A stone or masonry cistern stands on its own weight, so a free-standing wall holding 1.5 m of water must be about 0.67 m thick, and a 2 m by 2 m cistern holding 6 m³ then needs about 12 m³ of masonry. Build the cistern in a pit instead, so the surrounding earth props the walls, plaster the inside with a waterproof lime or cement coat, and keep it well clear of any latrine.
A stone or masonry cistern is a water tank built from stone, brick or block and made watertight with a lime or cement plaster. It is heavy and permanent, and strongest when buried.
Water Pushes Walls Outward With A Force That Grows With The Square Of The Depth
Water pressure rises with depth, so the force on a wall of water depth H per metre of wall length is ½ × ρ × g × H², acting a third of the way up from the base.
Worked example: a cistern 2 m × 2 m inside, water 1.5 m deep (6 m³), wall height 1.7 m. Inputs: ρ = 998 kg/m³, g = 9.81 m/s², H = 1.5 m.
- Force = 0.5 × 998 × 9.81 × 1.5² = 11,000 N per metre of wall = 11.0 kN/m.
- Overturning moment about the outer foot = 11.0 × (1.5 ÷ 3) = 5.5 kN·m per metre.
A wall that resists only by its own weight, without being tied to the floor or backed by earth, must have a resisting moment larger than this by a safety factor. Take stone masonry at 2,200 kg/m³ (an assumed round figure, since stone varies between about 1,800 and 2,600 kg/m³ as I recall) and a safety factor of 1.5 (also recalled as customary).
- Wall weight per metre = 2,200 × 9.81 × t × 1.7 = 36.7 t kN/m, with thickness t in metres.
- Resisting moment about the outer foot = weight × t/2 = 18.3 t² kN·m/m.
- Required: 18.3 t² ≥ 1.5 × 5.5 = 8.25, so t² ≥ 0.45 and t ≥ 0.67 m.
A second check, sliding: with a friction coefficient between wall and base of 0.5 (assumed), the wall resists 0.5 × 36.7 × 0.67 = 12.3 kN/m against a push of 11.0 kN/m, a factor of only 1.1. So sliding governs, and the wall must be keyed into the foundation or tied to the floor slab.
What That Means In Masonry
The outer dimension is 2 + 2 × 0.67 = 3.34 m square, so the masonry plan area is 3.34² − 2² = 7.2 m², and over a wall height of 1.7 m the volume is 7.2 × 1.7 = 12.2 m³, about 12 m³. That is about twice the volume of water held. Compare the ferrocement tank in Ferrocement Tanks, whose walls are 20-25 mm thick. A simple overturning calculation like this ignores the corners, which tie the walls together and help, and tensile cracks in the mortar, which hurt; treat the result as the scale of the problem, not a design.
Why Cisterns Go Underground
Firm earth packed against a buried wall resists the outward push of the water, so a buried wall can be much thinner than a free-standing one; the earth must be undisturbed or well tamped, and the pit wall must be firm. Dig a pit with firm sides, build the cistern walls against the earth with a stone or brick lining, and roof it. Underground Cisterns and Cistern Building give the construction, including a waterproof lime cistern: 1:2:0.5 hydraulic mortar and a cocciopesto plaster cured damp for 14 days, sited 15 m clear of any latrine. That distance assumes the soil between them filters the seepage; in fissured rock or karst it gives no protection, so do not rely on it there.
What You Need
- Stone, brick or block, sound and frost-resistant
- Mortar: lime (hydraulic if possible, Hydraulic Lime) or cement
- A foundation on firm ground, with a floor slab
- Waterproof plaster inside
- A roof or vault, with an inspection hatch, vent, inlet, overflow and drain (Storage Tanks & Cisterns)
Method: Dig, Lay, Plaster, Test
Step 1: Excavate a pit with firm walls and a level bottom, away from latrines, manure and trees whose roots can break the walls.
Step 2: Lay the floor as a slab, sloped slightly to the drain, before the walls.
Step 3: Build the walls in even courses with full mortar joints, bonded at corners.
Step 4: Plaster the inside in coats, curing each damp (Cement Plaster).
Step 5: Fill in stages. Fill one third, wait a day and check for leaks, then continue; a wall under load for the first time can fail at the base.
Step 6: Backfill the outside in layers with tamped earth.
Common Mistakes
- A thin free-standing wall.
- Building near a latrine. Seepage is possible both ways.
- Plastering over a leaking wall. Repair the mortar first.
- Filling it all at once.
Safety
Pits collapse, and a closed cistern is a confined space
A pit more than about a metre deep can cave in: slope the sides or shore them, and never work under an overhanging edge. A finished cistern with a roof can be short of oxygen or hold hydrogen sulphide or carbon dioxide; nobody enters without ventilation at two openings, a harness and line held by an attendant, and a second person outside who never climbs in. Do not test the air with a flame.
What’s Next
- Learn next: Tank Sizing & Placement — how big and how high
- Combine with: Underground Cisterns — the buried build in detail