Drainage is the controlled removal of excess water from the soil or a building, with the aim of preventing water damage, dampness and ground settlement. Drainage works by laying permeable pipes, channels or trenches through which water flows to a lower point, sewer or surface water.
How drainage works
Water in the soil flows under gravity to the lowest point. A drainage system offers water that point: a porous drain pipe lies in a gravel bed and is wrapped in geotextile to prevent silting. Water enters through the slots in the pipe and is conducted away.
Types of drainage
Garden drainage
A flat drain pipe at 60–100 cm depth around the garden, connected to an infiltration crate or rainwater tank. Prevents waterlogged lawns and plant die-off.
Foundation drainage
A drain pipe along the outside of the foundation wall, on the footing. Keeps groundwater away from basements and crawl spaces.
Road drainage
Beneath the road base or in the verge; carries away rainwater to prevent rutting and ice.
Agricultural drainage
A fine grid of drain pipes in a field, often 0.8–1.2 m deep. Keeps the groundwater table low for better crop yields.
Vertical drainage
Wells or gravel filters bored vertically into the ground, especially for ground improvement and dewatering of construction pits.
Applications
Drainage is installed at:
- Basements and souterrains — preventing water pressure on the wall
- Clay gardens — reducing puddles and waterlogging
- Sports pitches — keeping the pitch playable in rain
- Roads and car parks — preventing frost damage and ice
- Construction pits — keeping the works dry during excavation
Key components
- Drain pipe — perforated PVC or PE pipe, 50–125 mm
- Gravel bed — clean gravel (16/32) around the pipe
- Geotextile — prevents fine particles from clogging the system
- Inspection chamber — for checks and jetting
- Outfall — to sewer, ditch or infiltration crate
Maintenance
A drainage system needs periodic maintenance: at least every 5 years jet-flush to remove roots, silt and lime scale.
Related terms
- Groundwater level
- Infiltration
- Geotextile
- Dewatering
- Gravel bed
- Sewerage
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