Drilled Wells

Throughout the world, hundreds of thousands of hand- (or manually) drilled wells provide rural dwellers with water for agriculture, industry and domestic use. Hand drilling of water wells for domestic and productive use is very common throughout most of Bangladesh and along the Ganges plains of India. In parts of Latin America and Africa, hand-drilling technologies have been introduced more recently. In some cases, such as in Niger and Nigeria, hand drilling has become fairly well used (DANERT 2009). Despite the tremendous potential of hand drilling technologies to provide water at low cost, they are not well known by many WASH sector professionals, and in some cases are not considered to be acceptable (DANERT 2009). Hand drilling is also attractive as it can be undertaken by small, local enterprises.

Besides the manual construction, wells can also be drilled mechanically. Drilling equipment range from rather simple up to very sophisticated and powerful machines capable of drilling more than 200 metres in depth (SMET & WIJK 2002). 

Particularly in regions with limited access to construction material, equipment, financial means and knowledge, machine drilling only plays a minor role as it neither shortens time for construction nor provides low-cost solutions.

The basic principle of drilled wells is simple: a hole is drilled into a groundwater body and infiltrating water is then abstracted with the help of a manual or motorised pump. Compared to dug wells, diameters of drilled wells are much smaller: In a rural context, wells are usually 50mm in diameter, while high abstraction urban supplies require holes with up to 300mm in diameter (DANERT 2009; SMET & WIJK 2002). In terms of length, manual drilled wells technically range up to 200 metres, although, a maximum length of 35 metres for hand-drilled wells is more common. For wells ranging deeper than 50 metres, manual drilling is generally no longer an option making mechanical drilling necessary. 

The performance of the well in terms of quantity depends on its depth and diameter, the recharge capacity of the aquifer and the development of the well. As groundwater is usually of drinking water quality, purification is often not required. Yet, drilled wells are susceptible for contamination unless properly operated, maintained and protected.

Also, strict attention needs to be paid to the abstraction rate. Overdraft can give rise to various effects on the environment:

• The water table can be lowered beyond the reach of existing wells
• Wetland can dry up
• The loss of hydraulic pressure can cause collapse of the ground and formation of craters making agricultural use impossible
• Seawater intrusion may occur if the well is located in coastal areas.

Initial Assessment

Before starting to design and develop any kind of water sources, an initial assessment should be conducted in order to effectively and efficiently manage and develop water resources. This assessment includes a water balance estimation (i.e. the collection of hydrological data, determination of the current status and trends of water supply, an input-output-analysis, and a water and material flow analysis) and water resources assessment (i.e. the evaluation and systematic study of water resources in relation to human impact, e.g. demand evaluation, trend estimation, analysis of water use and quality). When analysing the demand side, water allocation issues play a vital role. Eventually, this assessment can lead to a water resources development plan describing the status, the management and future development of water resources in a particular area or watershed.

Identification of Location

As a first step of constructing a well, its location has to be evaluated. Proper siting of a well is crucial for its success and long-term sustainability. It is not only determined by physical factors (e.g. the prevailing geology, hydrology, proximity to the actual site of usage) but equally by ‘soft’ factors (e.g. acceptance by the community, capacities for O&M of the well including the pump, the type of water use, local habits, etc.). For this purpose several approaches exist ranging form simple to high-tech solutions. Villagers and members of the neighbouring communities can be a good source of information on the presence of shallow groundwater. Also, certain types of vegetation (e.g. banana plants, bulrushes, sugar cane, date palms) can indicate presence of groundwater (COLLINS 2000). Often, existing wells in close proximity indicate the presence of groundwater. If technical equipment is available, test borings can give detailed information on the groundwater level. For more detailed information see: CARTER et al. 2010.

Well Drilling Techniques

In order to access the groundwater, a hole has to be drilled and a permanent casing installed. The drilling must:

• Break or cut the formation,
• Remove the cut material from the hole, and, if necessary,
• Provide support to the walls of the hole, to prevent collapse during drilling (WAL 2010).
• Run pumping tests and develop the well.

Therefore, a vast number of different techniques exist. Generally, four distinct types of manual drilling are distinguished: augering, percussion, sludging and jetting. (Cf. WAL 2010). Increasingly, more than one or two techniques are used in combination (DANERT 2009). Nevertheless, it is important to recognise that a well drilled to the same depth, in the same aquifer and properly developed will be equally productive regardless of the method of drilling (UNICEF 2010: Financing).

The choice of the optimal drilling technique mostly depends on geological and hydrological conditions but also on experience and endowments of local communities or drilling contractors.

Drilling Techniques in Detail

Adopted from WAL (2010)

1. Augering

The hand auger consists of extendable steel rods, rotated by a handle. A number of different steel augers (drill bits) can be attached at the end of the drill rods. The augers are rotated into the ground until they are filled, then lifted out of the borehole to be emptied.

Above the water table, the borehole generally stays open without the need for support. Below the water table a temporary casing may be used to prevent borehole collapsing.

Drilling continues inside the temporary casing using a bailer until the desired depth is reached. The permanent well casing is then installed and the temporary casing must be removed. Augers can be used up to a depth of about 15-25 meters, depending on the geology. Diameters range from 50 to 200mm.

2. Percussion

Manual percussion uses a heavy cutting or hammering bit attached to a rope or cable and is lowered into the open bore hole or inside a temporary casing.

Usually, a tripod is used to support the tools. By moving the rope or cable up and down, the cutting or hammering bit loosens the soil or consolidated rock in the borehole, which is then extracted by using a bailer.

Just as with hand augering, a temporary casing of steel or plastic may be used to prevent the hole from collapsing.

When the permanent well screen and casing are installed, this temporary casing has to be removed. Manual percussion drilling is generally used up to depths of 25 meters.

3. Sludging

Sludging uses water circulation to bring the drilled soil up to the surface. The drill pipes are moved up and down. On the down stroke, the impact of the drill bit loosens the soil and on the up stroke, the top of the pipe is closed by hand (or valve), drawing up the water through the pipe and transporting the cuttings to the surface.

On the next down stroke, the hand (valve) opens the top of the pipe and the water squirts into a pit, in front of the well. In this pit, the cuttings separate from the water and settle out, while the water overflows from the pit back into the well. The borehole stays open by water pressure.

Thickeners (additives) can be added to the water to prevent hole collapse and reduce loss of working water. Sludging can be used up to depths of about 35 meters.

4. Jetting

Jetting is based on water circulation and water pressure. As opposed to sludging, water is pumped down the drilling pipes. The large volume of water has an erosive effect at the bottom and the ‘slurry’ (water and cuttings) are transported up between the drill pipe and the borehole wall.

A manual or motorised pump is used to achieve an adequate water flow.

The drill pipe may simply have an open end, or a drill bit can be added and partial or full rotation of the drill pipe can be used. Thickeners (additives) can be added to the water in order to prevent hole collapse and reduce loss of working water. Jetting (with rotation) is generally used up to depths of 35 meters.

5. Mechanical Drilling

Drilling methods using machine-mounted rigs are essentially of two types, the percussion method or one of several rotary methods. Both methods have particular advantages, limitations, performance and suitability (WURZEL 2001). See WURZEL (2001) for detailed information.

Completion of the Well

Having drilled the hole, several elements have to be installed before the well can be set into operation:

• The pump;
• The head works (apron) protects the opening at the surface from contaminants or particles entering the well;
• The sanitary seal is an impervious layer, preferably made out of concrete, preventing contamination of the well by holding back seepage through the gravel pack along the borehole;
• The well casing prevents the well from collapse and seepage of contaminants. Traditionally, steel pipes were used for lining but PVC pipe recently have replaced their use, as they are less expensive and easier to handle;
• The well screen holds back sediments while allowing water to infiltrate the well. The screen slots need to be smaller than the grain size of the surrounding soil. PVC pipes can easily be slit with special slitting saws to create thousands of fine cuts;
• The gravel pack, composed of graded gravel and sand, fills up the space between the screen and the borehole. It is only required if the surrounding soil has a grain size smaller than the slot size of the screen (WATERAID 2008; BALL 2001; WAL 2010).

Well Development and Testing

All drilling methods alter the hydraulic characteristics of formation materials in the vicinity of the borehole. Development procedures are designed to restore or improve these characteristics to maximise the performance (SMET & WIJK 2002). This is achieved by removing the fines and by consolidation of the gravel pack (WAL 2010). Over-pumping (that is, pumping at above the design-rate) can improve the efficiency of the packing by drawing further fine particles into it. Where the surrounding ground has many fine particles, the flow of water can be accelerated by back-flushing at a higher rate (WATERAID 2008) (see well rehabilitation).

Once a well has been developed and is free of any fines, the well should be test-pumped. Test pumping gives useful information about the performance of the well and indicates whether the well yield will be sufficient for its intended purpose (WAL 2010). It also indicates the maximum yield that can be drawn without risking overexploitation of the well. After having tested the water quality (see also water quality testing, the well can be set into operation.

Hand-drilled wells cost from US$ 20 to US$ 3,000, depending on the location, technology used, geology and hydrogeology. Given that conventionally drilled wells (i.e. mechanically drilled wells) in Sub-Saharan Africa can cost from US$ 2,000 to US$20,000, manual drilling, where feasible, could provide considerable savings for the provision of rural water supplies (DANERT 2009: HDD). Total costs mainly consist of costs for data, capital equipment, vehicles, fuel, materials, consumables and labour. A method to estimate well construction costs in your particular situation in more detail is provided by DANERT et al. (2010): Costing and Pricing.

Costs for operating and maintaining the drilled well are usually minor, unless well development or well rehabilitations become necessary due to an emergency, a contamination of the well or a reduction of the well yield. Operation and maintenance costs for manual pumping or mechanised pumping are generally dominant.

Operation of drilled wells comprises the pumping and cleaning of the surrounding area. Community members can easily operate a well on their own. Maintenance of a drilled well, however, includes maintaining the motor or electrical pump or manual pump as well, which needs more skills and experience. If the pump does not work, water abstraction is impossible. As for all groundwater sources, protection is vital to maintain water quality and hence an integral part of any maintenance plan.

In general, drilled wells are less susceptible to contamination compared to dug wells. Still, particularly in areas with rapid development, the risk of introducing contaminants to the groundwater body via the hole itself is considerably high. The protection of the well, e.g. by installing an adequate sanitary seal (i.e. an apron) of suitable material between the permanent casing and the surrounding soil, is vital for a safe water supply in the long-term (SMET & WIJK 2002). Additional protection measures, such as a roof and a fence are easy to install but very effective in maintaining the water quality appropriate. If arsenic-bearing rock formations are present, dissolving processes can lead to natural contamination with arsenic when water is exposed to oxygen at the surface making arsenic removal technologies necessary.