Casing Material
Water well casing may be constructed of steel or plastic (of various types). Permanent steel casing is not
advised as it may corrode in certain groundwaters and also encourage the development of biofilm slimes.
The driller should certify that the casing is suitable for the specific borehole. Factors that affect casing selection
include: cost, diameter, drilling method, water quality, strength requirements, and corrosion resistance. The
joints between lengths of casing must be watertight.
• Steel casing used in water wells should conform to the standards of the American Society for Testing
and Materials (ASTM), British Standards Institute (BSI) or American Petroleum Institute (API).
• Plastic casing may be of several types: Polyvinyl chloride (PVC), Acrylonitrile butadiene styrene (ABS),
rubber-modified polystyrene (SR), High Density PolyEthylene (HDPE), polyolefin, polypropylene, and
Glass-reinforced plastic (GRP or ‘glass fibre’).
• Plastic casing must have sufficient strength to withstand the pressure and setting temperature of the
cement grout that will be placed in the annulus outside the casing. Well casing should be specifically
designed for that purpose. Cheaper but weaker types of pipe, such as sewer pipe, should not be used.
Casing Depth
Casing should be installed to sufficient depth to fulfil the three functions listed above.
Where the aquifer is unstable, e.g. sands or gravels, the casing and well screen (see below) should normally
be installed to the full depth of the well.
• In rock, casing should be installed and grouted to a minimum depth of 20 metres below ground, or to 10
metres into bedrock, whichever is the greater.
• Casing should also be installed to at least 2 metres below the depth at which the pump will be set. This
will ensure that (a) the pump and rising main are protected from any collapse in the well, and (b) the
pump is set a good distance above any muddy sediment in the bottom of the well.