A soakaway is a pit filled with rocks, such as a 1 m × 1 m × 1 m pit, that takes a cistern’s overflow and lets it sink into the ground instead of running across the surface. In slow soil it fills in minutes at storm flow, so test how fast your ground drains and give heavy overflow a second, surface route to a safe place downhill.
Part of Cisterns and Rainwater Storage
A soakaway is a buried, rock-filled pit that stores water briefly and releases it into the surrounding soil. It handles occasional clean surplus water such as cistern overflow; it is not a disposal pit for sewage or dirty water.
A Pit Is Only As Good As The Soil Around It
Water leaves a soakaway by soaking out through its floor and walls. How fast depends entirely on the soil: sand and gravel take water quickly, clay hardly at all. The rocks do not move the water. They hold the pit open, give the water room to spread and keep the walls from caving.
The water-distribution page builds a standpipe soakaway as a 1 m × 1 m × 1 m pit filled with rocks, which works for the small flows of spilled tap water. A cistern overflow can be much larger.
What You Need
- A site downhill of the tank, its foundations and any well, away from latrines
- Rock or broken stone, fist-size or larger
- A pipe from the overflow entering the pit at the top
- A cover of flat stones or a slab with soil over it, so no one steps into the pit
- A bucket and a clock, for a percolation test
Method: Test, Dig, Fill, Cover
Step 1: Test the soil. Dig a small hole, fill it with water, let it drain once, fill again, and time how far the level falls in an hour.
Step 2: Dig the pit to the planned size.
Step 3: Fill it with rock, leaving the overflow pipe’s entry clear.
Step 4: Cover it with flat stones, then soil.
Step 5: Plan an overflow-of-the-overflow: a shallow ditch from the pit’s top to a safe lower place.
Worked Example
Overflow from the 40 m² roof at heavy rain: 0.5 L/s = 1.8 m³/h (see Overflow Pipe).
Assume, for illustration, that the rock fill leaves 40% of the pit as open space. The pit holds 1 m³ × 0.4 = 0.4 m³ in its gaps. Suppose the soil takes 10 mm/h through the pit’s walls and floor (about right for a slow soil; measure yours). The wetted area when full is 1 m² floor + 4 m² walls = 5 m², so it drains 5 × 0.010 = 0.05 m³/h.
Net filling rate = 1.8 − 0.05 = 1.75 m³/h. Time to fill = 0.4 ÷ 1.75 = 0.23 h = about 14 minutes. After that the pit overflows. In clean sand or gravel, which drain far faster, the pit keeps up much longer. The 40% void space and the 10 mm/h rate are assumptions to show the method, not measured values.
Common Mistakes
- Siting uphill of the tank. Water seeps back toward the cistern and the foundation.
- Digging in clay and expecting it to drain. Test first.
- No second route. A soakaway that fills in 14 minutes sends the rest across the ground.
- Sending greywater or sewage to it. That is a different design.
- Leaving it open. A hidden pit is a hazard to people and animals.
Safety
An open pit and a soft cover
A soakaway hidden under thin soil can collapse under a person. Cover it with flat stones or a slab strong enough to stand on, and mark it. Do not put the pit near a well or a latrine.
What’s Next
- Learn next: Drainage Design — the same ideas for spilled water at standpipes
- Combine with: Overflow Pipe — the pipe that feeds it