Well casing is the pipe that lines a drilled or augered hole: solid pipe through the upper layers to hold the hole open and keep surface water out, and slotted pipe, called the screen, in the water-bearing layer to let water in and keep sand out. PVC pipe of 110-160 mm diameter is the usual choice, because it is light, cheap, does not corrode and can be joined with solvent cement.
Part of Deep Wells
Casing is the permanent lining of a borehole well. Together with the screen, the gravel around it and the seal above it, it makes a narrow well that stays open, yields clean water and cannot be reached by surface contamination.
A Bare Hole Closes Up And Lets Dirt In
A drilled hole through sand or gravel fills in as soon as the bit is out, and even a hole through firm clay lets surface water seep down its sides into the aquifer. Casing solves both problems. Every borehole well needs it: solid casing through the upper layers, and a screen in the water-bearing zone.
The Parts Of A Cased Well
| Part | Where | What it does |
|---|---|---|
| Solid casing | From above ground down to the top of the screen | Holds the hole open; carries the pump pipe; keeps surface water out |
| Screen | Opposite the water-bearing sand or gravel | Admits water through narrow slots and keeps sand out (Screen Section) |
| Bottom cap | At the foot of the screen | Closes the pipe and may be extended as a sump for settling sand |
| Gravel pack | In the gap around the screen | Filters the water and supports the screen (Gravel Pack) |
| Clay or cement seal | In the gap above the pack, up to the surface | Stops surface water running down outside the casing (Surface Seal) |
What You Need
- Casing and screen pipe — see Casing Materials; PVC of 110-160 mm
- A hole wide enough — it must take the casing and a gravel pack around the screen; choose the casing first (Percussion Drilling has the worked clearance)
- Solvent cement — for PVC joints
- A tripod and hoist — to lower the string
- Clean gravel, clay or cement — for the pack and the seal
- A log of the hole — showing where the water-bearing layer lies
Method: Build The String, Lower It, Fill Around It
Step 1: Plan the string. From the log of the hole, put the screen opposite the water-bearing sand or gravel, solid casing above it, and a bottom cap below.
Step 2: Join the pipe. Clean the pipe ends, apply solvent cement, push together and hold until set, following the cement’s directions. Let the joints cure before lowering any weight on them.
Step 3: Lower the string. Lower it slowly on the hoist so it stays central and does not bend. Hold it with a clamp at the top while you add each length.
Step 4: Place the gravel pack and seal. Pour clean washed gravel of 3-6 mm into the gap around the screen, then seal above it with clay or cement.
Step 5: Finish the top. Cap the casing, set the pump over it, and pour the concrete apron (Sanitary Apron).
Step 6: Develop the well. Pump or bail hard for several hours so that fine particles are drawn out and the water runs clear.
Common Mistakes
- Screen in the wrong place. The screen must sit opposite the water-bearing layer, not a metre above it.
- Hole too narrow for the casing. Decide the casing size before the bit size.
- No seal. Without clay or cement above the pack, surface runoff runs down the outside of the casing.
- Not developing the well. Low yield and cloudy water follow.
Safety
A long pipe and an open hole
A string of casing is long and awkward: lower it with a hoist and several people, keep clear of the line of the pipe, and never stand with a foot near the open hole. Solvent cement gives off fumes: use it in the open air and away from any flame. Cover the hole whenever work stops.
What’s Next
- Learn next: Casing Materials — PVC, steel or concrete
- Combine with: Screen Section — the slotted part of the string