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Construction Method

Principle 3: The construction method chosen for the borehole is the most economical, considering the design and available techniques in-country. Drilling technology needs to match the borehole design.

Sub-Principles

  • Well depths should not be unnecessarily over-specified or under-specified.
  • A stepped approach to technology selection should be followed. Very low-cost methods, including protected hand dug wells and manual drilling, are considered first, if they are feasible, before mechanised drilling.
  • Subsequently, the use of small rigs, which provide the specified diameter and well depth and reach remote locations, should be considered.
  • Finally, the use of larger drilling rigs should be considered.

Discussion

It is important that well designs are properly specified. Stakeholders should avoid over-specifying depths and diameters on a ‘just in case’ basis as this then over-specifies the drilling rig required. This results in the mobilisation of excess equipment and raises costs. However, under-specification is also a problem. It can sometimes result in a failed borehole or in the need to mobilise a different drilling rig. Past experiences should be used to help determine the design by comparing previous contract specifications with actual drilled depths.

Tender and contract documents should enable the least expensive, but suitable drilling equipment to compete against larger, more expensive rigs. Tenders should specify the final product (i.e. the drilled water well) and thus avoid over-specifying the drilling equipment. Small, low-cost, mechanised rigs, which drill at lower cost than large rigs, can often be transported on the back of a four-wheel drive pickup or single axle trailer and can reach more remote locations, particularly where road networks are poor. If stakeholders do not know about small mechanised drilling technologies, explicit efforts should be made to raise their awareness.

It should however be noted that drilling with a smaller rig may be slower, and will thus require longer supervision. This needs to be fully considered when planning contract management and supervision requirements.

Hand-dug and manually drilled wells are an option in specific environments (soft formation and shallow groundwater). In areas where such techniques can provide boreholes in significant numbers, they should be fully considered. However, borehole verticality needs to be good enough to enable the installation and operation of the specified pump, and wells need to be deep enough to sustain supplies in prolonged dry periods and successive dry years. Therefore the use of such methods needs to incorporate an appropriate level of supervision and quality control.

Dissemination of information, exchange visits, pilot projects, support to the local private sector, and in-country studies may be necessary in areas where hand dug wells or manual drilling techniques are not well known or not commonly used.

The conventional mechanised drilling techniques to construct drilled water wells suitable for rural and peri-urban water supply projects are rotary drilling with mud-circulation, cable tool percussion drilling, and down-the-hole (DTH) air-percussion drilling. Combinations of these drilling techniques can also be used depending on the geology (e.g. mud-circulation rotary drilling through collapsible over-burden to hard rock, then air percussion drilling). Selection of the most suitable equipment depends on the depth of the well and the lifting capacity of the rig to pull out drilling tools and temporary casings, too. The full Code of Practice document (pdf) provides guidance with respect to drilling method selection.

References