Choose PVC of 110-160 mm for most drilled wells because it is light, cheap, rust-proof and joined with solvent cement; choose steel pipe where the casing must be driven, handles heavy loads or must stand above ground; and choose concrete pipe or rings for wide, shallow dug wells. Whatever you use, it must never have carried fuel, chemicals or sewage, and its pressure rating must be good for the external water pressure at the bottom of the well.
Part of Deep Wells
Casing materials are the pipes and rings used to line a well. Each trades weight, strength, cost and chemical behaviour, and in a rebuilding settlement the choice is often set by what can be salvaged.
The Material Must Be Strong, Clean And Joinable
| Material | Strengths | Weaknesses | Best use |
|---|---|---|---|
| PVC pipe, 110-160 mm | Light, cheap, no corrosion, easy to cut and slot, solvent-cement joints | Can crack on impact; softens in sun and heat above ground; may be hard to find; strength limited at depth | Drilled and augered wells, and screens (Slot Cutting) |
| Steel pipe | Strong in tension and bending; can be driven; stays rigid | Rusts and may colour or taste the water; heavy; needs threading or welding | Heavy casing, driving through hard layers, wellhead stub |
| Concrete pipe or rings | Cheap if cast locally; strong in compression; does not corrode | Heavy; joints leak unless sealed; needs a hoist | Wide shallow dug wells (Concrete Rings) |
| Fired clay pipe | Inert, rot-proof | Brittle; short sections; many joints | Shallow, wide holes only (Clay Pipes) |
Salvaged Pipe Needs Checking Before It Goes In A Well
Water in a well sits against the casing for years. Check each length:
- What it carried. Do not use pipe that carried fuel, solvents, pesticides, acid or sewage. Plastics absorb some chemicals and release them into the water later. If you do not know the history, do not use it for drinking water.
- Condition. Reject cracked, crushed or sun-damaged pipe, and pipe that has gone brittle: it rings dull when tapped, or snaps instead of bending.
- Straightness and fit. A bent length will not slide down the hole; sockets that do not mate will leak sand.
- Never burn PVC offcuts. Burning PVC releases hydrogen chloride and other toxic fumes.
Outside Pressure Rises With Depth
The pipe also has to withstand the water outside it. Water pressure rises by 9.81 kPa for every metre of depth: at 30 m below the water surface it is 30 × 9.81 = 294 kPa, nearly 3 bar. Casing that is open to the water on the inside, in a well full of water, is balanced. It is when the well is pumped low, or the casing is lowered empty and capped, that this pressure acts on it from outside. Check the pipe’s rated pressure (manufactured pipe states one) and do not cap a long empty string and lower it into deep water. If you do not know the rating, keep the casing flooded or vented as you lower it.
What You Need
- Pipe of one material and diameter for the whole string, plus the screen
- Joining materials — solvent cement for PVC, threads or welds for steel, mortar for concrete
- A way to check the pipe — a straightedge, a hammer to ring the pipe, and a long stick or rod dragged through to test the bore
- A record of source — where each length came from and what it carried
Method: Choose By Job
Step 1: List the loads. Depth, soil, whether driving is needed, and what the pipe will stand above ground.
Step 2: Pick the material. PVC for most drilled wells; steel where it must be driven or stand above ground; concrete or rings for dug wells.
Step 3: Inspect every length as above, and discard what fails.
Step 4: Match the joints. Use one joint type throughout. Mixed joints leak or fail.
Common Mistakes
- Using pipe of unknown history. A fuel line or chemical drain gives the well a taint that cannot be removed.
- Mixing diameters. A string with a step in it jams in the hole.
- PVC left in the sun. Above-ground plastic degrades; shield or paint it, or finish the top in steel or concrete.
- Light pipe in deep, heavy ground. Check the collapse rating.
Safety
Fumes from cement and from burning plastic
Solvent cement and primer give off fumes: use them outdoors, away from flames and sparks. Do not burn plastic pipe waste. Cutting PVC raises dust and sharp edges: wear eye protection and gloves.
What’s Next
- Learn next: Screen Section — turning part of the string into a screen
- Combine with: Well Casing — how the string goes together