Dig a cistern pit as a rounded bottle, bell or cylinder with no sharp corners, about 2.5 m across and 2.5 m deep for 10,000 L, and choose the shape by the soil: stable clay holds itself while you work, but sandy soil must be lined as you dig. Keep the site above the water table, because groundwater 1 m up the wall pushes an empty tank upward with about 48 kN while the roof slab weighs only about 12 kN.
Part of Cisterns and Rainwater Storage
Excavation design is the choice of a pit’s shape, size, depth and position for an underground cistern, based on the soil, the water table and the distance from contamination. A pit that stands up during digging and stays put after building is the foundation of the whole tank.
Round Shapes Resist Earth Pressure, And Corners Concentrate Stress
The cisterns page prefers a bottle or bell shape (wider at the bottom, narrower neck at the top) because earth pressure pushes inward on a curved wall and is carried around it. A cylinder works well too. Sharp corners concentrate stress; round them.
The bell is stronger once built, but it is a more dangerous shape to dig. A wall cut back under the neck is an overhang, and an overhang above a person is what collapses. Dig a bell only in firm clay, and shore or line everything else.
Stable clay: the walls hold themselves during construction. Sandy soil: line as you go, in stages from the top down, because a pit in sand can slump into itself. Other soils are not covered by the cisterns page; if you cannot say how a soil behaves, treat it like sand and support it.
Siting: Water Table, Distance And Trees
- Above the water table. Dig a test hole first, ideally after rain, and see whether water seeps in.
- 30 m clear of latrines, animal pens and waste (the Sphere default where soil permeability cannot be tested; WHO’s lower 15 m figure applies only to a pit sited down gradient, and in gravel or fissured rock no horizontal distance protects — see Source Protection), and 3 m from large trees, whose roots crack masonry (Cistern Building).
- Where the roof catches the rain by gravity, so the gutter runs downhill into the inlet.
What You Need
- Stakes and string — to mark a circle of the planned radius
- A trial hole — to read the soil and check for water
- Shoring — boards, poles or woven wattle for any non-clay wall
- A ladder and a lifting line — spoil goes up in buckets
- A rope and harness for anyone in the pit
Method: Mark, Test, Dig, Check
Step 1: Mark the circle. For 2.5 m across, drive a stake at the centre and swing a 1.25 m string.
Step 2: Dig the trial hole beside it. Note the soil and any water.
Step 3: Dig in layers, shaping to the string. Pile spoil well back from the edge.
Step 4: Shape the floor to a very slight dish, and round the wall-to-floor corner.
Step 5: Check the wall as you go and line or shore if it flakes or slumps.
Worked Example
Volume. π × 1.25² × 2.5 = 12.3 m³ = 12,300 L, of which about 10,000 L is usable (see Underground Cisterns).
Groundwater uplift. Suppose groundwater stands 1 m up the wall of the empty tank. Displaced volume is π × 1.25² × 1 = 4.91 m³. Upward force is density × gravity × volume = 1,000 × 9.81 × 4.91 = 48,200 N = 48 kN. The roof slab, 10 cm thick, is 4.91 m² × 0.10 m = 0.491 m³; at about 2,400 kg/m³ for reinforced concrete that is 1,180 kg, a weight of 1,180 × 9.81 = 11,600 N = 12 kN. The walls and the grip of the soil add some resistance, but the margin is thin, so an empty cistern in wet ground can lift and crack. Keep water in it when the ground is wet, or place the cistern above the water table.
Common Mistakes
- Digging a bell in sand. It collapses as you work.
- Piling spoil at the pit edge. The weight pushes the wall in.
- Siting by convenience, not by water table. A seasonal high water table only shows in the wet months.
Safety
A pit wall can bury you, and a deep pit is a confined space
Soil weighs far more than a person and falls in one piece. Keep spoil back from the edge (a US construction rule puts it at least 0.6 m back), shore or line any wall that is not firm clay, never work under an overhang, and never dig alone. A pit that is roofed, or deep and sheltered, can be short of oxygen: no entry without a harness, a line and an attendant at the surface who pulls the line and never climbs in after the entrant. Do not test the air with a flame.
What’s Next
- Learn next: Cement Plaster — making the pit watertight
- Combine with: Cistern Roof — covering what you have dug