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Note: in this Atlas, the word borehole is generally used to mean a drilled hole (by manual or mechanical rig), and the word well is generally used to mean a dug hole. Elsewhere, the words well is often used interchangeably with borehole to also mean a drilled hole.
There are two main approaches to drilling boreholes : drilling with a mechanised rig; and manual drilling. The most appropriate technique will depend on the hydrogeology, the required yield, and available funds.
An introduction to borehole drilling techniques that are appropriate for rural water supply can be found in the chapter Designing and constructing water points in MacDonald et al. (2001), which can be freely downloaded online.
Drilling with a rig
Most boreholes are drilled using a motorised drilling rig. There are different types of drilling rig and methods of drilling, and these should be chosen to suit the local hydrogeology. The main types are cable tool percussion (also known as shell and auger), and rotary drilling. Rotary drilling can be air flush, sometimes with down-the-hole hammer; mud flush; or reverse circulation.
Manual drilling is an approach that is appropriate in some hydrogeological environments, particularly in shallow unconsolidated aquifers with shallow water tables. It can reduce drilling costs and increase cost-effectiveness of groundwater development programmes. Manual drilling methods are being used to provide water for drinking and other domestic needs in at least 36 countries around the world, and in some places are already well established.
UNICEF has worked with a range of partners to develop a toolkit for African countries wishing to embark on the professionalisation of manual drilling. This toolkit includes technical notes and technical manuals, advocacy materials, case studies, and implementation and training manuals for manual drilling. There is also a series of mapsshowing areas suitable for manual drilling in 12 countries in West Africa, and a report on the mapping methodologies used.
The Rural Water Supply Network (RWSN) has produced a Manual Drilling Compendium, which provides a useful overview of the impacts and challenges of manual drilling, and support for improving practices on the ground.
Collecting Data during Drilling
Whichever drilling technique is used, collecting data during drilling is a very important part of successful groundwater development. Drilling is usually the only opportunity to look below the ground and find out what the geology and hydrogeology is at depth, where it is usually hidden. One of the main aims is to identify groundwater-production zones at depth in the geological sequence – at what depths is groundwater found? Data on the local geology is also invaluable for developing understanding.
A good introduction to what data to collect during drilling, and how best to collect it, is in the chapter Designing and constructing water points in MacDonald et al. (2001), which can be freely downloaded online.
The activities in the table below are a summary of good practice in data collection during water borehole drilling:
|Activity||Why do this?|
|A field logbook or notebook should be kept, with notes of all drilling activities and what data are collected||Recording all information while in the field means that nothing is forgotten. A detailed field notebook is an invaluable record in case any details need to be checked later.|
|The borehole should be flushed clear of cuttings at the end of every sampling interval (e.g. every 1 m or every 3 m; or every drill rod) and an accurate sample of drill (rock) cuttings/chippings collected for observation.||Rock samples collected at depth during drilling provide a record of the geology and how it changes with depth. The geology is the main control on the groundwater potential.|
|Rock chip samples from the borehole should be washed and described consistently: e.g. rock type, colour, texture. In this way, a geological log is built up showing changes in geology with depth.||This allows people to assess where the best groundwater potential is with depth in the borehole, and therefore make the best decisions on the final borehole construction; and also to estimate how successful the borehole is likely to be. It also provides hugely valuable hydrogeological information to allow a wider assessment of groundwater resources beyond just one borehole.|
|Information should be recorded in the field logbook on: the penetration rate of drilling (how long it takes to drill a given interval, e.g. every 1 m or every drill rod); breaks or irregularities in drilling observed or reported by the driller; water strikes and/or flows; and dust production.||This information supports the description of geology in the geological log and helps improve understanding of the geology and the groundwater potential of the rocks.|
|If possible, water conductivity (SEC) should be measured at regular intervals/depths during drilling, to observe any changes.||Water conductivity indicates how mineralised a water is. If water conductivity changes suddenly, it can indicate that the borehole has entered a different aquifer with depth.|
|An initial estimate of potential borehole yield should be made during airlifting (cleaning the borehole) at the end of drilling.||This can help assess if the borehole is likely to be productive enough to be worthwhile installing screen and casing, and what size of pump to use for a pumping test.|
Professional Water Borehole Drilling
For successful and sustainable groundwater development, it is very important that borehole drilling is done in a professional way. Collecting data during drilling, as summarised above, is part of this, but it is a much bigger issue. The Rural Water Supply Network (RWSN) does much work to promote professional water borehole drilling. On its website you can view and freely download RWSN's resources, which have been used to improve projects and have been incorporated into the curricula of some academic and training organisations.
Some of the specific resources produced by RWSN to support professional water borehole drilling are:
- - A guidance note on Professional Water Well Drilling, available in English and in French / en français File:Flag of france.png. This is written for people who manage or support water borehole drilling programmes and projects, and provides guidance on project design, implementation and monitoring; groundwater information; capacity; institutional frameworks; investment; and dialogue and awareness.
- - A series of short animated films, in English and in French / en français. These films explain borehole siting, drilling supervision, drilling contract management and why some boreholes are better than others.
- - National assessments of the water borehole drilling sector (its cost-effectiveness and professionalism) for a number of countries, including at least 14 African countries.
- - A professional drilling management online training course, providing an introduction to the professional management of water borehole drilling projects and programmes.