CHAPTER VI Tue BurtpInc—Its WATER SUPPLY (Part 2)

Reading beds, and the Thanet sands contain much sand and gravel and are full of water in many laces. Chalk Strata. The chalk around London forms a fine source of supply, the water being held up by the impervious strata beneath. If the London clay is bored through the water will rise nearly to the surface. The Upper Greensand contains a good deal of water but its area is limited. The Lower Greensand, underlying the Gault, is a good water-bearing stratum. The Hastings Sand furnishes a water of a somewhat chalybeate nature, Tunbridge Wells being situate upon it. Oolite. The Upper Oolite is a poor water-bearing stratum, but the Middle Oolite furnishes a good supply and wells are largely sunk into it; the Lower Oolite also yields a good water in places. In the case of the Trias, the lower portion of the Keuper series contains porous beds with considerable water, supported on seams of compact marl. Springs occur at the outcrop and plenty of - water is obtainable by boring, but it is very hard. The Bunter Sandstone. The Bunter sandstone is the most im- portant water-bearing stratum in England, except the chalk with greensand. The yield is largely due to the permeability of the strata, and the wells of Manchester and Salford alone yield - 164 THE BUILDING—ITS WATER SUPPLY 6,000,000 gallons per day of clear water, drawn from an area of not more than 7 square miles, largely covered with buildings, streets and boulder clay. Possibly much of the water percolates from the Rivers Irwell, Irk and Medlock, which traverse the strata, but, if so, the sandstone is a wonderfully effective filter, since the the rivers named are very foul. One spring from this source, the Wall Grange spring near Leek in Staffordshire, yields, 3,000,000 gallons a day. Permian Formation. The Permian formation also contains much water in its lower beds when they immediately overlie those of the carboniferous series. The Coal Measures contain alternating beds of grits and clay, the former being full of water. The Mill- stone Grit, where resting on shales, also gives a good supply. The Devonian and Old Red Sandstone rocks frequently furnish springs. The Silurian, Cambrian and Igneous rocks are only suitable for large collecting areas of surface water. Analysis of Sources. The sources from which water is usually obtained from a practical point of view, are (1) upland surface water, collected from large uncultivated tracts of land; (2) streams and rivers; (3) springs; (4) wells; and (5) rain water collected from roofs and other collecting areas. Springs. Springs are derived from that portion of the rainfall which has sunk into the soil. They may be divided into two classes, (1) surface springs, and (2) deep-seated springs. The water falling on the surface will percolate downwards until it is stopped by an impervious layer. It will then issue at the lowest point of the porous stratum, usually on the side of a hillor cliff. Fig. 134 shows such a case. It represents a section through a hill with a permeable stratum overlying an impermeable one. The surface water will percolate downwards till its descent is arrested, when it will issue at the point S on the hillside in the form of a surface spring. Fig. 135 shows another such case. In this a permeable stratum occurs between two which are impermeable, the former coming out to the surface, or outcropping, on either side of the hill. Surface water will pass over the upper impermeable layer, and with that collected on the exposed part of the permeable will percolate through the latter, issuing at the lowest point S in the form of a deep-seated spring. Fig. 136 shows a case where the continuity of the permeable stratum is interrupted by a fault. The water in such a case will collect in the permeable stratum and overflow in the form of a surface spring at the point S. The water from a surface spring is unlikely to be as pure as THE BUILDING—ITS WATER SUPPLY 165 that from a deep-seated spring, owing to the water in the latter ease having filtered downwards to a greater depth. Fig. 137 gives a section showing one example of a deep-seated spring. Owing to a fault or dislocation of the strata, the water in the per- meable stratum is obstructed in its downward flow and finds an outlet upwards through the line of fault, issuing at S. Springs, if they happen to be available, give a useful source of water supply for isolated houses in the country or small isolated blocks of cottages. The more deeply seated it is, the better, generally speaking, will the water be. Thus, in the case shown in Fig. 137 it will be seen that the water is less liable to surface con- tamination than in those shown in Figs. 1384 and 136, owing to its filtering through greater depths. Sometimes the outlet area is rather large, but this can be overcome by forming a channel, ora small gallery, in the hillside to tap the water as it reaches the sur- face. Springs do not normally provide a sufficiently constan source for the supply of towns, but there are a few cases in which towns are supplied in this way, notably Bath, Malvern, and Lan- caster, while part of London’s supply comes from springs in Hert- fordshire, a conduit about 40 miles long having been utilised by the New River Company for the purpose of conveying it. In supplying houses in the country, a spring may sometimes be obtained at such a height as to supply a storage tank by gravita- tion. Such tanks are usually made of sufficient size to contain from about three days’ to one week’s supply. If the spring is large, a ball-valve inlet should be used. The tank should be covered and ventilated and provided with an overflow. Such tanks can be formed of concrete backed by clay puddle and ren- dered inside in cement mortar. If the water is not of the best, it may be made to pass through a sand filter before reaching the tank, or, if it is not so filtered, pressure filters should be used on the taps supplying drinking water. These filters will be fully described later. If the level of the available spring does not permit of supplying the storage tank by gravitation, the water must be pumped up to a storage tank situated in the highest part of the house. Wells. Water is also obtained from wells sunk into the under- ground water to a level below that at which it would issue as springs. Wells are classed as shallow and deep respectively, and it is important to note that these terms, as used in this connection, have no direct reference to depths in feet. Thus, a shallow well may be a greater number of feet in depth than a deep one, the term shallow meaning that the source of supply is the subsoil 166 THE BUILDING-——-ITS WATER SUPPLY water, while the supply for deep wells is derived from a water- bearing stratum beneath an impermeable one and often at a great depth. Shallow Wells. The water from shallow wells is always open to suspicion, owing to the liability of pollution from defective drains and cesspools. The underground water is always moving, and by its lateral motion or by its rise and fall in times of heavy rainfall or drought, may place the water in the well in direct communication with sewage-sodden soil. In an attempt to obtain a pure supply from a shallow well, therefore, the well must be as far as possible from any likely sources of con- tamination, with the dip of the strata towards any neigh- bouring cesspools, lined (or “‘steined”’) with brickwork (as shown in Fig. 188), concrete, stoneware tubes, or iron cylinders, be covered over and a permanent pump fixed. The old-fashioned “draw” or “dipping” well is not permissible under any circum- stances, if only owing to the possibility of polluting matters finding their way in at the top. Steining. There are many ways of steining a shallow well with brickwork; three will be described: If the ground is reasonably firm it should be possible to excavate successive depths of 3 feet or 3 feet 6 inches, and then to timber each length immediately with poling boards, walings and struts, until a depth is reached several feet below the lowest level of the subsoil water, the excavation being kept dry by continuous pumping. When the bottom is reached a 6-inch diameter pipe is sunk vertically in a hole to act as a sump for the pump and around it is built a floor of conerete. The walls are then built up, timbering being removed as required. The brickwork for some distance up from the bottom will have open vertical joints, to allow water to pass in; above this level it will be solidly built in cement mortar, with clay backing, to exclude surface water. At the top the diameter is reduced by corbelling to about 2 feet and the opening covered by a stone slab with lifting ring, the top of the slab being about 1 foot above the level of the ground, whose surface should be concreted around the cover. A permanent pump is fixed and the temporary sump in the floor concreted in. The second method, like the first, is possible only when a reason- able depth of soil can be excavated without setting timbering. Excavation is carried down as far as is safe and a circular kerb of oak or elm, 9 inches wide, is laid to form a support for the wall. The wall is then built up in cement mortar and backed with clay puddle, 6 inches thick. Holes are then dug in the ground below THE BUILDING—ITS WATER SUPPLY 167 for the insertion of inclined struts for the temporary support of the walling, with their lower ends resting on solid wooden foot- blocks near the centre of the well and with their upper ends wedged under the wooden kerb. Excavation then proceeds down to the level of the foot-blocks, where another kerb is laid. Brickwork with clay backing is built up upon it, as before, to a level as close as possible to the kerb above, after which the space between is wedged tight with pieces of slate and cement mortar. In building these lower lengths gaps must be left around the inclined struts, but when the brickwork is sufficiently set to carry the weight, the struts are removed and the remainder of the brickwork filled in. Successive lengths follow in similar manner, but the last length or two will have open vertical joints and no clay backing. The third method is one which can be used even where the soil is insecure, so that it would not be possible to excavate any appreciable depth without timbering. Its disadvantage is that it is not possible to form a clay backing to the steining, so that percolation of surface water down the outside of the wall is more likely than where the other methods are used. A circular iron kerb, with bevelled cutting edge below, as shown in Fig. 138, is sunk a little way into the ground and carefully levelled. The brickwork is built upon it by a bricklayer working at ground level; when he has got the work up 3 or 4 feet another man excavates the ground within the kerb to a depth of about 1 foot and then gradually works the soil away from under the cutting edge evenly all the way round, so that the kerb may sink under the weight of the wall above. Another foot of soil is then excavated and the lowering repeated, whilst the bricklayer continues to build up the wall. Very great care must be taken to keep the kerb quite level as, if the steining gets out of the vertical, it may become impossible to sink it further. Whichever method of construction is used, the thickness of the walling will usually be 9 inches, though occasionally it is more. The upper part, which is solidly built in cement mortar, is usually built of radiated bricks, but this is quite unnecessary for the lower part, which has open joints. The internal diameter of the well will generally be about 4 feet. ee If iron cylinders are used as steining they are made up of sec- tions, with machined flanges on the inside. Before these _are bolted together their faces are smeared with a mixture of iron filings and sal ammoniac. This form of steining could usually be sunk into position on a kerb. The lowest section would of course have perforations. 168 THE BUILDING—ITS WATER SUPPLY Wells of small diameter are sometimes lined with concrete tubes, the lowest one or two being perforated. Reinforced con- crete is also sometimes used, the thickness being about 4 inches with vertical and horizontal steel rods in the centre of it; the hori- zontal rods are bent to the form of hoops and are wired to the vertical rods wherever they cross. Deep Wells. Wells which are sunk to considerable depths are dug out to only a depth of about 4 or 5 feet. From that depth they are completed by boring (also called “ drilling”) with special tools, the diameter being usually much less than that which is usual with a well, although bores of quite large diameter are possible. The borehole is almost always lined with steel tubing, which serves the purposes of holding the subsoil in position and of excluding surface water. The lining usually need not extend to the bottom of the borehole. The methods of drilling may be classified into two distinct systems, the Percussion System and the Rotary System. In the Percussion System the breaking up of the rock or soil is done by a chisel bit, which is screwed to a vertical rod, suspended from a derrick set up over the site of the borehole. As the work proceeds the rod must be extended by screwing on additional lengths of rod. The rods and tool are raised by a winch and allowed to drop under their own weight, so that the chisel digs into the stratum. When 3 or 4 feet of soil has been penetrated the tool is brought to the surface and replaced by a shell-bucket, which is a contrivance for hauling out the debris after this has been softeneel by water. The chisel is afterwards substituted once more and drilling continued. The objection to this method is the time which is lost in raising the tools and changing them and in using the shell-bucket. To avoid this it is sometimes the practice to use hollow rods and a hollow chisel with oblique holes at its sides and to foree water down inside these by a pump at the surface. The water carries up (outside the rods) to the surface the cuttings or debris, which are settled in a settling tank and the water decanted off for use again. If this “hydraulic flushing” is used the drilling chisel has to be brought to the surface only when lining tubes have to be inserted. In the Rotary System tubular boring rods, screwed together in 6-foot lengths, are suspended from a derrick and rotated by bevelled gearing at the surface. At the bottom the circumference of the rod has either saw-like teeth, fixed diamonds, or a serrated face resting on chilled steel shot. By these means a circular groove is cut in the bottom of the borehole and the inside core THE BUILDING—ITS WATER SUPPLY 169 works up into the tubular boring rod and can be brought to the surface for an examination of the nature of the strata. To facili- tate drilling, water is circulated down the interior of the rods as a lubricant and in some cases enough water is pumped down to wash loose debris to the surface on the outside of the rods. As already stated, in boring through the upper formations it is usually necessary to line the borehole with steel casing, whose thickness is generally from 3 to } inch, the tubes being 10 or 12 feet long with screwed ends, so that they can be fitted together to form watertight joints. The lowest length has at the bottom a steel shoe with cutting edge. Until some considerable depth is reached there will be no difficulty in lowering the lining into posi- tion, but after that it will need to be driven down with a ram or monkey. The lowering and driving is done in stages while drilling is suspended. It is important to note that water will not always be obtained when a boring is made through an impermeable stratum to one which is permeable. Fig. 139 shows both a shallow and deep well, and so far as one can say from the data furnished by the sketch, water should be obtainable in both wells. The circum- stances shown in Fig. 140, however, are quite different. The fault, or dislocation of the strata, ensures water being collected so that well A is ensured a supply, but with the permeable stratum dipping downwards, as it does, there will be no certainty of supply to well B, which would probably be a dry or dumb one. Deep wells are often termed artesian well, but the term does not correctly apply to all deep wells. Artesian Wells. A true artesian well is one formed in a valley or “basin” under such conditions that the water rises up through it and discharges with some foree. Thus, Fig. 142 shows the con- ditions favouring an artesian well. Assuming the water level, in the lowermost permeable stratum shown, to be at the line AB, water will rise approximately to that level through the well. The name artesian is derived from the fact that the first such well was sunk in the province of Artois,