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Tubewells and boreholes

Yoram Lembris has been the pump attendant for a borehole in Tanzania for the past 2 years

Yoram Lembris is the pump attendant for a borehole in Tanzania.

Credit: WaterAid / Alex Macro

Depth for depth on any given site, hand-dug wells may yield more water than tubewells, but tubewells 100 or 150 mm diameter are usually quicker and cheaper to sink, need no dewatering during sinking, require less lining material, are safer in construction and use, and involve less maintenance. From a hygienic point of view, the fact that a pump is needed to lift water from a tubewell is an asset, not a liability.

This section describes some features of tubewells which have handpumps, and two methods of sinking them in generally soft ground using only man-powered low-technology equipment, namely, hand auguring using a Vonder rig, and sludging.

Salient features

Some salient features of a simple tubewell are shown in the following diagram:

tubewell

The casing - often in PVC, which is both cheap and inert - houses the inlet, cylinder, piston valves and rising main of a "down-the-hole" type handpump, which can be lifted out for maintenance or repair.

Seepage down the tubewell bore is prevented by the sanitary seal. Seepage from the ground above the aquifer is excluded by the lengths of plain casing. Water to be pumped is admitted through slots in the lower lengths of casing.

Casing to support the external surfaces of the borehole against collapse may be needed, either temporarily or permanently, but is not shown.

Water abstracted from aquifers in relatively soft ground usually contains sand or silt particles, which are liable to cause rapid wear to pump valves and cylinders (and dissatisfaction among consumers).

Methods of preventing these particles from reaching the pump are of two general types:

Screening
The drawing shows slots in the PVC casing which can be cut on site, using a hacksaw. More elaborate, and far more compact, screens are available commercially; some can be bolted on to pump inlets. Materials used include woven wire and man-made fabric; the latter can be wrapped around the pump inlet assembly.

Sand/gravel packing
The drawing shows graded sand and gravel, which is placed from the top of the borehole. More compact, pre-bonded, packs of sand and/or gravel are available commercially; some of these can also form part of the pump inlet assembly.

Screening is nearly always needed, in some form. Sand and/or gravel packing is meant to eliminate particles from the water before they reach the screen and would otherwise have passed through the screen.

Developing the well

Over-pumping (that is, pumping at above the design-rate) before the well enters service 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. This over-pumping and back-flushing is known as developing the tubewell.

Sinking tubewells

At least two experienced operators are required; communities will usually supply unskilled labour. Samples of the excavated material should be taken at regular intervals of depth (and also if the strata changes) and a borehole record should be kept.

Particular attention is needed to maintain verticality. A "down-the-hole" pump which has been installed out of the vertical may be hard to operate and subject to excessive wear.

Auguring

Auguring cuts earth away by the rotation of a cylindrical tool with one or more cutting edges. The excavated earth feeds upwards inside the tool body, which needs lifting to the surface for emptying at intervals. augering

This requires the whole auguring (drilling) train to be uncoupled and lifted; the weight involved can be considerable, and puts a limit to the depth of hand-operated auguring.

Auguring using the Vonder Rig

The Vonder Rig, manufactured in Zimbabwe and possibly elsewhere, can sink a tubewell hole up to 170mm in diameter and about 115m deep in about two days in ground which is predominantly soft. The next drawing shows its salient parts, all of which are made of mild steel and can be carried by hand between sites.

The crossbar is friction-bolted to a stem, at a height suitable for pushing round by hand. Helpers can sit on it if auguring needs extra weight (or even if it doesn't). Additional stem sections are added as auguring proceeds.

Several shapes and sizes of auger-bit are provided, including a "hole-saw"; this is intended to tackle soft rock, but has rarely been successful. Thin layers of rock have been penetrated, however, by an improvised arrangement including a slow-speed diesel drive to the drilling train.

vonderrig

Sludging

Sludging is an effective method of sinking tubewells.

Credit: WaterAid / Caroline Penn

Sludging is a cheap but effective method of sinking small-diameter tubewells to a great depth in the water-logged silts and fine sands which underlie some flat river plains and deltas, notably those in the Indian subcontinent

Tubewells 25mm and upwards in diameter (the larger ones are able to accommodate a "down-the-hole" pump) are sunk to depths of 60m or more. A boring pipe, usually a galvanised mild steel tube fitted with a case-hardened open socket at its base, moves vertically under the action of a bamboo lever pivoted on an H-frame.

The boring pipe rests initially in a shallow pit filled with a water/cow-dung mixture, which acts as a drilling mud and helps to stabilise the walls of the bored hole during drilling. Using a lever, two men raise and drop the pipe successively.

For the duration of each upstroke, another man seals the open top of the pipe with his hand, creating a partial vacuum inside it, so that the water within the pipe rises with it. He removes his hand for the downstroke, during which the pipe drops faster than the water inside it. As this hand-on / hand-off cycle repeats, water starts to gush from the top of the pipe and the whole assembly begins to work as an elementary force pump.

Soil, fluidised by repeated strokes of the case-hardened socket, is entrained into the upward flow of the water and the boring pipe sinks further into the ground with each stroke. Boring rates of 20m per hour have been achieved.

Additional lengths of boring pipe are attached successively until the required depth is reached. The whole pipe is then withdrawn and replaced by PVC rising main (for a suction pump) or PVC casing (for "down-the-hole" pumps).

The process of sludging is illustrated in the next diagram:

sludging

Anything that may be termed 'rock' is rarely met within these types of strata, but isolated stones, or groups of them, do occur occasionally. They totally inhibit sludging operations as just described, but a technique has been evolved which will usually deal with them.

On being stopped by a stone, the sludger boring pipe (with its open socket end) is withdrawn and replaced by a butt-jointed 'hammering pipe' which is fitted, at its base, with a cone of the same diameter as the open socket.

The latter can be used to protect the top end of the hammering pipe, which is driven down by a two-man operated weighted sleeve driver. When the stone has been successfully broken and passed, the hammering pipe is winched or jacked out, the sludge boring pipe with its open socket replaced, and
normal sludging operations re-started.

Other methods of drilling

WaterAid prefers the simplest methods of drilling, particularly those which can be operated by villagers themselves. However, there are several other, more complicated, techniques which can be used and the next few diagrams illustrate the following methods: