considerable depth, and by far the larger part of them through a great length of filtering material, and must have taken so long a time to reach the well that their organic character has disappeared. The principal water-bearing formations, utilized in Great Britain by means of deep wells, are the Chalk and the New Red Sandstone. The Upper and Middle Chalk are permeable almost through their mass. They hold water like a sponge, but part with it under pressure to fissures by which they are intersected, and, in the case of the Upper Chalk, to ducts following beds of flints. A well sunk in these formations without striking any fissure or water-bearing flint bed, receives water only at a very slow rate; but if, on the other hand, it strikes one or more of the natural water-ways, the quantity of water capable of being drawn from it will be greatly increased.
It is a notable peculiarity of the Upper and Middle Chalk formations that below their present valleys the underground water passes more freely than elsewhere. This is explained by the fact that the Chalk fissures are almost invariably rounded and enlarged by the erosion of carbonic acid carried from the surface by the water passing through them. These fissures take the place of the streams in an impermeable area, and those beneath the valleys must obviously be called upon to discharge more water from the surface, and thus be brought in contact with more carbonic acid, than similar fissures elsewhere. Hence the best position for a well in the Chalk is generally that over which, if the strata were impermeable, the largest quantity of surface water would flow. The Lower Chalk formation is for the most part impermeable, though it contains many ruptures and dislocations or smashes, in the interstices of which large bodies of water, received from the Upper and Middle Chalk, may be naturally stored, or which may merely form passages for water derived from the Upper Chalk. Thus despite the impermeability of its mass large springs are occasionally found to issue from the Lower Chalk. A striking example is that known as Lydden Spout, under Abbot’s Cliff, near Dover. In practice it is usual in chalk formations to imitate artificially the action of such underground watercourses, by driving from the well small tunnels, or “adits” as they are called, below the water-level, to intercept fissures and water-bearing beds, and thus to extend the collecting area.
Next in importance to the Chalk formations as a source of underground water supply comes the Trias or New Red Sandstone, consisting in Great Britain of two main divisions, the Keuper above and the Bunter below. With the exception of the Red Marls forming the upper part of the Keuper, most of the New Red Sandstone is permeable, and some parts contain, when saturated, even more water than solid chalk; but, just as in the case of the chalk, a well or borehole in the sandstone yields very little water unless it strikes a fissure; hence, in New Red Sandstone, also, it is a common thing to form underground chambers or adits in search of additional fissures, and sometimes to sink many vertical boreholes with the same object in view.
As the formation approaches the condition of pure sand, the
water-bearing property of any given mass increases, but the
difficulty of drawing water from it without admixture
of sand also increases. In sand below water there are,
of course, no open fissures, and even if adits could be
Wells in sand.
usefully employed, the cost of constructing and lining them
through the loose sand would be prohibitive. The well itself
must be lined; and its yield is therefore confined to such water
as can be drawn through the sides or the bottom of the lining
without setting up a sufficient velocity to cause any sand to
flow with the water. Hence it arises that, in sand formations,
only shallow wells or small boreholes are commonly found.
Imagine for a moment that the sand grains were by any means
rendered immobile without change in the permeability of their
inter spaces; we could then dispense with the iron or brickwork
lining of the well; but as there would still be no cracks or fissures
to extend the area of percolating water exposed to the open
well, the yield would be very small. Obviously, it must be very
much smaller when the lining necessary to hold up loose sand
is used. Uncemented brickwork, or perforated ironwork, are
the usual materials employed for lining the well and holding up
the sand, and the quantity of water drawn is kept below the
comparatively small quantity necessary to produce a velocity,
through the joints or orifices, capable of disturbing the sand.
The rate of increase of velocity towards any isolated aperture
through which water passes into the side of a well sunk in a deep
bed of sand is, in the neighbourhood of that aperture, inversely
proportional to the square of the distance therefrom. Thus, the
velocity across a little hemisphere of sand only 12 in. radius
covering a 1-in. orifice in the lining is more than 1000 times the
mean velocity of the same water approaching the orifice radially
when 16 in. therefrom. This illustration gives some idea of the
Artificial increase
of yield.
enormous increase of yield of such a well, if, by any
means, we can get rid of the frictional sand, even from
within the 16 in. radius. We cannot do this, but
happily the grains in a sand formation differ very
widely in diameter, and if, from the interstices between the larger
grains in the neighbourhood of an orifice, we can remove the
finer grains, the resistance to flow of water is at once enormously
reduced. This was for the first time successfully done in a well,
constructed by the Biggleswade Water Board in 1902, and now
supplying water over a large area of North Bedfordshire. This
well, 10 ft. diameter, was sunk through about 110 ft. of surface
soil, glacial drift and impermeable gault clay and thence passed
for a further depth of 70 ft. into the Lower Greensand formation,
the outcrop of which, emerging on the south-eastern shore of
the Wash, passes south-westwards, and in Bedfordshire attains
a thickness exceeding 250 ft. The formation is probably more
or less permeable throughout; it consists largely of loose sand
and takes the general south-easterly dip of British strata.
The Biggleswade well was sunk by processes better known in connexion
with the sinking of mine shafts and foundations of bridges
across the deep sands or gravels of bays, estuaries and great
rivers. Its full capacity has not been ascertained; it much
exceeds the present pumping power, and is probably greater
than that of any other single well unassisted by adits or boreholes.
This result is mainly due to the reduction of frictional resistance
to the passage of water through the sand in the immediate
neighbourhood of the well, by washing out the finer particles
of sand and leaving only the coarser particles. For this purpose
the lower 45 ft. of the cast-iron cylinders forming the well was
provided with about 660 small orifices lined with gun-metal
tubes or rings, each armed with numerous thicknesses of copper
wire gauze, and temporarily closed with screwed plugs.
On the removal of any plug, this wire gauze prevented the sand
from flowing with the water into the well; but while the finer
particles of sand remained in the neighbourhood of the orifice,
the flow of water through the contracted area was very small.
To remove this obstruction the water was pumped out while
the plugs kept the orifices closed. A flexible pipe, brought
down from a steam boiler above, was then connected with any
opened orifice. This pipe was provided, close to the orifice,
with a three-way cock, by means of which the steam might be
first discharged into the sand, and the current between the cock
and the well then suddenly reversed and diverted into the well.
The effect of thus alternately forcing high-pressure steam among
the sand, and of discharging high-pressure water contained in
the sand into the well, is to break up any cohesion of the sand,
and to allow all the finer particles in the neighbourhood of the
orifice to rush out with the water through the wire gauze into
the well. This process, in effect, leaves each orifice surrounded
by a hemisphere of coarse sand across which the water flows
with comparative freedom from a larger hemisphere where the
corresponding velocity is very slow, and where the presence
of finer and more obstructive particles is therefore unimportant.
Many orifices through which water at first only dribbled were thus
caused to discharge water with great force, and entirely free from
sand, against the opposite side of the well, while the general
result was to increase the inflow of water many times, and to
entirely prevent the intrusion of sand. Where, however, a
firm rock of any kind is encountered, the yield of a well (under
a given head of water) can only be increased by enlargement
