BEWARE “BUDGET” BOREHOLES!

Many will not be surprised to learn that the drilling industry in Zimbabwe has some unethical players. The good news for customers is that the major pitfalls can be avoided:
1. Inadequate disclosure in quotations. Many quotes do not reveal exactly what is being provided. The quote should separate the cost of drilling per metre from that of casing per metre and clearly state what casing is to be used.
2. The siter is also the driller. A driller who sites his own holes faces a number of ethical dilemmas. Firstly his revenue increases with the depth of the hole so that he may be tempted to advise the customer to drill deeper than necessary. Secondly the most convenient location for the driller (the driveway?) is not always the most favourable location geologically. Thirdly there is the temptation to advise the customer that the location is a good prospect for underground water whilst well knowing that the chances of a reliable water supply are remote. For these reasons the siter should be an independent person.
3. Casing not deep enough. Casing is expensive so that there is a considerable incentive for a driller to reduce his costs (and his quote) by using less casing than necessary. A borehole owner may not know until months or years after the event that the driller cut a corner. There is debate amongst drillers as to what constitutes an adequate depth but one thing is certain – a hole which is cased to the bottom hardly ever collapses. Pumps are very often lost because the casing was not deep enough.
4. Casing substandard. PVC casing is classified by class so that the bigger the number the stronger the casing. The industry standard for a 140mm casing is class 10. Class 6 casing has a tendency to distort or crack if the hole is drilled into expansive soil or there is later subterranean movement or there is deep sediment. This can happen during the next wet season after drilling so that a pump which was inserted shortly after drilling can never be removed. Some drillers prefer to use class 6 or even class 4 casing because it is cheaper. Sales statistics from a major casing manufacturer show that almost half the casing it is selling substandard.
5. Casing diameter is inadequate. The smaller the diameter of the hole, the smaller the casing has to be – so that it is tempting for a driller to cut costs in this way. The majority of submersible pumps used for domestic and commercial applications are known as 4” pumps which have a diameter (including the power cable) of 97 mm. Casings are marketed according to their exterior diameter so that the internal diameter of a 110mm casing is not greater than 105mm – which is a very tight fit. The internal diameter of a 125mm casing is 115mm which is also tight for a 97mm diameter pump. 140mm casing is a sensible size for a 4” pump.
6. The space (or annulus) between the sidewalls of the hole and the casing is not properly backfilled. If a borehole is cased then the water pumped will be drawn into the casing through slots which are normally 1mm wide. Particles smaller than 1 mm can be sucked into the casing and pump whereas larger particles remain trapped outside the casing. The annulus should be filled by the driller with screened +1mm river sand. This initiates the development of a filter. If this is not done and the sidewall of the hole is made up primarily of < 1mm fines, then the pump will continue to pump the fines until the hole collapses or the pump fails. The majority of drillers do not backfill the annulus with the correct product and many do not backfill at all because this is time consuming and expensive.
7. Hole unnecessarily deep. It is rare to find reliable underground water in the Greater Harare area below 75 metres. This means that most of the drilling which takes place below 80 metres is wasted drilling. There is a mythology surrounding deep holes. Your neighbour may have drilled a 150 metre hole and he may have water but is that water coming from 150 metres or from 40 metres?
8. Insufficient development of the hole. When the target drilling depth is reached the driller should develop the hole by continuing to flush out the fines generated by the drilling process using compressed air. If this is not done depth can be lost and problems may be encountered when casing. Also the hole may produce less than it’s optimum yield. A compressor uses about 50 litres of fuel per hour so a driller may be very tempted to stop drilling as soon as the target depth is reached.
9. No recourse in the event of collapse. The depth of the hole should be measured on completion of the casing and backfilling by the driller in the presence of the customer or his representative. This should be done using a length of pipe (suspended on a calibrated rope) which is slightly larger in diameter than a submersible pump. It often happens that the casing is not inserted to the depth originally drilled because the hole was collapsing during casing. In that event the depth of the hole is the depth of the casing. An ethical driller will refund any portion of the deposit paid for drilling beyond the final measured depth of the hole using the method described.
10. No driller’s report. On completion of drilling, a driller is required to prepare a report in the prescribed form and submit it to the customer and the Catchment Council. This very detailed form contains information which is essential to the pump installer and very useful to the customer. Many drillers prefer not to produce a report because it should contain information which they would prefer not to share. Before accepting a quote, a customer should request confirmation that the driller will provide a report on completion.
– RICHARD BRIDGES – GET WET –

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