Solutions for Reducing Borehole Costs in Rural Africa - International ...

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Figure 4: Hydro-geological domainsof Sub-Saharan AfricaPreCambrian"basement" rocksVolcanic rocksUnconsolidated sediments(mainly Quaternary)Consolidated sedimentaryrocks (post PreCambrian)Lakessome 220 million people in rural Africa.Groundwater is mostly stored in theweathered overburden and feeds intofractures in the underlying rock. A goodwell site therefore needs to be locatedon an adequate depth of weatheredground. There must also be a goodchance of accessing a fractured layer inthe underlying rock. In some locations,weathering is deep and rock is heavilyfractured, making successful drillingeasy. In other places, the weatheringcan be thin and the rock fractureshard to locate, leading to very poorsuccess rates. Geophysics can helpto site boreholes by indicating areas ofdeep weathering and likely fractures,but this should be done in conjunctionwith a good common-sense appraisalof the site and location. Drilling to hitdeep weathering and shallow fracturesis the key: simply drilling deep is not asolution.Volcanic rocksIt is hard to generalize on drillingspecifications for volcanic rocks– on which some 45 million Africansare settled – as they often occurin mountainous areas. Generally,groundwater would be expectedbetween lava flows or in lava thatis heavily faulted or even porous.Sometimes water is located deepunderground and the water qualitycan be poor. Skilled geologicalassessment, possibly assisted withsome geophysics, can help to locategood drill sites.Consolidatedsedimentary rocksSome 110 million people in Africaoccupy areas of consolidatedsedimentary rocks which includelimestone and sandstone, and containwater in a mixture of circumstances. Itis more important to first understandthe general geology of an area to knowthe depth horizon at which water willbe located than to select a drilling siteon the basis of fractures. In this class ofrock types there are rocks and mixturesthat do not readily store groundwater,and in these cases more detailedinvestigations involving geophysics canassist.Unconsolidated sedimentsSome 60 million rural Africans liveon unconsolidated sediments wherewater will be located in layers of sandsor gravels. When unconsolidatedsediments cover a large area, it is3(Driscoll 1986).consistently easy to drill good sitesrepeatedly by drilling deep enoughinto the sand or gravel layer. Thereis also a useful range of sites basedon very localized sediments (in smallriver valleys, for example), particularlyin basement rock areas, as thesecan store appreciable amounts ofgroundwater from relatively shallowsediment layers.What iscurrent technologyoffering Africa?Borehole diameterThe DTH hammer has clearly carveda niche for itself in quarrying, andthere are numerous benefits for thistechnology in tapping groundwatersupplies. In quarrying, operators havelearned that - where fuel and capitalequipment are expensive to obtain andmaintain they can reduce operationalcosts by reducing the diameter oftheir blast holes and by improving theperformance of their explosive charges(Box 2).In setting a suitable bore diameter fora water well, the first principles of welldesign should be followed. Basically,adequate room is required to installthe water pump and to supply it witha flow that matches its maximumoutput. The India MKII technology,now in its third decade of operation,requires a minimum diameter of 4”(10.2 centimeters) for insertion. NewMKIII cylinders combined with theAfridev handpump will fit in boreholes6
Solutions for ReducingBorehole Costs in Rural AfricaBox 2. Borehole Efficiency and CostsA borehole is defined by its diameter and depth, calculated as volume. Thelarger the volume, the more the construction work. The surface area of a 4”diameter hole is close to half that of a 6” diameter hole. When multiplied byits depth, this will equate to a 4” hole being half the volume of a 6” hole.existing practice and would lead to thedisappearance of high-cost equipmentand high-value contracts. A realignmentof existing markets would ensue andequipment that is too large and costly tooperate would fall into disuse.Hard rock drilling spoil is cleared out by blowing compressed air up theborehole in the space between the drill pipe and drilled hole (annular area).The compressed air must travel above a certain speed (drillers refer to thisas ‘up-hole velocity’) to achieve sufficient lift of the spoil. This means thatthere is a direct ratio between the compressed air flow and the boreholediameter (a 6” hole requires twice the volume of compressed air than a 4”hole).The same ratio applies to the direct cost of constructing a 4” diameterhole which should be half the cost of a 6” diameter hole when the properselection of plant is made. This is because a 4” hole is half the work of a 6”hole. The drilled diameter has little effect on the volume of water availablefrom the borehole. It has been established that every time a boreholediameter is doubled, the available water inflow will increase only by 10percent. (Driscoll 1986).with a diameter of less than 4”, and it isfeasible to make installations for 3” (7.6-centimeter) holes. 3TraditionThe DTH hammer is still relatively newtechnology in a notoriously conservativeindustry. Without a DTH hammer,there are two options for penetratingrock. The first is to use a tricone bitwith heavy drill collars and torque.But to get enough weight of steel intoa hole in a short length, the optimumdiameter would have to be at least 6”(15.2 centimetres). Alternatively, a cabletool with a chisel and sinker bars couldbe used. This set up calls for a similarcritical mass and also leads to drill bitsof 6” or larger. Prior to the introductionof DTH technology, it was easier andcheaper to drill boreholes at 6” diameteror above than it was to drill smallerdiameters.Drilling regulations and standards,worldwide, are based on the applicationof these now superseded technologies.With DTH technology, however, it iseasier and cheaper to drill smallerdiameters. A DTH hammer will producethe lowest cost per meter in hard rockby a wide margin. An important provisois that the production rate has to bekept high to amortize the high capitalcosts of the equipment (Table 1).Vested interestsClearly, the adoption of alternativeborehole designs constructed withsmaller equipment departs fromWhile such a scenario would bebeneficial in terms of service coverage,it would clearly not go unopposed.Radical realignment of the businessenvironment in favor of smaller boreswould oblige established rig ownersand contractors to make a starkchoice – adapt to the change, orgo out of business. New investorswould be presented with considerableopportunities to win contracts, requiringa fraction of the resources currentlyneeded. With smaller bore diameters,the establishment of rural-based Africancontractors who could drill directly fortheir own communities would becomean attainable goal.The diameter of the DTH hammerdefines the drill-pipe diameter,compressor capacity, engine power,and rig size, weight and transportationmethod. Basically, the diameter ofthe borehole determines whether thecompressor is towed by a small pick-upor mounted on a four-wheel-drive truck,and whether the rig package costsUSD$60,000 or USD$150,000.By adopting new borehole designs thatspecify smaller holes, large equipmentis effectively made economicallyinefficient even though a small-diameterdrill pipe can still be used on a large rig.The increased use of commercial drillingcontractors would lead to construction7
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