The borehole diameter and depth.
Leading on from the borehole design will be an assessment of the average depth with contingency for a maximum depth, a diameter of borehole to be drilled and in what sort of geology.
PAT drilling machines construct holes with the following main drill methods.
Mud Drilling
This is used in sedimentary formation such as sands, gravels, clays and very soft or lightly cemented or weathered rocks. It uses a circulation of water from mud pits into a mud pump and through the drill pipe is wash to surface the drilled rock material – the fluid in the hole also hydraulically supports the borehole walls from collapse until casing is inserted. The normal drill bit used is a drag blade and these are manufactured by  PAT in a huge variety of diameters – sized to drill inside casing and outside – ie to make a hole big enough to insert casing. Once a drag blade finds the rock too hard to penetrate then the ground is better drilled with ancompressor and down the hole hammer.(see below) |
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Click for more explanation on Mud drilling Chapter 2 |
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Compressed air drilling.
This is used in hard consolidated rock – ground that is free standing and will not  collapse easily. With this method compressed air is blown down the centre of the drill pipe and the air column in the drilled hole blows drilled material to the surface. If the rock is soft enough to be cut with a drag blade type drill bit this is a quick drilling method – as soon as the penetration rate slows a down the hole hammer is used this is a percussive drilling tool that drives a piston onto a percussive rock drilling bit to pulverize the very hardest of rock. A drilling compressor is carefully selected to provide enough air volume to clean the hole and operate the down the hole hammer
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Click for more explanation on compressor air drilling Chapter 3 |
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