Bore Construction
All Bores need to be constructed to a minimum standard,
specified in the minimum construction requirements for
water bores in Australia (ARMCANZ, 1997) and to the
satisfaction of the relevant water authority, here in
Victoria generally Southern Rural Water or Goulburn
Murray Rural Water.
However the minimum construction standard is a guideline
only and should be used as a MINIMUM requirement. All
new bores should in reality exceed this requirement.After
locating a suitable aquifer, drilling is commenced with
a variety of techniques, usually either Air Rotary or
Mud. Air rotary relies on high volumes of compressed
air to operate the cutting bit called the “hammer”
and remove detritus from the bore. Air rotary is suitable
for all hard rock formations as well as some unconsolidated
formations such as sand and clay, providing the operator
is sufficiently skilled in this type of drilling. Air
rotary is by far the most common as it is the fastest
and most efficient method in the above formations.
The Mud Rotary drilling technique relies upon mud as
the media to not only remove detritus from the bore
but also to stabilize and support the walls of the hole,
thus preventing collapse and in some cases the outbreak
of pressure water (artesian) from occurring. Mud drilling
is not as fast or efficient as air or as popular for
water boring, however it is often the only technique
to use in certain formations, such as fine drift sands,
running gravels and in artesian areas to prevent the
outbreak of water under pressure. Mud drilling is predominantly
used in the oil and gas industries for reasons of pressure
control as well as hole stabilization.
Once water is encountered, the bore is cased, that
is lined with PVC bore casing, generally of class9
wall thickness or larger. The bore must be lined the
full depth of the bore to prevent collapse of the
bore or the inflow of detritus. If the bore is not
cased all the way to the bottom, then there is a high
probability that the bore will collapse on top of
the pump, locking the pump in the bore resulting in
the loss of the pump and no water. Retrieval of a
pump in such cases can be a costly and time consuming
task with the chances of retrieving the pump undamaged
quite slim.
Once the casing has been installed full depth in
the bore, the bore must then be developed to remove
detritus and drilling slurry which will allow the
bore to produce to its full capacity. Development
of the bore is also necessary to produce clean clear
sediment free water. If the drilling technique involved
was mud rotary, then development can take significantly
longer than for air rotary drilled holes. This is
because the mud needs to be firstly broken down with
detergents, into a less viscous format, then removed
with an air lift technique. With an air blast drilled
bore, generally development can proceed directly to
air lift.
During development, the yield of the bore will be
flow tested to determine water volume and flow rate
available. This information is of course necessary
to be able to correctly size a pump to the depth and
yield of the bore.
After development and flow testing, the bore will
then have a concrete surface pad erected around the
top of the casing, for support and to impede surface
contamination and is then capped. The bore is now
ready for a pump/windmill to be installed.
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