Design and Construction

Code of Practice for Cost Effective Boreholes Principle 5: The borehole design is cost-effective, designed to last for a lifespan of 20 to 50 years, and based on the minimum specification to provide a borehole which is fit for its intended purpose.

Sub-principles

Minimum specification for “fit for purpose“ well in terms of yield, diameter, depth, casing and screen, gravel pack/formation stabiliser, verticality, drilling additive and sanitary seal. Over-design of boreholes, especially excessive depth or diameter, is wasteful and should be avoided.

The procedures for well development are agreed and clearly specified in the drilling contract. The drilled well must be developed until the water is free of solids and fine materials (fines) and any turbidity for a continuous period of 30 minutes.

The procedures for well development and for pumping tests are agreed and clearly specified in the drilling contract. Pumping test requirements for a handpump should be realistic and not over-specified.
Water quality testing for specified chemicals and microbiological content is undertaken, particularly for areas at risk and boreholes serving institutions (e.g. health centres and schools).

Discussion

A fit-for-purpose design is summarised below. In the case of motorised systems (if deemed cost-effective), design must account for the precise requirements of the system.

In terms of deliverable well yield for a handpump, 1m³/hour is sufficient, although this may be dropped to 400 litres per hour in areas where groundwater is difficult to find [1]. Wells should only be drilled to the required depth for this output. Requirements for motor pumps depend on the system design, which is based on user requirements and can be significantly higher than that of a hand pump.
Handpump borehole diameter requirements and the small diameter pumps now on the market mean that 4” (~100mm) internal diameter cased boreholes are sufficient for the handpump cylinder and rising main. Normally uPVC casing/screen is specified which has a minimum internal diameter of 103mm and an outside diameter of 113mm. Motorised boreholes may require more space to permit the installation of a submersible pump, and can be completed with a casing/pump housing of 4” or 5” nominal diameter.
Depth: Firstly, the current rest water level needs to be measured. Then the low rest water level as a result of seasonal or longer term fluctuations needs to be determined. The expected drawdown due to pumping needs to be factored in, together with the requirements for screen length and sump. This will enable the total depth to be calculated. In locations where superficial weathered formations are likely to be of a sufficient thickness, permeability and storage to support the required yield, and deal with fluctuations in the water level, the use of a relatively shallow well (screened and installed with gravel pack or formation stabiliser), constructed by a small drilling rig, or manually drilled may be cost-effective.
Plain casing and screen: The screen should be installed in the water-bearing formation and should have sufficient open area (determined by slot size and length) to allow the water to flow freely into the well. Screen length should not be compromised to save cost as it can result in a dry borehole. In locations where boreholes are drilled into stable basement formation, it is possible to make savings by casing the collapsing formation only. However, the interface between the collapsing formation and hard formation must be sealed (e.g. with grout). Plastic (uPVC) casing and screen should be used in preference to steel where wells are less than 100-120m in depth.
Gravel pack or formation stabiliser⁴ should have a proper grading with a quality of >95% silica. It needs to be installed slowly and carefully, preferably with a tremie pipe and funnel.
Verticality and alignment need to be specified as a condition of the well being denoted as successful (e.g. <100mm for every 30m). Verticality and alignment should be such that the pump and rising main can be lowered into the well without meeting any resistance.
Chemical foam and biodegradable mud should be utilised as drilling additives in preference to bentonite and other non-degradable mud. Once the drilling progresses below the water table, bentonite should not be used as a drilling mud as it tends to clog the inlet of the water bearing formation and is very difficult to remove.
Backfill of the annulus with drilling spoil is essential.
A sanitary seal of grout to a depth of at least 5m from the ground level is required. It is essential that no contaminated water (e.g. from the surface or from pit latrines) can leak into the well or the aquifer.
A concrete apron/platform is required which drains spilt water away from the borehole. Drainage ditches, as well as a fence to keep out animals are also necessary.

Download the Code of Practice full document (pdf) to see sample borehole designs for the major hydrogeological formations.

Well development must be undertaken before the drilling contractor moves to the next site. If the screen surrounding formation/gravel pack area is not properly flushed, well efficiency can drop and the screen can block over time. Wherever possible, natural well development should be utilised. Well development is best undertaken with compressed air or water jetting thoroughly applied to all of the screened areas. Surging with a bailer may also be suitable. The borehole should be flushed until it is free of fines and turbidity for a continuous period of 30 minutes. Chlorine can also be introduced before well development to help break down the drilling polymer. Once the lifted water is clear, then the amount of water being voided from the well by the air-lift should be quantified and the measured “airlifted yield” recorded.

Pumping tests are undertaken to establish well efficiency and assess the aquifer properties. Requirements appropriate for a hand pump are continuous pumping for 3 to 6 hours. Normally, the discharge rate should be at least 10% above the design discharge for a successful borehole. Recovery should be measured. National or international standards (e.g. BS ISO 1468:2003) should be used here). In the case of motorised wells, more comprehensive drawdown and recovery tests should be undertaken (e.g. step drawdown and 24-hour constant discharge pumping tests).

Water quality analysis in line with national guidelines should be undertaken and results submitted to the appropriate authority. On site testing of temperature, pH, turbidity, conductivity, colour, taste, arsenic, iron, manganese, total coliform and E. coli should be undertaken when possible.

Pump installation: The pump choice should be in line with national standards and specifications, if these exist. The pump should be positioned at the correct level above the screened section, taking into account the drawdown and seasonal variations. This is generally 2m below the lowest dynamic water level.

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.

Doyen (2003) estimates that 7% savings would be possible in Kenya if a 3-hour, rather than a 24-hour discharge and 12 hour recovery was used to test pump rural handpump wells. The high standards test pumping requirements are intended to obtain as much hydrogeological information about the aquifer in the vicinity of the borehole as possible. Doyen (2003) states that although per meter drilling costs in Kenya fell by 35% between 1988 and 1996, the increased standards for well development, pump testing and well design increased costs by as much as 36% with the result that there were no net savings.

Tanzania specifies a 24-hour pumping test (Baumann, 2005). In Nigeria, pumping tests have been matched to borehole purpose for several years. In the basement complex pumping takes 2 to 6 hours followed by 6 hours of recovery monitoring.