TUBEWELLS AND BOREHOLES
depth on any given site, hand-dug wells may yield more water
than tube wells, but tube wells 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 tube well is an asset, not a liability.
This section describes some features of tube wells which
have hand pumps, 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
Some salient features of a simple tubewell are shown in the
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.
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:
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
Sand / gravel packing
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 are 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 tube well.
Sinking tube wells
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 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.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
Vonder Rig, manufactured in Zimbabwe and possibly elsewhere,
can sink a tube well 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
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.
Sludging is an
effective method of sinking tube wells.
Sludging is a cheap but effective method of sinking
small-diameter tube wells 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
Tube wells 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 galvanized
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 stabilize the walls of the bored hole during
drilling. Using a lever, two men raise and drop the pipe
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 down stroke, 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, fluidized 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:
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.