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rwsn » Flagships » Cost-effective Boreholes » Key Principles » Borehole Designs

Borehole Designs

Boreholes should be designed and constructed to fit their purpose in terms of diameter, depth, casing and screen.

What do we know?

Many rural handpump boreholes are being constructed to give high yields, and are forced to conform to higher standards that are appropriate for boreholes in more densely populated areas. 

Drilled wells should be designed so that they are fit for their intended purpose, meaning that their depth, diameter, lining and backfill materials, screen open area and other design features are well-matched to need (ie water demand, longevity, hydraulic efficiency and cost).  Differentiating between different magnitudes of abstraction requirements is particularly important. 

Handpump boreholes diameter requirements and the small diameter submersible pumps that are now on the market mean that 4” (102mm) internal diameter boreholes are sufficient.  Country specifics are as foollows:

  • In Tanzania the internal diameter for deep and shallow wells are specified at 150mm and 117mm respectively. 
  • In Mozambique 4” casing is installed. 
  • In Uganda 4-5” casing is specified. 
  • 6” casing is used in Ethiopia. 
  • Malawi specifies the installation of 110 mm casing. 
  • Final drilling diameters in Burkina Faso and Senegal are 8” and 12” respectively. 
  • In Nigeria, there are five different borehole designs depending on the terrain and expected yield. 

In countries where boreholes are drilled into stable basement formation, it is possible to make savings by casing the collapsing formation only, grouting at the joint to the hard formation only and not casing the hole drilled into the basement.  This is the policy and practice in Uganda.  In Tanzania, all boreholes are fully cased and gravel packed, although Baumann et al (2005) state that the specifications are not very precise.   A study in Malawi (Mthunzi, 2004) of 60 PC and 23 fully cased (FC) boreholes found that 73% of the PC boreholes had no depth reduction over 4-6 years and that 5% of boreholes showed an increase exceeding 5% of datum depth.  Borehole yields were comparable for both types. 

In Kenya, drillers lobbied Government for six years to relax the drilling specifications and thus drilling and rig costs but did not succeed.  Plans to upgrade these sources to motorised pumps with small piped distribution systems may explain this but, such forward thinking may be too advanced for the needs of rural people today and, even if the well yields are sufficient, the water resources may not be.

Drilling beyond the optimum yield depth is common in Ethiopia (Carter et al, 2006) and Kenya (Doyen, 2003), which raises costs significantly.  In order to avoid this, there is need for close on-site supervision, with the supervisor having the confidence and authority to decide when depth is sufficient.  It is envisaged that the increased cost of better supervision would ultimately be offset by reduced drilling costs and improved construction quality.

References