown in Figures 2 (a) and (b).
Working and Construction of Infiltration Galleries
- Infiltration galleries are constructed near a permeable aquifer with a high-water table and a continuous recharge of perennial flow.
- They are laid parallel to the riverbeds at depths of 3m to 6m.
- These galleries collect water by intercepting and collecting water through gravity flow.
- The horizontal pipe may be made of vitrified clay or concrete or brick of 0.5 m to 1.5 m in diameter.
- The pipe in the infiltration gallery is perforated and is surrounded and packed by means of a permeable boundary which is formed by a 3-layer graded filter. This graded filter is made of:
- 16 cm ballast (25 to 50 mm size) around the pipe.
- 16 cm large pebbles (12 mm to 24 mm) form the intermediate layer.
- 24 cm pebbles (3mm to 10 mm size) forming the outer layer.
- The water enters the tunnel either through the side or bottom.
- The purpose of filter arrangement in varying sizes of aggregates is to allow clear water percolation with low entrance velocities.
- The gallery is laid at a slope leading to a central collecting shaft, from where the water is pumped to the surface for use or distribution.
- For long-length distribution from infiltration galleries, manholes can be provided at intervals of about 100 m for inspection and maintenance.
Calculation of Flow/Yield Capacity of Infiltration Gallery -
q = k [H2 – Ho2]/2L
- Soil is isotropic, homogeneous, and incompressible
- The tangent of the angle of inclination of the water table i.e., the slope is equal to its sine
- The flow is uniform and horizontal throughout the depth of the aquifer.
Features of Infiltration Galleries
- One or more galleries are constructed that together connect to a central point like a spring box or a hand-dug well. Hence these center-point water collection structures are called collector wells.
- Infiltration galleries are constructed in combination with other means of water supply systems so as to meet the increasing water demand as the gallery alone cannot meet the demand of a big population.
- They are constructed in a way that is prevented them from getting contaminated. A safe distance of generally 30 meters is maintained from latrines and any contaminated areas. The safe distance varies with the site and location of the gallery.