A common modification of this method consists in excavating to such a depth as the strata will admit without falling in. A wooden curb is laid at the bottom of the excavation, the brick steining laid upon it and carried to the surface. The earth is then excavated flush with the interior sides of the well, so that the earth underneath the curb supports the brickwork above. When the excavation has been carried on as far as convenient, recesses are made in the earth under the previous steining, and in these recesses the steining is carried up to the previous work. When thus supported the intermediate portions of earth between the sections of brickwork carried up are cut away and the steining completed.
In sinking with a drum curb, the curb, which may be either of wood or iron, consists of a flat ring for supporting the steining, and of a vertical hollow cylinder or drum of the same outside diameter as the steining, supporting the ring within it and bevelled to a sharp edge below. The rings, or ribs, of a wooden curb are formed of two thicknesses of elm plank, 11⁄2 inch thick by 9 inches wide, giving a total thickness of 3 inches.
Figs. 19, 20.
[Fig. 17] is a plan of a wooden drum curb, and [Fig. 18] a section showing the mode of construction. The outside cylinder or drum is termed the lagging, and is commonly made from 11⁄2-inch yellow pine planks. The drum may be strengthened if necessary by additional rings, and its connections with the rings made more secure by brackets. In large curbs the rings are placed about 3 feet 6 inches apart. [Fig. 19] is a plan, and [Fig. 20] an enlarged segment of an iron curb. When the well has been sunk as far as the earth will stand vertical, the drum curb is lowered into it and the building of the brick cylinder commenced, care being taken to complete each course of bricks before laying another, in order that the curb may be loaded equally all round. The earth is dug away from the interior of the drum, and this, together with the gradually increasing load, causes the sharp lower edge of the drum to sink into the earth; and thus the digging of the well at the bottom, the sinking of the drum curb and the brick lining which it carries, and the building of the steining at the top, go on together. Care must be taken in this, as in every other method, to regulate the digging so that the well shall sink vertically. Should the friction of the earth against the outside of the well at length become so great as to stop its descent before the requisite depth is attained, a smaller well may be sunk in the interior of the first well. A well so stopped is said to be earth-fast. This plan cannot be applied to deep wells, but is very successful in sandy soils where the well is of moderate depth.
The curbs are often supported by iron rods, fitted with screws and nuts, from cross timbers over the mouth of the well, and as the excavation is carried on below, brickwork is piled on above, and the weight of the steining will carry it down as the excavation proceeds, until the friction of the sides overpowers the gravitating force or weight of the steining, when it becomes earth-bound; then a set-off must be made in the well, and the same operation repeated as often as the steining becomes earth-bound, or the work must be completed by the first method of underpinning.
When the rock to be sunk through is unstratified, or if stratified, when of great thickness, recourse must be had to the action of explosive agents. The explosives most frequently used for this purpose are guncotton, dynamite, lithofracteur, and gunpowder. Lithofracteur is now often employed, and always with considerable success, as its power is similar to that of dynamite, but, what is particularly important in vertical bore-holes, its action is intensely local; it is, moreover, safe, does not generate fumes more harmful than ordinary gunpowder, requires smaller holes, and but little tamping. The dangerous character of guncotton has hitherto prevented its adoption for ordinary operations, while the comparatively safe character and convenient form of gunpowder have commended it to the confidence of workmen, and hence for sinking operations this explosive is generally employed. We shall therefore, in treating of blasting for well sinking, consider these operations as carried out by the aid of gunpowder alone.
The system of blasting employed in well sinking is that known as the small-shot system, which consists in boring holes from 1 to 3 inches diameter in the rock to be disrupted to receive the charge. The position of these holes is a matter of the highest importance from the point of view of producing the greatest effects with the available means, and to determine them properly requires a complete knowledge of the nature of the forces developed by an explosive agent. This knowledge is rarely possessed by sinkers. Indeed, such is the ignorance of this subject displayed by quarrymen generally, that when the proportioning and placing the charges are left to their judgment, a large expenditure of labour and material will produce very inadequate results. In all cases it is far more economical to entrust these duties to one who thoroughly understands the subject. The following principles should govern all operations of this nature.
The explosion of gunpowder, by the expansion of the gases suddenly evolved, develops an enormous force, and this force, due to the pressure of a fluid, is exerted equally in all directions. Consequently, the surrounding mass subjected to this force will yield, if it yield at all, in its weakest part, that is, in the part which offers least resistance. The line along which the mass yields, or line of rupture, is called the line of least resistance, and is the distance traversed by the gases before reaching the surface. When the surrounding mass is uniformly resisting, the line of least resistance will be a straight line, and will be the shortest distance from the centre of the charge to the surface. Such, however, is rarely the case, and the line of rupture will therefore in most instances be an irregular line, and often much longer than that from the centre direct to the surface. Hence in all blasting operations there will be two things to determine, the line of least resistance and the quantity of powder requisite to overcome the resistance along that line. For it is obvious that all excess of powder is waste; and, moreover, as the force developed by this excess must be expended upon something, it will probably be employed in doing mischief. Charges of powder of uniform strength produce effects varying with their weight, that is, a double charge will move a double mass. And as homogeneous masses vary as the cube of any similar line within them, the general rule is established that charges of powder to produce similar results are to each other as the cubes of the lines of least resistance. Hence when the charge requisite to produce a given effect in a particular substance has been determined by experiment, that necessary to produce a like effect in a given mass of the same substance may be readily determined. As the substances to be acted upon are various and differ in tenacity in different localities, and as, moreover, the quality of powder varies greatly, it will be necessary, in undertaking sinking operations, to make experiments in order to determine the constant which should be employed in calculating the charges of powder. In practice, the line of least resistance is taken as the shortest distance from the centre of the charge to the surface of the rock, unless the existence of natural divisions shows it to lie in some other direction; and, generally, the charge requisite to overcome the resistance will vary from 1⁄15 to 1⁄35 of the cube of the line, the latter being taken in feet and the former in pounds. Thus, suppose the material to be blasted is chalk, and the line of least resistance 4 feet, the cube of 4 is 64, and taking the proportion for chalk as 1⁄30, we have 64⁄30 = 22⁄15 lb. as the charge necessary to produce disruption.