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GlossaryUnconsolidatedPoorly cemented or not at all (in reference tosediments).WadiA stream that fills up after rainfall, but which isusually dry the rest of the time.WeatheredEarth rocks, soils and their mineral content whichunderwent decomposition by direct contact withthe planet's atmosphere, water, light, frost andheat.112Adapted from “Geology dictionary” at http://geology.comEntry on “fracture (geology)” from Wikipedia at www.wikipedia.orgBoxes with this formattinghighlight experiencesfrom the field or practicalsuggestions.Boxes with thisformatting containinformation critical forsuccessful operationsor for the safety of thestaff.11ICRC Borehole Technical Review.indd 1119/02/2010 17:14

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1 Introduction andexecutive summaryIntroduction andexecutive summary1The International Committee of the Red Cross (ICRC) is animpartial, neutral, and independent humanitarian organization.Its mission is to protect the lives and dignity of victims of warand internal conflict and to provide them with assistance.Through its Water and Habitat unit, the ICRC provides waterand sanitation in dozens of countries and conflict zones aroundthe world, meeting the needs of millions of people. TheWater and Habitat unit has drilled or rehabilitated hundreds ofboreholes, sometimes employing contractors and sometimestheir own machines.This technical review is aimed at project coordinators, waterengineers, and technicians. It is intended to be of assistanceto everyone, from planners in offices to on-site personnel, inthe making of technically correct and cost-effective decisionsin the field when the drilling or rehabilitation of boreholes isrequired. An attempt has been made to orient the contentstowards problems that might be encountered in the field.Nevertheless, some consideration of theoretical information hasbeen necessary, because engineers will not be able to functionwithout it. The authors hope that they have struck a balancebetween the practical and the theoretical, a combination that isrequired in professional water engineers.13ICRC Borehole Technical Review.indd 1319/02/2010 17:14

TECHNICAL REVIEWThe review begins with an overview of the benefits of utilizinggroundwater and a consideration of various drilling methods,in Sections 2 to 4. Techniques are compared and details of thedrilling equipment associated with each are provided to assistthe user in selecting appropriate equipment.The review focuses on mud and air rotary drilling, as theyare the most common methods of borehole drilling foundin the field. Details on borehole construction, design anddevelopment using these two methods are found in Sections5 and 6. Construction costs are considered in Section 7. Keyfactors influencing borehole deterioration and aspects ofmonitoring and maintenance are outlined in Sections 8and 9. When borehole deterioration reaches a stage whereproduction is severely hampered, rehabilitation becomesunavoidable: this subject is treated in Section 10. Finally,Section 11 deals with issues that might arise while working withcontractors and with minimizing the unpredictability of thataspect of drilling.14ICRC Borehole Technical Review.indd 1419/02/2010 17:14

2 Groundwater and theadvantages of boreholesGroundwaterand the advantagesof boreholes2Easy access to safe, potable water is a basic human need,important for health and quality of life. A statement like thisis regarded now as being something of a cliché. However, itmust be said that even with the growing prevalence of watershortages throughout the world, a reliable water supply is stilltaken for granted, with no real thought about its sustainabilityand quality. This attitude is most starkly evident in areaswhere there is a reliance on water from boreholes, which,it is assumed, will keep on producing – at the same rate –continuously and forever. Groundwater is out of sight, andhence, largely out of mind, but it is one of the best sources ofwater that man has been able to utilize.2.1Exploiting groundwaterThe principal source of inland groundwater is rainfall. Aproportion of rain falling on the ground will percolatedownwards into an aquifer if the conditions are right. A greatdeal of rain water ends up as run-off in streams and rivers, buteven here there is often a direct hydraulic connection with alocal aquifer. Indeed, in arid areas with ephemeral streams, highgroundwater levels may be able to sustain surface flow alongdrainages.15ICRC Borehole Technical Review.indd 1519/02/2010 17:14

TECHNICAL REVIEWIf water is not flowingvisibly along a drywadi, it may be movingunseen slowly throughthe sediment, and canbe accessed by digginga well in the riverbed– a fact well known tomany elephants.It is obvious that a hole dug or bored into a saturated ‘sponge’will release water from storage. This water can be sucked orpumped out, and all being well, more water will enter the holeto replace that which has been withdrawn. This is the basicprinciple behind a water borehole.2.1.1Geological constraintsThe Earth’s crust has often been compared to a sponge, in thatit can soak up and hold water in pore spaces, fractures andcavities. This ability to store water depends very much upongeological conditions and on the host formation. For example,fresh, unfractured, massive granite – a crystalline rock – hasvirtually no space available for water, whereas unconsolidated,or loose, river gravel and highly weathered cavernous limestonecan store large quantities of groundwater and are capable ofreleasing it relatively freely. Sandstone and mudstone may beable to hold significant groundwater resources, but becauseof differences in grain size – and hence porosity – will release itat different rates. One may be a good aquifer, the other a poorone. The rate at which water flows through a formation dependson the permeability of that formation, which is determinedby the size of pores and voids and the degree to which theyare interconnected. Permeability and porosity should notbe confused, porosity being the ratio between the volumeof pores/voids to the bulk volume of rock (usually expressedas a percentage). Table 1 provides a range of porosities andpermeabilities for common soil profiles.16ICRC Borehole Technical Review.indd 1619/02/2010 17:14

2 Groundwater and theadvantages of boreholesTable 1: Typical porosities and permeabilities for various materials(various references)Porosity (%)Permeability (m/day)ClaySilt/Fine sandMedium sandLithology4243-463910-8-10-210-1-55-20Coarse sandGravelSandstoneCarbonate (limestone, dolomite)Fractured/Weathered rockVolcanics, (e.g. basalt, rhyolite)Igneous rocks (e.g. granite, 00010-3-110-2-10-3000-1000 10-5The three principal characteristics of aquifers are transmissivity,storage coefficient, and storativity. Transmissivity is a meansof expressing permeability, the rate at which water can flowthrough the aquifer fabric. Storage coefficient and storativityexpress the volume of water that can be released from anaquifer.Hydrogeology is the science of groundwater, and it is thejob of a hydrogeologist to assess the groundwater resourcesin any given area. This is accomplished through the useof maps (topographic, geological), satellite images, aerialphotos, field observations (geological mapping, vegetationsurveys, etc.), desk studies (literature, field reports, etc.) andground geophysics. Ground geophysical surveys are nowquite effective in locating water-bearing formations at depthsdown to around 100 metres. Methods include resistivity(vertical electrical profiling), natural-source self-potential andelectromagnetic methods (such as VLF), magnetic methods,and micro-gravity surveying.17ICRC Borehole Technical Review.indd 1719/02/2010 17:14

TECHNICAL REVIEW2.1.2Borehole sitingChoosing a borehole site is a critical part of the process ofproviding a safe and reliable supply of groundwater. Thebest sites are those in which catchment (natural water input)may be maximized. Such locations are not necessarily thosethat receive the highest rainfall (which may occur in uplandwatersheds). ‘Bottomlands’ – such as river valleys and lakebasins – tend to be areas of maximum catchment as bothsurface water and groundwater migrate towards them undergravity. Fracture zones, although not always directly relatedto bottomland, can also be good reservoirs for groundwater,and may be located by ground observation or satellite images/aerial photographs, and by geophysical methods.Another aspect of borehole siting that demands carefulconsideration in populated areas is the potential forcontamination by cattle and pit latrines or other wastedisposal facilities. Because near-surface groundwater migratesdownslope, a shallow dug well or a borehole tapping shallowgroundwater should be sited as far away as possible (whilebearing in mind the human need for proximity to a source ofwater) and upslope of potential sources of pollution (latrinesor sewage pipes, for instance). Deeper aquifers confined byimpermeable layers are at less risk of contamination fromsurface pollutants. One final consideration is the nature ofthe shallow aquifer. If the host formation is made of fine ormedium-grain-sized sand, it will act as a natural filter forparticulate pollutants, whereas fissured limestone, with ahigh rate of water transmission (transmissivity) will carry awaypollutants faster and to greater distances from the source. It isestimated as a rule of thumb that most microorganisms do notsurvive more than 10 days of transportation by undergroundwater.18ICRC Borehole Technical Review.indd 1819/02/2010 17:14

2 Groundwater and theadvantages of boreholes2.1.3Types of geological formationFigure 1 shows a hypothetical geological situation in whichdifferent sources of groundwater may (or may not) be tappedby dug wells or boreholes (also called ‘tube wells’).A) Perched aquifersAt site A, a shallow dug well may provide a little waterfrom a ‘perched’ aquifer in the weathered zone aboverelatively impervious (low porosity) mudstones. Ifthis well was extended into the mudstones it mightproduce very little additional water. A perched aquiferis normally limited in size and lies on an imperviouslayer higher than the area’s general water table.Figure 1: A hypothetical hydrogeological scenarioWeathered zoneEABLow porositymudstonesCRiver FissuredlimestoneMetamorphic basement19ICRC Borehole Technical Review.indd 1919/02/2010 17:14

TECHNICAL REVIEWB) Shallow unconfined aquifersThe term ‘unconfined’ refers to an aquifer within whichthe water is open to atmospheric pressure: the socalled piezometric surface (pressure head level) is thesame as the static water level (SWL) in the borehole. Atsite B, a borehole extracts water from an unconfinedsandstone aquifer, the SWL of which is somewhatlower than the level of flow in the river. This sandstoneaquifer is in a good catchment area because ofrecharge from the river.C) Confined aquifersA ‘confined’ aquifer may hold groundwater undergreater pressure, so that when punctured by aborehole, the SWL rises to the higher piezometric level.If the piezometric surface happens to be above groundlevel (which is not uncommon), water will flow out ofthe borehole by itself: this is known as ‘artesian water.’Deep borehole C intersects the sandstone aquiferand a deeper confined aquifer in fissured limestone;because of overpressure in the limestone aquifer, theSWL in C may be at the same or higher elevation thanin B. The limestone aquifer may have no source ofreplenishment; so the water in it is ancient, or ‘fossil,’and could be exhausted if over-exploited.D) Fracture zoneBorehole D, which has been drilled into fracturedgranite (shaded area), finds water held in the fracturezone. Fracture zones develop during geological timesas a result of the severe mechanical stress, caused bytectonic movements, that is exerted on non-plasticformations.20ICRC Borehole Technical Review.indd 2019/02/2010 17:14

2 Groundwater and theadvantages of boreholesE) Hydrogeological basementSite E, a borehole sunk into massive granite on top of ahill, is dry. In this situation, it would be a waste of timeand money to extend a deep borehole (such as C) intothe metamorphic basement, which is generally knownas the ‘hydrogeological basement’ or ‘bedrock.’ Thebedrock marks the level below which groundwater isnot likely to be found.2.2Groundwater extractionA water borehole is not just a hole in the ground. It has to beproperly designed, professionally constructed and carefullydrilled. Boreholes for extracting water consist essentially of avertically drilled hole (inclined and horizontal boreholes arerare and will not be discussed here), a strong lining to preventcollapse of the walls, which includes a means of allowing cleanwater to enter the borehole space (screen), surface protection,and a means of extracting water. Drilling by machine isan expensive process, and boreholes require professionalexpertise for both their design and their construction. Thereare, however, compensations: this method of extracting waterhas a number of significant advantages.The common alternatives to drilled boreholes, availableto everyone with basic knowledge and simple tools, aresurface water sources, springs, and dug wells. Where shallowgroundwater emerges at a seepage site or at a spring, watercatchment systems can be constructed to provide waterof reasonable quality. Catchment boxes, that include sandor stone filters, and collector sumps are extremely effectivemeans of collecting water. Gravity may be used to effect pipenetwork distribution from upland springs. Shallow dug wellsusually exploit near-surface groundwater. Wells down to adepth of five metres are relatively simple to construct (giventime and willing local labour), and there are many publications21ICRC Borehole Technical Review.indd 2119/02/2010 17:14

TECHNICAL REVIEWdescribing this process. Furthermore, because of their relativelylarge diameter, wells provide valuable storage volume. Thewater supply can be protected by lining the well, covering itwith a lid, and fitting a hand-pump to it. .However, dug wells, and surface water catchment in general,are very vulnerable to contamination caused by agriculturalactivities, animals, poor sanitation and refuse. In addition,surface or shallow groundwater catchment is vulnerable topoor rainfall and declines in water level caused by drought,because it usually taps the top of the aquifer. Borehole water,by contrast, often requires no treatment and is less susceptibleto drops in water level during periods of drought or limitedrainfall.2.2.1Advantages of drilled boreholesIf they are properly designed and maintained, drilled boreholes: Are less vulnerable to drought or drops in water level whendrilled into deep water-bearing formations Can be designed to exploit more than one aquifer (whenindividual aquifers are vertically separated and nothydraulically connected) Are less vulnerable to collapse Are less vulnerable to contamination Are, if properly sited, capable of producing large yields; so,mechanically or electrically powered pumps can be used Are amenable to quantitative monitoring and testing, whichenables accurate assessment of aquifer parameters (as inaquifer modelling), water supply efficiency, and optimaldesign of pump and storage/distribution systems Can be used to monitor groundwater levels for otherpurposes, e.g. environmental studies or waste disposal22ICRC Borehole Technical Review.indd 2219/02/2010 17:14

2 Groundwater and theadvantages of boreholes2.2.2 Disadvantages of drilled boreholes High initial material costs and input of specialized expertise,i.e. construction, operation, and maintenance may requireskills and expensive heavy equipment Vulnerable to irreversible natural deterioration ifinadequately monitored and maintained Vulnerable to sabotage, can be irreparably destroyed withlittle effort if inadequately protected Require a source of energy if water extraction pumps areused (unlike gravity feed systems) Do not allow direct access, for maintenance or repairs, toconstructed parts that are underground23ICRC Borehole Technical Review.indd 2319/02/2010 17:14

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3 Methods of drilling boreholesMethods ofdrilling boreholes3Once a suitable site has been selected and borehole drillingdecided on, the proper drilling method must be chosen.Another primary consideration in project planning is theavailability of existing water sources and water points. Theremay be completed dug wells and boreholes already in the area.Are they in use? If not, can they be rehabilitated to augmentwater availability or to reduce the cost of the programme?Drilling equipment, such as compressors, can be used to bringdisused boreholes back into use; the question of rehabilitationwill be addressed in Section 10 of this review. This sectionoutlines the factors that must be considered when choosing adrilling method.25ICRC Borehole Technical Review.indd 2519/02/2010 17:14

TECHNICAL REVIEW3.1Common drilling methodsEssentially, a drilling machine consists of a mast from whichthe drilling string components (tools plus drill pipes or cable)are suspended and, in most cases, driven. Modern systemsare powered rotary-driven, but it is probably worth a shortdigression to describe some methods of manual drilling forwater. Simple, low-cost methods include:A) Hand-auger drillingAuger drills, which are rotated by hand, cut into the soilwith blades and pass the cut material up a continuousscrew or into a ‘bucket’ (bucket auger). Excavatedmaterial must be removed and the augering continueduntil the required depth has been reached. Augerdrilling by hand is slow and limited to a depth of about10 metres (maximum 20 metres) in unconsolidateddeposits (not coarser than sand, but it is a cheap andsimple process.B) JettingA method whereby water is pumped down a stringof rods from which it emerge

exceeding the rock strength. 2: Glossary: 9: ICRC Borehole Technical Review.indd 9 19/02/2010 17:14: Groundwater: Water that exists below the water table in the zone of saturation. Groundwater moves slowly, and : follows the water table's slope. 1: Igneous: Formed by the crystallization of magma or lava.