An underground cistern is a watertight lined pit with a roof that stores rainwater below ground, where it stays cool and protected from damage and takes up no surface space. For about 10,000 L dig roughly 2.5 m across and 2.5 m deep, line it with plaster or plastic, and cover it with a reinforced slab that has a 60 cm lockable hatch. It suits volumes of 10,000 L and above, and entering it is a confined-space hazard that can suffocate a person in seconds.
Part of Cisterns and Rainwater Storage
An underground cistern is a below-grade tank for rainwater, built by digging a pit, making it watertight and roofing it over. It is the usual choice for large volumes because the surrounding earth keeps the water cool and the roof keeps it clean.
Cool, Covered And Out Of The Way Is Why Large Volumes Go Underground
Underground water stays cool, which slows algae and bacteria growth. The tank takes no surface space, and it is protected from damage. Cost and effort rise with depth, so this is the choice from about 10,000 L upward; smaller volumes are easier in an above-ground ferrocement tank.
A cistern is a set of parts that all have to work:
- The pit — shape and size for the soil (Excavation Design)
- The lining — cement plaster or a plastic liner
- The roof — slab, ring beam and a 60 cm hatch (Cistern Roof)
- The pipes — inlet near the top, overflow at the same level as the inlet, outlet 10 cm above the floor so the draw does not stir up sediment
- A way to draw water that never involves a bucket (Water Extraction)
What You Need
- Digging tools and people — a pit this size is days of work
- A site clear of latrines and animal pens by at least 15 m (Cistern Building), and away from large trees
- Cement and clean sand, or a plastic liner
- Rebar and a form for the roof slab
- A rope, harness and a second person, every time anyone goes near the hole
Method: Dig, Line, Roof, Pipe, Test
Step 1: Dig to the planned shape. Stable clay stands by itself during construction; sandy soil must be lined as the dig goes down.
Step 2: Line the walls and floor with two coats of cement plaster, or a plastic liner with 30 cm seam overlaps.
Step 3: Roof with a reinforced concrete slab on a ring beam, leaving the hatch.
Step 4: Fit the pipes and seal every penetration watertight.
Step 5: Fill, check and disinfect before drinking: chlorine at 50 mg/L for 24 hours is the dose used for a new well (Well Development); that is the dose added, not a residual. Then drain it fully and refill.
Worked Example
A cylinder 2.5 m across and 2.5 m deep has volume π × 1.25² × 2.5 = π × 1.5625 × 2.5 = 12.3 m³, which is 12,300 L. The usable 10,000 L is less than this because the water cannot rise past the inlet and overflow level below the roof, and the bottom 10 cm below the outlet stays as a sediment zone.
Common Mistakes
- Building in a high water table. An empty cistern can float or the lining can be pushed in; see Excavation Design.
- Sealing the cistern completely. Leave a screened vent.
- Letting people dip buckets through the hatch. That introduces dirt and bacteria every time (No Bucket Dipping).
Safety
Entering a cistern can kill, and the rescuer dies second
A sealed or partly sealed tank can hold air that is short of oxygen, and may also hold hydrogen sulphide or carbon dioxide, which is heavier than air and pools at the bottom. A person who steps in can collapse in seconds with no warning, and the commonest fatality pattern is a second person climbing in after the first collapses. Nobody enters unless the tank is open to the air at two points and has been ventilated, the entrant wears a harness tied to a line held by an attendant at the surface, and a second person stays outside to pull the line and never climbs in. Do not judge the air by smell, by sight, or by a flame: hydrogen sulphide numbs the sense of smell, and a flame can ignite vapour and burn on at oxygen levels already low enough to impair a person. Never run an engine pump or generator near the hatch; its exhaust is carbon monoxide.
An open pit collapses
A pit wall can fall without warning and bury a worker. Keep spoil and tools back from the edge, and do not work under an undercut wall.
What’s Next
- Learn next: Excavation Design — shape, size and soil
- Combine with: Inlet Filtering — what keeps the stored water clean