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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 Assessing the yield of a source 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, drilling is a crucial time for collecting groundwater data. A very important part of successful groundwater development is collecting data during drilling. 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.
The following list is a summary of activities that should be carried out for good practice in drilling data collection:
- a logbook should be kept with notes of drilling activities and what data are collected.
- the borehole should be flushed clear of cuttings at the end of every sampling interval (e.g. every 1 m or every drill rod) and an accurate sample of drill (rock) cuttings/chippings collected for observation.
- rock chip samples should be washed and logged (described)consistently: e.g. rock type, colour, texture,
- information should be recorded in the 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.
- if possible, water conductivity (SEC) should be measured at regular intervals/depths during drilling, to observe any changes.
- an initial estimate of potential borehole yield should be made during airlifting (removing rock chippings) 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 (see next section).