Concrete well rings are cast at the surface, 1 m across, 0.5 m tall and 5-8 cm thick, then lowered into the shaft or sunk by digging out the soil beneath them, so that the digger always works inside finished lining. Each ring weighs roughly 180-280 kg, which is too heavy to lift by hand, so cast them with lifting loops, move them with a tripod and hoist, and never have anyone below when a ring is lowered.
Part of Deep Wells
Concrete rings are short cylinders of concrete stacked one on another to line a well. Sinking them is the safest way to dig a hand well, because the lining is in place before the soil beneath it is removed.
The Ring Is Cast Above Ground So The Worker Is Under Finished Lining
The Deep Wells guide specifies pre-cast rings 1 m across, 0.5 m tall and 5-8 cm thick, placed on the surface, with the soil dug out beneath them so that they sink under their own weight and the next ring is added on top. The method works best in soft, uniform ground; in boulders or mixed ground a ring can jam, and the other methods in Lining As You Go suit better.
Each Ring Weighs A Few Hundred Kilograms
Treat the 1 m figure as the outside diameter. Concrete is about 2,400 kg/m³ (a recalled typical value for ordinary concrete; lime concrete and your aggregate may differ, so weigh a test block). For a wall 6.5 cm thick, the middle of the 5-8 cm range:
- Inner diameter = 1.00 − 2 × 0.065 = 0.87 m
- Wall area = π ÷ 4 × (1.00² − 0.87²) = 0.191 m²
- Volume = 0.191 × 0.5 = 0.0955 m³
- Mass = 0.0955 × 2,400 = 229 kg; weight = 229 × 9.81 = 2,250 N
Across the thickness range the same arithmetic gives about 180 kg at 5 cm and about 280 kg at 8 cm (1,760 N to 2,720 N). A well 10 m deep needs 20 rings: 20 × 0.0955 = 1.9 m³ of concrete and about 4.6 tonnes of rings. One ring dropped down the shaft is a weight of about 2.2 kN falling onto the digger.
Two consequences follow. The bore of such a ring is 0.84-0.90 m, which is cramped: the Phase 0 well-lining guide treats a bore under 0.9 m as one where a digger risks getting stuck. And the rings must be handled with a hoist.
What You Need
- Moulds — an outer and an inner form, of timber or sheet metal, sized so the ring is true and the wall an even 5-8 cm
- Concrete — cement or lime concrete (Concrete Making, Lime & Cement)
- Reinforcement — wire or bar hoops and vertical pieces, with lifting loops cast in at three or four points
- A tripod and hoist — rated for the ring weight with margin (Windlass)
- A cutting edge — a steel-shod or sharp-edged lowest ring
- A damp-curing area — to keep rings wet for days after casting
Method: Cast, Cure, Lower, Dig, Stack
Step 1: Cast. Set the inner form in the outer form, fill in layers and compact each layer so the concrete has no voids. Cast in the lifting loops and any water holes needed for the lower rings.
Step 2: Cure. Remove the forms when the concrete can hold its shape, and keep the rings damp. Thin concrete shells dry out quickly and weaken. The Concrete Curing guide keeps ferrocement tanks wet for at least 14 days; treat rings the same way unless you have strength data (Curing).
Step 3: Place the first ring. Dig a shallow starting pit, set the sharp-edged ring level and plumb, and check with a plumb line and a level.
Step 4: Dig inside it. The digger works inside the ring and removes soil from under the lower edge evenly, keeping the ring level. Never dig outward beyond the edge to undercut. If the ring tilts, dig more on the high side.
Step 5: Stack. As the ring sinks, add the next on top, bed it in mortar, and seal the joint. Add or lower a ring only when nobody is in the shaft.
Step 6: Zone the lining. In the water-bearing zone, use rings with holes or open joints to admit water, with gravel packed around them. In the upper 3 m use solid rings with sealed joints and fill the gap behind with clay or grout (Surface Seal).
Worked Example: The Gap Behind The Rings
If the shaft is marked out 1.2 m across and the rings are 1.0 m across, each side has a 0.1 m gap that must be filled behind the lining after sinking. For a ring 0.5 m tall that is π × (0.6² − 0.5²) × 0.5 = 0.17 m³. Ten rings need about 1.7 m³ of backfill, divided between gravel in the lower zone and clay or grout in the top 3 m. A ring sunk into a hole cut to its own diameter has less gap but more friction on the ground.
Common Mistakes
- Uncured rings. Rings that crack or chip when lifted were allowed to dry too fast.
- Tilted start. A first ring set out of level carries the error to the bottom. Check it before the second ring goes on.
- Forcing a jammed ring. Piling load on a stuck ring can crack it or make it drop suddenly when the obstruction gives way.
- Mortar skipped. Rings left dry-stacked leave open joints that admit surface water in the top 3 m.
Safety
A single ring weighs a few hundred kilograms and crushes whatever it lands on
One ring is about 229 kg; a stack of rings weighs tonnes. Lift only with a rated hoist, keep hands and feet clear of the edges, and never have anyone in the shaft while a ring is lowered or added. The digger works inside the ring, not under its edge, and stops at once if it creaks, tilts or sinks suddenly. Every entry needs ventilation, a harness and line, and two people above (Safety Protocol); a collapsed digger is hauled out from above and nobody goes down to rescue.
What’s Next
- Learn next: Dewatering — working below the water table
- Combine with: Surface Seal — sealing the upper rings