The frustrations of looking down a deep dark borehole.

You have just visited a water point, you have surveyed 3 in the morning, checking the condition of the rope pump, depth, and condition of guide block (by pulling it up from the bottom),  you have 3 more to do that afternoon.  Everything seems fine up on top and you start removing the guide block, it becomes stuck.   You cant move it up or down, you are looking down a 28m deep 100mm diameter hole in the ground.  What is wrong? How do we fix it?

Looking down a problem borehole, with rising main and rope.

In the last month we have had two cases of apparent borehole collapse, our submersible camera is not functioning so, from our past experience, we have carried out optioneering to both establish why the borehole collapse occurred and now our potential repair work.

There are numerous potential causes of borehole casing collapse, including:
  •           Insufficient casing strength to resist earth and groundwater pressures.
  •           Poor PVC material Quality
  •           Gravel Pack Collapsing
  •           Ground formation shifting
  •           Poor casing placement – including jointing, filter and bending
Having reviewed our borehole logs it appears that the collapses are likely to have occurred where filter screen sections were cut in sections of Class B PVC whereas usually these section are only cut in stronger Class C.  Filter screen sections have small slots to allow water to flow into the borehole,.  This shows the importance of close supervision, clear communication and on going training.

Given the above we will over the coming months be  reviewing the PVC quality we are procuring, check stores are being rotated properly/out of sunlight and having training sessions with drillers focussing on keeping basic skills including casing socket creation and filter screen provision.  Also having all the drillers together will let us gather and share their valuable lessons learnt.

Unfortunately once a casing has collapsed below ground there is not much to help.  There is a lot of friction on the pipe which makes extraction difficult.  We will first attempt to do this, by flushing the borehole with water (and possibly soap!) to reduce this friction.  If this fails we will attempt  to break the collapsed area, bail out materials and reline with a smaller casing, this will have a lower capacity but still provide sufficient water.  

If you have any queries or recommendations on the above please do contact us.

Kiran Gowda and Hashim Mtoi