A borehole is a narrow hole through soil and rock. If its walls aren't supported in the right places, they can cave in or let sand and silt in, slowly choking the borehole and the pump.
What casing does
Casing is the steel or PVC pipe installed from the surface down through the soft, weathered ground. It:
- Stops loose soil and gravel from collapsing into the hole.
- Keeps surface water and contamination out.
- Protects the pump and cable.
Below the casing, in solid rock, the hole may be left open. In unstable ground, slotted casing or screens let water in while holding back sand.
Signs of trouble
- Sand, silt or mud in the water, getting worse over time.
- Falling yield: the pump runs dry sooner or the tank fills more slowly.
- The pump gets stuck when pulled, or keeps wearing out.
- Cloudy water after heavy rain, which can point to surface water getting in.
See also borehole pump problems and borehole water smell or colour.
Common causes
- Too little casing for the depth of soft ground at that site.
- Corroded or damaged casing, especially in older boreholes.
- Pump set too low, close to the bottom of the hole, where silt collects.
- Pumping harder than the borehole can yield, which pulls in fine material. See choosing the right borehole pump.
- On the East Rand, dolomitic ground can be unstable. That's one reason the City of Ekurhuleni checks dolomite risk before approving drilling. See borehole rules in Ekurhuleni.
Repair options
| Option | When it helps |
|---|---|
| Pull the pump and clean the borehole (air-lifting or flushing) | Silt build-up in an otherwise sound hole |
| Reset the pump higher | Pump sitting in silt near the bottom |
| Install a liner or new casing | Damaged or too-short casing, if the hole is still open |
| Drill a new borehole | Collapsed hole that can't be recovered |
A borehole company can often run a camera down the hole to see the casing and where water and sand are coming in before deciding.
Preventing it
Ask for enough casing in the original quote, a correctly sized pump set at the right depth, and dry-run protection. Our borehole maintenance checklist covers yearly checks, and what a borehole quote should include helps you compare drillers.