Probing for mine workings | Drilcorp

“Vertical means at right angles to a horizontal plane; in a direction such that the top is directly above the bottom.”

“Alignment for our purposes means ‘straightness’. The borehole must be straight to install ridged steel casings (and extend the life of shaft driven pumps which are very rarely used these days). An ‘aligned’ or straight borehole is not necessarily vertical.”

Most major water supply borehole tender documents specify that the borehole to be drilled must be vertical to 1:300. In simple terms, this means that at a depth of 300 metres the borehole can not be more than one metre off vertical.

Staying within these parameters requires years of experience, genuine skill and attention to detail. Think of it this way: most people find it difficult to drill a hole with an electric drill through a piece of 4”x4” timber and come out at a pre-marked spot vertically opposite.

Not only does the task of drilling a borehole require great precision, it must also contend with the large number of variables that can be found in the ground once drilling commences. We drillers must often drill through superficial deposits consisting of clay, sand, gravel, and boulders with a big numb tricone bit before we even reach a competent bedrock in which to set our ‘perfectly vertical’ superficial casing. We then continue drilling through the bedrock.

The bedding plane of the bedrock itself often dips at a steep angle rather than lying conveniently flat. Then, after negotiating all these challenging obstacles, we must ensure that the final hole is within one metre of perfectly vertical when we reach 300-metre depth (30cm at 100-metre depth). If not, the drilling team may be required to drill the hole again at their own cost.

We have seen instances where boreholes are 20–30 metres off vertical by a depth of 200 metres when checked with verticality instrumentation – even though the driller believes the bit to be exactly plumb below the top of the borehole. Indeed, there are many instances where Drilcorp have been contracted to correct mistakes made by other drilling companies.

Start drilling as you mean to go on

It may seem obvious but if a borehole is not started vertically, there is no chance that it will be vertical when it is completed. The first 10 metres or so must be perfect if the rest of the borehole stands any chance of meeting the requirements. During this critical period, the drilling team will continually check and monitor progress.

If there are any issues, it is within the first 10–20 metres that corrections need to be made. If there is an issue with verticality at this depth then work to rectify the issue is much simpler than would be the case if drilling continued to full depth.

Note: If a borehole is started 1 degree off vertical and is drilled in a straight line for 300 metres it will be 5.23 metres off vertical at the bottom.

Deviation and misalignment

If stabilisation is not fitted behind the bit, it is highly likely that the trajectory of the borehole will deviate offline. The result is a curved (misaligned) borehole that could be 30 metres or more off vertical with depth.

A badly misaligned drill can make it difficult to ensure the accurate construction of a borehole. Placement of casings, annular grout, filter material and seals can all become a challenge when alignment is not maintained.

These days electric submersible pumps will work at any angle with little or no detriment to performance, so it could be argued that alignment is more important than verticality.

What to look for when commissioning borehole drilling

At Drilcorp we always ensure that clients are provided with a written detailed method statement for maintaining verticality and alignment before we commence any work. We take all necessary steps to demonstrate due care and attention, testing verticality and alignment throughout the construction process. Using a REFLEX EZ-TRAC survey tool we monitor progress throughout drilling. A Multi-Shot survey determines the azimuth, dip, and regional coordinates along a borehole path.

Dave Gowans

M.D. Drilcorp

To find out more about verticality and alignment services provided by Drilcorp, please get in touch via info@drilcorp.vm or by calling 0191 5273970.

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