BY JOHN GEO. SWINDELL
CHAPTER III. PRACTICE OF WELL-DIGGING.
THE practice of well-sinking may be properly divided into two divisions, digging or excavating being one, and steining or lining with brickwork or stone the other; in the case of hard chalk or rock, the latter operation is dispensed with, the work being confined solely to excavating a lining of brickwork being quite unnecessary for the stability of the work. Wells are usually of a circular form, and those which are merely picked in the solid strata, lack the regularity of the nearly perfect cylinder of brickwork: such wells, however, generally require steining to some depth from the surface of the ground, owing to the looseness of the surface soil; this is exemplified in many parts of Hertfordshire and elsewhere, where a gravelly surface soil overlies the chalk. The mere excavation of a well requires but little skill, though at times it is a matter of great labour, requiring, in hard rock, blasting the plumb-bob, and a rod marked with the diameter of the hole, being sufficient to insure accuracy. Buckets, a windlass, and ropes are required to remove the products of the excavation. Plate 4 contains these things in sufficient detail to render any description unnecessary. Where the well is sunk through stiff clay, as, for instance, that in the London basin, steining of half-brick thick, or four inches and a half, is required for small wells, and of nine inch work for wells of large diameter. Great improvements have latterly been made in the method of carrying on, and also in the stability of this description of brickwork, owing to the use of Roman and other descriptions of cement entirely superseding wedges of slate, bond timber, and common mortar: the two latter are especially injurious, as the timber will decay and the lime in the mortar, unless it be blue lias or equally hydraulic lime, will dissolve out into the water contained in the well, rendering the same very hard; besides, as will be seen when describing the manner of steining, the slow setting of the mortar is a bar to its general use. Loose wet sand, or loam, try the skill of the well-diggers; in such cases, however, it may become necessary to puddle behind the brickwork, and care must be taken that the upper steining should not slip while so doing.
Again, in passing through land springs, they must be carefully walled out, by executing the brickwork entirely in cement-an operation which can only be clone when the quantity of water entering from the spring is limited; where the rush is enormous, as in sinking through the main sand springs of the plastic clay formation, the water must be dammed out, by substituting for brickwork cylinders of iron, which may be either cast or wrought; the latter are the most modern, and have been applied in some large wells; the former are the most convenient for handling, being bolted together in segments, or in divisions. When the sinking such cylinders is necessary, digging will most probably be precluded altogether, and boring alone will be admissible, the cylinders sinking as the sand is bored out : when they have been sunk to a sufficient depth in the solid clay beneath, digging and steining may go on as before. If it be determined to bore, near London, into the chalk, boring should commence before the sand spring be entered, the expense of large cylinders being thereby saved, as their place would be taken by the small bore pipe; and as the water from the chalk will generally rise higher than the level of the sand spring itself, no advantage is gained commensurate with the increased outlay by sinking cylinders. The position of the sand spring can be determined by boring in advance of the well itself, while the latter is being sunk through the plastic clay: by driving a bore hole very small, and thus feeling the way, no danger of a surprise may then be anticipated. Steining is executed in a variety of ways, as regards its manner of application, its thickness, and its bond. The bricks used should be hard, square, and well burnt; if the cost will allow, malm paviors should be used, and if stocks, they should be the very best. As the work is for the most part laid dry, unless the backs run of one uniform thickness, a great waste of time and trouble will unnecessarily take place during the steining: again, as the bricks are laid so as only to touch each other at the edges, a soft crumbling brick would manifestly be useless. The old method of carrying on the steining was by building on a curb of wood shod with iron. The earth being removed from the bottom, the curb and its superstructure sunk down; the brickwork was then added from the top, and this method of proceeding continued till the curb would sink no longer, owing to the swelling of the ground; a new curb and new excavation smaller than the last was then begun.
This method is seldom now used except in peculiar circum stances, all bricks being added under the executed steining the latter being kept from slipping by artificial means when the natural swelling of the ground is insufficient; a circumstance unlikely to take place when the bricks are worked close to the sides of the excavation: in clayey soils especially, the friction preventing slipping is most enormous. The steining is usually executed partly in dry and partly in cemented work, the latter occurring as rings laid at intervals between the portions of the work laid dry: these are regulated by the nature of the ground; in London clay, the intervals generally vary from five to twelve feet, though sometimes the work requires to be laid for some distance entirely in cement. The rings are usually three courses thick, averaging about nine inches; the bricks are laid flat, as in Plate 5, Fig. 2, the courses alternately breaking joint; it is often desirable to insert cement or small wedges in the open spaces at the back of the touching edges of the bricks. The thickness of the steining itself depends on the diameter of the well and the nature of the ground to be passed through; some use nine inch work laid dry, and radiating as in Fig. 3, Plate 5; this is evidently not so strong as four-and-a-half-inch work laid in cement, or even backed with the same in the manner described above; therefore, if nine-inch work is ever used, it should be laid in cement, as being in a situation where four-and-a-half inch work in cement will not do. In commencing an excavation from one cement ring to another, the hole is dug as far as is safe or practicable; the nature of the ground will determine this; a line is then plumbed (see Fig. 1, Plate 5, which represents a section of the steining of a well) from the brickwork above, which will give the position of the face of the brickwork in the lower ring; the cement is usually gauged with half sand, as in works above ground. Too quick setting a cement is not desirable, as it partially sets in being conveyed down the well to the workmen; Roman, blue lias, Portland, or any approved water cement, may be used for the purpose. In many cases, even where the work does not absolutely require it, the steining is clone entirely in cement, a practice which makes excellent work; but which is attended with a further disadvantage than the extra cost of execution, viz., occasioning much trouble and loss of time in fixing the permanent pumps, and temporary ones also, if any are used. In sandy soils, should the well not be deep, the old plate of working on a curb may be adopted, but in deep wells that is inadmissible; here the steining should be set entirely in cement, and, to prevent slipping, the work should be laid, in quarters, care being taken to well hang up the steining do the completion of the work by the insertion of an iron curb, secured in its place by tie rods, which are carried up the shaft and bolted to cross timbers or another curb fixed into the brickwork. In some wells that have been executed in sandy soil cast iron curbs have been inserted at intervals,, each curb slung to the one above it by tie rods; the gravel, or sand can then be excavated under the curb, as the clay can under the brickwork rings set in cement. The curbs, in fact, bearing the same relation to the cemented brickwork, in the case of sandy soils, as the cemented rings do to the dry brickwork in clayey ground. The method of bond or laying the bricks remains to be considered: Fig. 2, Plate 5, shows this. The bricks, though they do not touch exactly at the edges, for practically that is impossible, yet are set in but a mere trifle, and the harder the description of brick the more nearly may the edges abut, the swelling of the ground will soon fill up the spaces at the back of the edges when the bricks are laid dry; this method induces fewer joints than if the work were laid, as in the manner usually adopted for half-brick arches, above ground, and for other reasons is more fit for this purpose. The ground behind them prevents any displacement of the bricks, for, the tendency of the pressure being to twist them, a compressing of the ground must necessarily take place before such occurrence; thus the bricks, aaa, &c., in the figure, before they can be moved nearer to the centre of the well, or alter their position, must force outwards one or other of their two neighbours, G.G., these cannot evidently be so moved without compressing the solid ground behind; here, again, we see the advantage of working as close as possible to such ground, and, if at any time, owing to a stone or otherwise, the excavation be not perfectly round, care should be taken to puddle with solid clay behind the steining to prevent displacement, by thus forming a sufficient abutment.
The work when 9 in. thick is laid either radiating, as in Fig. 3, Plate 5, or in separate 4 1/2 inch rings, Fig. 4; the latter plan is usually adopted, and may be considered the best, for the following reason, it being understood that the work in both cases is laid in cement. Considering the strength as that of a compound of bricks and cement in Fig. 4, fracture of the cement must take place before any failure, while in Fig. 3 a slipping of the bricks away from the cement might arise; and again, in executing the work it might be considered advisable- indeed, it generally is- to execute the back steining first, for a certain distance, and afterwards to complete the inner. Even work, not wavy, but strictly vertical, constitutes good steining, and looking upwards from the bottom of a well will at once detect if the work be true or not, the eye in such case being placed close to the steining. Welldiggers, after attaining a certain depth, find the confined air very unpleasant and noxious. The carbonic acid from the breath, being specifically heavier than common air, soon stagnates at the bottom of the excavation; lime water is sometimes recommended, as this will absorb the carbonic acid; it is, however, a dirty and unworkmanlike expedient. A pair of bellows or a fan blast should be used in such cases, and the air conveyed clown the well in pipes; thin zinc ones answer the purpose very well, they are about two inches diameter. The depth of hole at which artificial supply of air is desirable will depend on the diameter of the well, and the position of the aperture. If it be open to the air, with no temporary shed or other erection over it, a supply may not be required, with a four feet excavation, till about 130 feet from the surface. In this question, however, the extreme limits should not be sought for, as the sooner a plentiful supply is given the better, the workmen getting on more comfortably to themselves, and also much faster. A few words must be said on the construction of iron steining or the cylinders before alluded to. The wrought iron ones are riveted with internal ribs of angle or T iron, so as to be flush on the outside, the rivets being countersunk to attain this end; lowering rings are also riveted inside them, for convenience in fixing. Cast-iron cylinders being much thicker, therefore heavier, will sink into the bole with less driving; they are cast in about five-feet lengths, and are joined together with bolts and internal flanges. In sinking cylinders, their vertical position must be insured by letting them travel or slide between four battens, fixed as guides, and secured to the brickwork. When iron cylinders are used, it is generally necessary to secure up the lower part of the brickwork, as the sand and water will give it no support; an elm or iron curb is therefore used for the purpose, which is secured by iron rods to wood beams let across the well, or iron curbs inserted some distance up the shaft. The space between the cylinders and brickwork should also be well concreted, so as to shut out the water, which would otherwise rise up from the sand. To prevent land springs or drains from percolating into a well, it is well to execute the first ten or twelve feet from the surface, in nine-inch work, the same being well puddled behind. When the surface soil itself is close upon the stiff clay, this may be neglected; and, when the land springs are very strong, they must be shut out by the use of cylinders as previously described.