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Types of Bore Casing

Types of Casing

– Water Well Casing –

The main types of casing used in bores are:

Steel
PVC-U
ABS
FRP and FRE
Stainless steel

Each of these has different properties in relation to column,
collapse and tensile strengths, resistance to corrosion,
reaction to ground and water chemistry, and temperature.


Steel

Steel is a commonly used casing material because of its greater strength.
When used as a casing it can be butt welded or screwed.
Steel has the following advantages over other types of materials.

It is stronger than other materials.
It can be pressure-cemented to greater depths because of its higher collapse strength.
It can withstand high temperatures.
It is available in large diameters.
It can withstand rougher treatment.

A disadvantage of steel is that its life can be reduced in a corrosive environment.
This can be through corrosive soils, water, or by galvanic action
arising from the use of dissimilar materials in the bore.

Some potable waters can be very corrosive to steel
because of the dissolved gases they contain.
Carbon dioxide (CO2) is the most common of these gases.
Groundwater with high levels of dissolved CO2 and oxygen (O2)
can accelerate the corrosion of steel (see Table below).

The reactivity shown in the table can vary,
depending on the chemistry of the particular water.

Reactivity of steel casing to corrosive waters:

Reactivity of steel casing to corrosive waters - Recommendations By Water Bore Drillers - Direct Drilling

Non-ferrous or plastic materials are commonly used as casing materials
where corrosive waters preclude the use of steel.


PVC-U (Unplasticised polyvinyl chloride)

PVC-U piping is made for a wide range of uses including drainage
and general water distribution.
It is made in a variety of wall thicknesses and internal diameters.

The only PVC-U piping suitable for use as bore casing is pressure-rated pipe
manufactured to conform to AS 1477.
This Australian standard is for pipe that is rated for potable water supply.

Because of recent changes in AS 1477, PVC-U pipe now uses
a pressure nominal (PN) rating instead of a class.
PN indicates the pressure rating of the PVC-U,
which provides a guide to the external collapse pressure.

The collapse pressure of PN 6 pressure pipe has been found to have insufficient strength
and shall not be used as bore casing.
PN 9 piping can be used with care for shallow bores.
PN 12 piping is the recommended casing for most bore construction applications.

PVC-U has the following advantages over other types of materials.

It is:

Non-corrosive
Readily available in some sizes, particularly small diameters
Light and easy to handle and join
Inert

PVC-U casing is low in compressive strength relative to steel casing.

The actual strength for any situation will depend
on the uniformity of the wall thickness, the roundness of the casing,
the rate of loading, and the temperature of the casing when the loading is applied.

PVC-U material is much more flexible than steel,
and temperatures greater than 20°C reduce the pressure rating of the casing.
It should be de-rated in accordance with the manufacturer’s specifications
when used with elevated water temperatures and when cementing.

The following factors should also be considered.

PVC-U casing requires care in handling, storage,
and installation to prevent breakage and/or distorting its shape.

Plastic parts installed above the ground must be protected from damage
(e.g. from moving vehicles, contact with drilling tools, fire).

The impact strength of PVC-U casing may be reduced significantly
over time from extended exposure to UV rays.

Occasionally PVC-U casing will float in a bore during installation,
thus creating special handling problems.

The short-term strength of PVC-U casing is much higher than its strength over time.
Therefore the driller should consider the long-term forces of the formation on the casing.

Changes to differential pressure and temperature ratings
may result from cementing.

If volatile organic chemicals make contact with PVC-U casing,
they may permeate the casing and enter the bore.

It should be centred in the borehole during backfilling or gravel packing.
Any voids in the backfill or gravel pack may lead to a sudden collapse
of formation materials against the casing, causing the casing to collapse.


ABS (Acrylonitrile butadiene styrene)

ABS is a low-density material with a higher tensile strength
and lower temperature rating than PVC-U.

Any ABS piping to be used shall comply with AS 3518.1.
ABS can be supplied with a flush internal and external screw joint,
as well as a tapered solvent-weld flush internal and external joint.


FRP (Fibreglass-reinforced polyester)
and FRE (Fibreglass-reinforced epoxy)

FRP casing is usually referred to as fibreglass casing
as the reinforcing fibre is normally glass strands.
Fibreglass casing has a higher collapse strength to weight ratio than steel.
FRP does not require de-rating for temperatures below 80°C.
It can also be custom-made for particular application requirements
(e.g. with specific collapse and tensile strengths).
FRP is used for deep water supply bores in some areas.

FRE is stronger and has higher chemical resistance than FRP.


Stainless steel

A range of grades of stainless steel can be matched to soil water chemistry
and temperature to provide a higher corrosive resistance than steel.

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