Dru’s System.
Fig. 158.
The system applied by Dru is worthy of attention, not so much on account of the novelty of the invention, or of any new principle involved in it, as on account of the contrivances it contains for the application of the tool, “à chute libre,” or the free-falling tool, to Artesian wells of large diameters. It has been already explained that under Kind’s arrangements the trepan was thrown out of gear by the reaction of the water which was allowed to find its way into the column of the excavation; but that it is not always possible to command the supply of the quantity necessary for that purpose; and even when possible, the clutch Kind adopted was so shaped as to be subject to much and rapid wear. Dru, with a view to obviate both these inconveniences, made his first trepan similar to that shown in [Fig. 101], in which it will be seen that the tool was gradually raised until it came in contact with the fixed part of the upper machinery, when it was thrown out of gear. The bearings of the clutch were parallel to the horizontal line, and were found in practice to be more evenly worn, so that this instrument could be worked sometimes from eight days to fourteen days without intermission; whereas, on Kind’s system, the trepan was frequently withdrawn after two days’ or three days’ service.
We take the following complete account of the system from a paper read by M. Dru at the Conservatoire des Arts et Métiers, Paris, 6th June, 1867.
It will be seen from Figs. [158], [159], that the boring rod A is suspended from the outer end of the working beam B, which is made of timber hooped with iron, working upon a middle bearing, and is connected at the inner end to the vertical steam cylinder C, of 10 inches diameter and 39 inches stroke. The stroke of the boring rod is reduced to 22 inches, by the inner end of the beam being made longer than the outer end, serving as a partial counterbalance for the weight of the boring rod. The steam cylinder is shown enlarged in [Fig. 160], and is single-acting, being used only to lift the boring rod at each stroke, and the rod is lowered again by releasing the steam from the top side of the piston; the stroke is limited by timber stops both below and above the end of the working beam B.
The boring tool is the part of most importance in the apparatus, and the one that has involved most difficulty in maturing its construction. The points to be aimed at in this are,—simplicity of construction and repairs; the greatest force of blow possible for each unit of striking surface; and freedom from liability to get turned aside and choked.
Figs. 159-162.
The tool used in small borings is a single chisel, as shown in [Figs. 161, 162]; but for the large borings it is found best to divide the tool-face into separate chisels, each of convenient size and weight for forging. All the chisels, however, are kept in a straight line, whereby the extent of striking surface is reduced; and the tool is rendered less liable to be turned aside by meeting a hard portion of flint on a single point of the striking edge, which would diminish the effect of the blow.
Figs. 163-167.
Fig. 168.
Fig. 169.
The tool is shown in Figs. [163] to [169], and is composed of a wrought-iron body D, connected by a screwed end E to the boring rod, and carrying the chisels F F, fixed in separate sockets and secured by nuts above; two or four chisels are used, or sometimes even a greater number, according to the size of the hole to be bored. This construction allows of any broken chisel being easily replaced; and also, by changing the breadth of the two outer chisels, the diameter of the hole bored can be regulated exactly as may be desired. When four chisels are used, the two centre ones are made a little longer than the others, as shown in [Fig. 167], to form a leading hole as a guide to the boring rod. A cross-bar G, of the same width as the tool, guides it in the hole in the direction at right-angles to the tool; and in the case of the larger and longer tools a second cross-bar higher up, at right-angles to the first and parallel to the striking edge of the tool, is also added.
Figs. 170-173.
If the whole length of the boring rod were allowed to fall suddenly to the bottom of a large bore-hole at each stroke, frequent breakages would occur; it is therefore found requisite to arrange for the tool to be detached from the boring rod at a fixed point in each stroke, and this has led to the general adoption of free-falling tools. M. Dru’s plan of self-acting free-falling tool, liberated by reaction, is shown in side and front view in [Figs. 170 to 173]. The hook H, attached to the head of the boring tool D, slides vertically in the box K, which is screwed to the lower extremity of the boring rod; and the hook engages with the catch J, centred in the sides of the box K, whereby the tool is lifted as the boring rod rises. The tail of the catch J bears against an inclined plane L, at the top of the box K; and the two holes carrying the centre-pin I of the catch, are made oval in the vertical direction, so as to allow a slight vertical movement of the catch. When the boring rod reaches the top of the stroke, it is stopped suddenly by the tail end of the beam B, [Fig. 159], striking upon the wood buffer-block E; and the shock thus occasioned causes a slight jump of the catch J in the box K; the tail of the catch is thereby thrown outwards by the incline L, as shown in [Fig. 172], liberating the hook H, and the tool then falls freely to the bottom of the bore-hole, as shown in [Fig. 173]. When the boring rod descends again after the tool, the catch J again engages with the hook H, enabling the tool to be raised for the next blow, as in [Fig. 171].
Figs. 174-178.
Another construction of self-acting free-falling tool, liberated by a separate disengaging rod, is shown in side and front view in [Figs. 174 to 178]. This tool consists of four principal pieces, the hook H, the catch J, the pawl I, and the disengaging rod M. The hook H, carrying the boring tool D, slides between the two vertical sides of the box K, which is screwed to the bottom of the boring rod; and the catch J works in the same space upon a centre-pin fixed in the box, so that the tool is carried by the rod, when hooked on the catch, as shown in [Fig. 175]. At the same time the pawl I, at the back of the catch J, secures it from getting unhooked from the tool; but this pawl is centred in a separate sliding hoop N, forming the top of the disengaging rod M, which slides freely up and down within a fixed distance upon the box K; and in its lowest position the hoop N rests upon the upper of the two guides P P, [Fig. 174], through which the disengaging rod M slides outside the box K. In lowering the boring rod, the disengaging rod M reaches the bottom of the bore-hole first, as shown in [Figs. 174, 175], and being then stopped it prevents the pawl I from descending any lower; and the inclined back of the catch J sliding down past the pawl, the latter forces the catch out of the hook H, as shown in [Fig. 176], thus allowing the tool D to fall freely and strike its blow. The height of fall of the tool is always the same, being determined only by the length of the disengaging rod M.
The blow having been struck, and the boring rod continuing to be lowered to the bottom of the hole, the catch J falls back into its original position, and engages again with the hook H, as shown in [Fig. 177], ready for lifting the tool in the next stroke. As the boring rod rises, the tail of the catch J trips up the pawl I in passing, as shown in [Fig. 176], allowing the catch to pass freely; and the pawl before it begins to be lifted returns to the original position, shown in [Fig. 177], where it locks the catch J, and prevents any risk of its becoming unhooked either in raising or lowering the tool in the well.
The boring tool shown in [Figs. 163, 164], which was employed for boring a well of 19 inches diameter, weighs 3⁄4 ton, and is liberated by reaction, by the arrangement shown in [Figs. 170 to 173]; and the same mode of liberation was applied in the first instance to the larger tool, shown in [Figs. 166 to 169], employed in sinking a well of 47 in. diameter at Butte-aux-Cailles. The great weight of the latter tool, however, amounting to as much as 31⁄2 tons, necessitated so violent a shock for the purpose of liberating the tool by reaction, that the boring rods and the rest of the apparatus would have been damaged by a continuance of that mode of working; and M. Dru was therefore led to design the arrangement of the disengaging rod for releasing the tool, as shown in [Figs. 174, 175]. In this case the cross-guide G fixed upon the tool is made with an eye for the disengaging rod M to work through freely. For borings of small diameter, however, the disengaging rod cannot supersede the reaction system of liberation, as the latter alone is able to work in borings as small as 31⁄4 inches diameter; and a bore-hole no larger than this diameter has been successfully completed by M. Dru with the reaction tool to a depth of 750 feet.
The boring rods employed are of two kinds, wrought-iron and wood. The wood rods seen in Figs. [159], [179], are used for borings of large diameter, as they possess the advantage of having a larger section for stiffness without increasing the weight; and also when immersed in water the greater portion of their weight is floated. The wood for the rods requires to be carefully selected, and care has to be taken to choose the timber from the thick part of the tree, and not the toppings. In France, Lorraine, or Vosges, deals are preferred.
Fig. 179.
Figs. 180-182.
The boring rods, whether of wood or iron, are screwed together either by solid sockets, as in [Fig. 181], or with separate collars, as in [Figs. 180, 182]. The separate collars are preferred for the purpose, on account of being easy to forge; and also because, as only one half of the collar works in coupling and uncoupling the rods, while the other half is fixed, the screw-thread becomes worn only at one end, and by changing the collar, end for end, a new thread is obtained when one is worn out, the worn end being then jammed fast as the fixed end of the collar.
The boring rod is guided in the lower part of the hole by a lantern R, [Fig. 159], shown to a larger scale in [Fig. 179], which consists of four vertical iron bars curved in at both ends, where they are secured by movable sockets upon the boring rod, and fixed by a nut at the top. By changing the bars, the size of the lantern is readily adjusted to any required diameter of bore-hole, as indicated by the dotted lines. In raising up or letting down the boring rod, two lengths of about 30 feet each are detached or added at once, and a few shorter rods of different lengths are used to make up the exact length required. The coupling screw S, [Fig. 158], by which the boring rod is connected to the working beam B, serves to complete the adjustment of length; this is turned by a cross-bar, and then secured by a cross-pin through the screw.
Fig. 183.
Fig. 184.
In ordinary work, breakages of the boring rod generally take place in the iron, and more particularly at the part screwed, as that is the weakest part. In the case of breakages, the tools usually employed for picking up the broken ends are a conical screwed socket, shown in [Fig. 183], and a crow’s foot, shown in [Fig. 184]; the socket being made with an ordinary V-thread for cases where the breakage occurs in the iron; but having a sharper thread, like a wood screw, when used where the breakage is in one of the wood rods. In order to ascertain the shape of the fractured end left in the bore-hole, and its position relatively to the centre line of the hole, a similar conical socket is first lowered, having its under surface filled up level with wax, so as to take an impression of the broken end, and show what size of screwed socket should be employed for getting it up. Tools with nippers are sometimes used in large borings, as it is not advisable to subject the rods to a twist.
When the boring tool has detached a sufficient quantity of material, the boring rod and tool are drawn up by means of the rope O, [Fig. 158], winding upon the drum Q, which is driven by straps and gearing from the steam-engine T. A shell is then lowered into the bore-hole by the wire-rope U, from the other drum V, and is afterwards drawn up again with the excavated material. A friction break is applied to the drum Q, for regulating the rate of lowering the boring rod down the well. The shell shown in [Figs. 186, 187], consists of a riveted iron cylinder, with a handle at the top, which can either be screwed to the boring rod or attached to the wire-rope; and the bottom is closed by a large valve, opening inwards. Two different forms of valve are used, either a pair of flap-valves, as shown in [Fig. 186], or a single-cone valve, [Fig. 187]; and the bottom ring of the cylinder, forming the seating of the valve, is forged solid, and steeled on the lower edge. On lowering this cylinder to the bottom of the bore-hole, the valve opens, and the loose material enters the cylinder, where it is retained by the closing of the valve, whilst the shell is drawn up again to the surface. In boring through chalk, as in the case of the deep wells in the Paris basin, the hole is first made of about half the final diameter for 60 to 90 feet depth, and it is then enlarged to the full diameter by using a larger tool. This is done for convenience of working; for if the whole area were acted upon at once, it would involve crushing all the flints in the chalk; but, by putting a shell in the advanced hole, the flints that are detached during the working of the second larger tool are received in the shell and removed by it, without getting broken by the tool.
Figs. 185-187.
The resistance experienced in boring through different strata is various; and some rocks passed through are so hard, that with 12,000 blows a day of a boring tool weighing nearly 10 cwt., with 19 inches height of fall, the bore-hole was advanced only 3 to 4 inches a day. As the opposite case, strata of running sand have been met with so wet, that a slight movement of the rod at the bottom of the hole was sufficient to make the sand rise 30 to 40 feet in the bore-hole. In these cases Dru has adopted the Chinese method of effecting a speedy clearance, by means of a shell closed by a large ball-clack at the bottom, as shown in [Fig. 186], and suspended by a rope, to which a vertical movement is given; each time the shell falls upon the sand a portion of this is forced up into the cylinder, and retained there by the ball-valve.
Borings of large diameter, for mines or other shafts, are also sunk by means of the same description of boring tools, only considerably increased in size, extending up to as much as 14 feet diameter. The well is then lined with cast-iron or wrought-iron tubing, for the purpose of making it water-tight; and a special contrivance, invented by Kind, and alluded to at p. [110], has been adopted for making a water-tight joint between the tubing and the bottom of the well, or with another portion of tubing previously lowered down. This is done by a stuffing-box, shown in [Fig. 188], which contains a packing of moss at A A. The upper portion of the tubing is drawn down to the lower portion by the tightening screws B B, so as to compress the moss-packing when the weight is not sufficient for the purpose. A space C is left between the tubing and the side of the well, to admit of the passage of the stuffing-box flange, and also for running in concrete for the completion of the operation. The moss-packing rests upon the bottom flange D; but this flange is sometimes omitted. The joint is thus simply made by pressing out the moss-packing against the sides of the well; and this material, being easily compressible and not liable to decay under water, is found to make a very satisfactory and durable joint.
Fig. 188.
M. Dru states that the reaction tool has been successfully employed for borings up to as large as about 4 feet diameter, witness the case of the well at Butte-aux-Cailles of 47 inches diameter; but beyond that size he considers the shock requisite to liberate the larger and heavier tool would probably be so excessive, as to be injurious to the boring rods and the rest of the attachments; and he therefore designed the arrangement of the disengaging rod for liberating the tool in borings of large diameter, whereby all shock upon the boring rods was avoided and the tool was liberated with complete certainty.
In practice it is necessary, as with the common chisel, to turn the boring tool partly round between each stroke, so as to prevent it from falling every time in the same position at the bottom of the well; and this was effected in the well at Butte-aux-Cailles by manual power at the top of the well, by means of a long hand-lever fixed to the boring rod by a clip bolted on, which was turned round by a couple of men through part of a revolution during the time that the tool was being lifted. The turning was ordinarily done in the right-hand direction only, so as to avoid the risk of unscrewing any of the screwed couplings of the boring rods; and care was taken to give the boring rod half a turn when the tool was at the bottom, so as to tighten the screw-couplings, which otherwise might shake loose. In the event of a fracture, however, leaving a considerable length of boring rod in the hole, it was sometimes necessary to have the means of unscrewing the couplings of the portion left in the hole, so as to raise it in parts instead of all at once. In that case a locking clip was added at each screwed joint above, and secured by bolts, as shown at C in [Fig. 180], at the time of putting the rods together for lowering them down the well to recover the broken portion; and by this means the ends of the rods were prevented from becoming unscrewed in the coupling sockets, when the rods were turned round backwards for unscrewing the joints in the broken length at the bottom of the bore-hole.
When running sands are met with, the plan adopted is to use the Chinese ball-scoop, or shell, [Fig. 186], described for clearing the bottom of the bore-hole; and where there is too much sand for it to be got rid of in this way, a tube has to be sent down from the surface to shut off the sand. This, of course, necessitates diminishing the diameter of the hole in passing through the sand; but on reaching the solid rock below the running sand, an expanding tool is used for continuing the bore-hole below the tubing with the same diameter as above it, so as to allow the tubing to go down with the hole.
In the case of meeting with a surface of very hard rock at a considerable inclination to the bore-hole, M. Dru employs a tool, the cutters of which are fixed in a circle all round the edge of the tool, instead of in a single diameter line; the length of the tool is also considerably increased in such cases, as compared with the tools used for ordinary work, so that it is guided for a length of as much as 20 feet. He uses this tool in all cases where from any cause the hole is found to be going crooked, and has even succeeded by this means in straightening a hole that had previously been bored crooked.
The cutting action of this tool is all round its edge; and therefore in meeting with an inclined hard surface, as there is nothing to cut on the lower side, the force of the blow is brought to bear on the upper side alone, until an entrance is effected into the hard rock in a true straight line with the upper part of the hole.
Although as regards diameter, depth, and flow of water in favourable localities, some extraordinary results have been obtained with this system of boring by rods worked by steam power, yet, as Dru himself observes, “in some instances his own experience of boring had been, that owing to the difficulties attending the operation, the occurrence of delays from accidents was the rule, while the regular working of the machinery was the exception.” A further disadvantage to be noticed is that, owing to the time and labour involved in raising and lowering heavy rods in borings of 10 inches diameter and upwards, there is a strong inducement to keep the boring tool at work for a much longer period than is actually necessary for breaking-up fresh material at each stroke. The fact is that after from 100 to 200 blows have been given, the boring tool merely falls into the accumulated débris and pounds this into dust, without again touching the surface of the solid rock. It may therefore be easily understood how much time is totally lost out of the periods of five to eight hours during which with the rod system the tool is allowed to continue working.