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Borehole

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A borehole is a deep and narrow shaft in the ground used for abstraction of fluid or gas reserves below the earth's surface. If the fluid reserve is under pressure, such as oil or gas, then extra machinery may be required. For water a special type of submersible pump is used to pump water up the rising main.

A borehole is also a commonly used term in the Environmental consulting and Engineering industries. A borehole is a small diameter hole drilled from the land surface to collecting soil samples. Soil samples are often then, tested in a laboratory to determine geotechnical properties or to assess levels of various chemical constituents. If the boreholes are installed for the purpose of assessing environmental conditions they may be completed as a Monitoring well to collect water samples.

Boreholes are most often used as a substitute for water wells, which tend to be shallower and wider than a borehole.

Boreholes are most heavily used in industrialised nations by water utilities, and by high volume water consumers such as golf courses and factories, for whom an independent water supply is an economical substitute for the metered supply of piped water. Although private domestic boreholes are usually free to operate, heavier users of a region's water table may be taxed by local authorities.

Boreholes are most important and widespread in the developing world, in regions where piped water supplies are not extensive. They can be the main water supply for a community, or sustain livestock and crops in difficult conditions. In either case, they require extremely robust design and implementation strategies to achieve sustainability.

Contents

Features

Boreholes consist of a drilled hole, up to 20cm in diameter, descending between 4-250 m below ground level. A water-tight rising main is inserted into this hole, with inlets in the bottom section to allow water to flow into the pipe. The pipe is assembled in sections. Bracing brackets may be attached at intervals to its exterior, to contact the walls of the bore and reduce vibration during pumping.

At ground level, the borehole consists of the pump head, set in a concrete base, with concrete surrounding the immediate area. The borehole around the water-tight rising main is packed and sealed to prevent the ingress of surface pollutants. A capped access point enables the water level in the bore to be inspected. A fixed spigot delivers pumped water with enough clearance to enable a variety of containers to be placed beneath it.

Concrete surrounds the borehole, to channel spillages away from the head of the bore. This prevents 'seepage' - surface water trickling back into the borehole, carrying pollutants or bacteria into the well and contaminating it. It also safeguards easy and hygienic access to the pump, however heavily used.

At depths up to 6 m, simple suction pumps at ground level can pull the water column up the main. Also known as pitcher pumps, these are a low cost solution, with ease of access to all moving parts for maintenance and service.

Beyond 7 m depth, constraints of atmospheric pressure prevent this type of pump from functioning. Instead, the water column must be pushed from beneath, using a valve and cylinder device resting inside the main, at the bottom of the borehole. A series of interlocking pump rods connects this device to the surface, where energy is usually imparted to the pump by hand, via a lever. The 'downhole' equipment requires special skills to maintain.

For depths greater than 50 m, hand power becomes less viable, and wear increases. Some of the stress of lifting the heavier water column can be offset by buoyancy aids on the pump rods.

Installation

Boreholes may be drilled using a hired plant, or by a hand-operated rig. Machinery and the precise technique vary considerably according to manufacturer, geological task, and job specification.

Sludger Technique

In alluvial plains areas such as Bangladesh, with sedimentary mud and little rock, boreholes may be 'sludged' using hollow bamboo or metal scaffold poles. The sludge (a low-tech drilling lubricant typically a mixture of water and cow dung) is mixed in a shallow pit, into which the pole is inserted. The pole is pinned vertically to the short end of a lever, set in a frame, to be moved up and down by one or two drillers holding the long end of the lever. Another stands on the lever frame with his hand over the top of the hollow tube. On the upstroke, this seal creates suction at the base of the pole, lifting mud into the tube. On the downstroke, he removes his hand. The pole drops faster than the water column inside, and the mud underneath is pulverised, to be collected on the next upstroke. Sludge soon fills the pole, and flows out of the top. In favourable circumstances, this technique realises sink rates of up to 20 metres per hour, and works for depths up to 50 or 60 metres.

Development

However the hole is sunk, the borehole requires 'development' before going into regular service. Depending on the type of borehole and borehole pump, development describes a process that might include the testing of water for corrosive and abrasive properties, and the subsequent specification of coated pump components; priming of the system with water; backflushing to remove particulates.

Advantages

Boreholes have three primary advantages over traditional hand-dug wells:

Speed

Boreholes can be drilled and functioning in less than a week, meeting emergency needs. A relief organisation can apply hired plant to drill a medium-depth borehole in around one day, fitting parts, back-flushing and testing, and putting into service at very short notice. Large population movements, for example, can in the right circumstances have their water needs provided for with a rapidly-installed grid of boreholes and pumps.

Precision

Boreholes can descend over 100 m, to remote water-bearing rock strata or low water tables. Consequently, a borehole's location is less decided by geological features than a shallow well has to be. It may therefore be possible to locate it more conveniently for its users.

Further, the borehole permits extraction from a particular depth, without mixing the supply with potentially contaminated surface water. For example, contamination of parts of Bangladesh's many-layered aquifer system [link] with arsenic makes it essential that water be drawn from the correct depth.

Hygiene

Boreholes are significantly more sanitary than open wells. The water source itself is protected from contamination by its depth, and by basic precautions at the well head to prevent dirty water returning into the bore. Since users do not dip containers or rope that they have handled into the water source, any organisms they carry cannot spread to the water source.

Basic rope and bucket wells are regularly contaminated by bacteria from the containers and hands of their users. Typically, they are also open to insect infestation. Water-born diseases are a significant cause of death and disability in the developing world.

Disadvantages

Boreholes have one severe disadvantage: the necessity of regular servicing. Whereas none of the technology involved in an open, rope-and-bucket well is beyond the means of its users to repair, worn metal, plastic, rubber and nylon parts in a borehole pump cannot be replaced by typical users, who lack spare parts, tools and expertise.

In regions where government support networks are unreliable, and local industry does not support metalworking, boreholes can thus rapidly fail, and fall out of use. In this way, boreholes regularly fail the basic sustainability test.

Typical failures include:

There is significant regional variation to these sustainability issues. In India and Pakistan, for example, the widespread use of bicycles has created a network of repair shops which have the materials and skills to fashion spare parts for borehole pumps. This indigenous industrial resource is on average much less developed in Africa, where bicycles are relatively underused.

Sustainability

A worldwide research effort by the World Bank in the 1970s and 1980s sought to discover ways to overcome the sustainability barrier to borehole use. It concluded that: According to some commentators, this approach still fails too regularly, and improved rope and bucket wells - despite their higher risk of contamination - would in many cases provide a more sustainable solution. [link] A working example of such improved open well systems, the Rope Pump, is widely deployed in Nicaragua [link]. A survey of failure rates among World Bank approved Afridev borehole pumps is provided in the appendices of Gabriele, below.

Borehole Pump types

Main article: Pump

References

External links

 

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