BOREHOLE

A 'borehole' is the generalized term for any narrow shaft drilled in the ground, either vertically or horizontally. A 'borehole' may be constructed for many different purposes including the extraction of water or fluid (such as oil) or gases (such as natural gas or methane), as part of a geotechnical investigation or environmental site assessment, for mineral exploration, or as a pilot hole for installing piers or underground utilities.
In the engineering and environmental consulting fields, the term is used to collectively describe all of the various types of holes drilled as part of a geotechnical investigation or environmental site assessment (a so-called Phase II ESA). This includes holes advanced to collect soil samples, water samples or rock cores, to advance insitu sampling equipment, or to install monitoring wells or piezometers. Samples collected from boreholes are often tested in a laboratory to determine their physical properties, or to assess levels of various chemical constituents or contaminants.
Typically, a borehole used as a well is completed by installing a vertical pipe (casing) and well screen to keep the borehole from caving. This also helps prevent surface contaminants from entering the borehole and protects any installed pump from drawing in sand and sediment. When completed in this manner the borehole is then more commonly called a well: whether it is a water well, oil well or natural gas extraction well. The following section describes boreholes used as wells, especially water wells in more depth.

Contents

Features

Installation

Sludger Technique

Development

Advantages

Speed

Precision

Hygiene

Disadvantages

Sustainability

Borehole Pump types

Climate proxy

References

See also

External links

Features

Boreholes used as wells consist of a drilled hole descending below ground level to a geologic layer containing water or other fluids to be extracted. A casing is inserted into this hole, with inlets in the section in the water (or other fluid) bearing layer to allow water to flow into the pipe. The section of the casing above the fluid-bearing layer is sealed to prevent loss of fluid and contamination from other strata.
At ground level, the borehole consists of the pump head, pump motor or, in the case of a submersible pump, the piping and electrical connections, set in a permanent installation. The borehole around the water-tight casing is packed and sealed to prevent the ingress of surface contaminants. 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 often 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 and low maintenance pumping solution, with easy access to all moving parts for service. Atmospheric pressure prevents this type of pump from drawing the water column higher than approximately 7 m of depth. Instead, when the groundwater level is below this depth the water column must be pushed from beneath, using a variety of different types of water well pumps including turbine pumps (basically enclosed fan blades) and valve and cylinder devices. Valve and cylinder pumps use a series of interlocking pump rods to connect the pump 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 drilling rig, or by a hand-operated rig. The machinery and technique to advance a borehole varies considerably according to manufacturer, geological conditions, fluid to be extracted, and job specification.

Sludger Technique

In alluvial plain 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

Whatever method is used to drill the borehole, it will require 'development' before going into regular service as a water well. Development describes the entire process of preparing the finished water well for use, including to test the water for corrosive and abrasive properties (to specify pump components and coatings), to prime the system with water, and most importantly to flush the well and surrounding aquifer which helps remove sediment, particulates and contaminants such as the drilling fluids used to advance the borehole.

Advantages

Especially in undeveloped areas, 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 [1] 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:

★ Loss of pressure in rising main, due to rubbing by moving pump rods, or vibration-induced cracks.

★ Dropped parts inside the main (eg rod joints), requiring special 'fishing' tools for retrieval.

★ Clogged filters or inlets at the bottom of the rising main.

★ Worn bearings at the pump handle.

★ Worn valve parts.

★ Water corrosion or abrasion throughout the mechanism.
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:

★ Borehole pump parts should be designed to be easily replicable by local industry (as the World Bank sponsored, public domain Afridev pump design is intended to be).

★ Standardisation within a region is desirable, to encourage a reliable supply of spare parts and skilled mechanics.

★ Full capacity and responsibility for borehole maintenance should rest with a committee of users, not outside agencies (a principle referred to as Village Level Operating and Maintenance, or VLOM).
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. [2] A working example of such improved open well systems, the Rope Pump, is widely deployed in Nicaragua [3]. 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''

★ Manpowered

★
★ Direct action - eg the Tara pump [4], the Malda pump [5]

★
★ Leve le - eg the open source Afridev pump [6], the Consallen [7]

★
★ Flywheel - eg the Dutch-built Volanta [8]

★
★ Treadle (various suppliers, especially in India and Pakistan)

★
★ Playground roundabout [9]

★ Animal powered

★ Windmill powered

★ Solar powered [10]

★ Com ion engined

Climate proxy

Borehole temperatures can be used as temperature proxies. This is because heat transfer through ground is slow, so that by measuring temperature (and using the proper mathematical formulas) past temperatures can be inferred several hundred years prior. (eg Huang, et al. 2000 ).

References

★ Stewart, E, ''Selection of a pump for lifting water in a developing country'', (PDF at Michigan Technological University [11])

★ Gabriele, J de, ''Improving Community Based Management of Boreholes: A Case Study from Malawi'' (PDF at University of Wisconsin-Madison [12])

★ Hankin, P, ''The Afridev Pump - Problems and Solutions'' (PDF at Loughborough University [13])

★ Huang, S. P., Pollack, H. N., Shen, P. Y. ''Temperature trends ever the past five centuries reconstructed from borehole temperatures''. Nature, 403, 6771, pp 756-758, 2000. doi:10.1038/35001556.

See also

★ Boring (mechanical)

★ Tunnel boring machine

★ Water well

★ Drilling rig

External links

★ Borehole Monitoring

★ Freshwater Action Network

★ Water Aid

★ Rural Water Supply Network

★ Water and Sanitation Management in Developing Countries

★ Intermediate Technology Development Group

★ Water for People

★ Water Can