The old tubes being three-ply, three of these knives or cutters were required to cut out the three tubes, the inner one being detached first, and then the two exterior ones; and so soon as these latter were cut out as far as they had been forced into the clay, the work became simplified into following down the interior tubing by the new tubes, as shown by the dotted lines. From d at the lower end, it was found that the old inner tube had been so damaged or torn, either by the putting in or hammering it down, as to leave a vent or fissure for the sand to descend, and thus spoil the whole of the work for all future success in the boring, to say nothing of the very great cost of lifting the sand out, and subsequent most arduous labour to put the hole right.
Boring was recommenced after about a month’s labour in taking out the old tubings, leaving the new ones firmly bedded into the lias formation, 112 feet from the surface, and the hole was subsequently bored to a depth of 710 feet in the new red sandstone formation, proceeding at the rate of about 3 feet in the twelve hours, and leaving the hole so as, if requisite, it might be widened out to 4 inches diameter. [Fig. 92] shows the action of the knife and spring-cutter when forced down into the tubing, ready to commence cutting. It also shows the lower end of the new tubing, enclosing the others at the commencement of the work. The joints of the new tubes were made by means of a half-lap screw. [Fig. 94] is a back view of the knife or cutter b. [Fig. 93] shows the action of the spring and cutter when the requisite length is cut through and ready for lifting; the position of the tube being maintained perpendicular, or nearly so, by the ball or thickening on the rods at K, and the lower end of the tube being supported by the projecting steel cutter at b, the dotted lines from d showing the position of the new steel-ended tube when screwed down ready for another operation. In boring deeper after the tubes were removed, three wooden blocks were used round the rods in the new tube to keep them plumb.
In some cases it is necessary to widen out holes below the sharp edge of tubing, so as to permit its descent. This is effected with a rimer, Figs. [81] and [82], and is an operation requiring great care and attention.
To reduce the stoppages for the withdrawal of débris the system of Fauvelle was introduced, but it is now very little practised on the Continent, and not at all in Great Britain. The principles upon which it was founded were: first, that the motion given to the tool in rotation was simply derived from the resistance that a rope would oppose to an effort of torsion; and therefore that the limits of application of the system were only such as would provide that the tool should be safely acted upon; and, secondly, that the injection of a current of water, descending through a central tube, should wash out the débris created by the cutting tool at the bottom. The difficulties attending the removal of the débris were great; and though the system of Fauvelle answered tolerably well when applied to shallow borings, it was found to be attended with such disadvantages when applied on a large scale, that it has been generally abandoned. The quantity of water required to keep the boring tool clear is a great objection to the introduction of this system, especially as in the majority of cases Artesian wells are sunk in such places as are deprived of the advantage of a large supply.
In the ordinary system of well boring, innumerable breakages and delays occur when a boring is required to be carried to any depth exceeding 200 or 300 feet, owing to the buckling of the rods, the crystallization of the iron by the constant jarring at each blow, and particularly the increased weight of the rods as the hole gets deeper. It follows from this, that where the excavation is very deep, there is considerable difficulty in transmitting the blow of the tool, in consequence of the vibration produced in the long rod, or in consequence of the torsion; and, for the same reason, there is a danger of the blows not being equally delivered at the bottom. It has been attempted to obviate this difficulty, but without much success, by the use of hollow rods, presenting greater sectional area than was absolutely necessary for the particular case, in order to increase their lateral resistance to the blows tending to produce vibration.
Boring is usually executed by contract. The approximate average cost in England may be taken at 1s. 3d. a foot for the first 30 feet; 2s. 6d. a foot for the second 30 feet; and continue in arithmetical progression, advancing 1s. 3d. a foot for every additional 30 feet in depth. This does not include the cost of tubing, conveyance of plant and tools, professional superintendence, or working in rock of unusual hardness, such as hard limestone and whinstone. A clause is usually inserted in the contract, to the effect that, if any unforeseen difficulty is met with in the course of the work, it is then paid for by the day, at a rate previously determined upon, until the difficulty has been overcome.
[CHAPTER V.]
AMERICAN TUBE WELL.
This well consists of a hollow wrought-iron tube about 13⁄4 inch diameter, composed of any number of lengths from 3 to 11 feet, according to the depth required. The water is admitted into the tube through a series of holes, which extend up the lowest length to a height of 21⁄2 feet from the bottom.