Infiltration gallery construction

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.

 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.