Table of Contents
- Square Set System
- Life of Timbers
- Cost of Timber
- Later Systems
- Underground Open Cut System
- Sloping Stope System
- Block System
- Comparative Remarks
- Conclusion of Underground Stoping
- Open Excavations
- Incline Tramways
- Flying Fox
- Capacity of Skips
- Ore Body
- Drive Sets
- Stables Underground
As this district is so far distant from the other mining centres and its mode of approach so circuitous, I am afraid that many residents of other portions of the Commonwealth never think of paying this “ Australia’s Silver City” a visit, to learn personally the many and various lessons everywhere available, and to become familiar with the really wonderful and interesting features and deposits (which nature has so lavishly bestowed in the mineralogical world, and yet seemed to almost ignore or overlook when disposing of topographical beauties of landscape and vegetation) unless professional or business duties require their presence here. So it is to those whose life is necessarily spent in other parts of the States where they answer their call of duty, and yet feel an interest, either for business or other reasons, that my paper is specially addressed and prepared, and for that reason I will endeavour to make my remarks more of an explanatory than a dictatorial nature. I feel sure that there, are many members of our Institute who are much more familiar, and have had many more years’ experience in the various methods of timbering, filling, and ore extraction, etc., than myself, but I hope that over 16 years’ experience in connection with mines and engineering works, the last six of which have been spent in Broken Hill, where I have been very closely connected with all the details of mining and ore extraction as Mining Surveyor and Civil Engineer to the Sulphide Corporation (the output of whose mine, “ The Central,” is over 5000 tons of sulphide ore per week, and comes next to the Great Broken Hill Proprietary Company) will merit the favour of your careful and, I hope, interested attention ; and if my paper, with its accompanying drawings, etc., is of interest or profit to any member of our Institute and in any way advances its interest and influence, I shall feel amply repaid. As the majority of our members are unable to attend this meeting and have not visited Broken Hill to inspect for themselves. I hope this will be accepted as an explanation for my going into sundry details which, many present may be quite familiar with. At the same time, as this gathering only represents a very small quota of the members, I have not excised these details, and hope that they maybe of interest and assistance to those who are unable to be present with us to hear any explanation that may be deemed necessary given viva voce by myself and other local members of the Institute.
I must confess that the impressions of a new arrival on coming to the Barrier are anything but pleasant or reassuring, for when, after spending a whole night in rattling over 300 miles of almost desert country, one seeing the line of lode, about one and a half miles in length, with its long chain of chimney stacks, poppet heads, engine houses, concentrating mills, and immense mullock and tailings dumps, etc., finds it hard to imagine that this is the now world-famed Barrier Range or Broken Hill, from which almost countless fortunes have been extracted- and as many more remain awaiting consummation-unless a mining man who has been on similar fields. The original Broken Hill is now a thing of the past, having been entirely removed by the large open cuts, from which the oxidised ore is being extracted down to about 200 to 250-feet; but I have fortunately, secured a copy of a photo of the original outcrop on Block 13, so that the nature or form of the wonderful outcrop after which the city is named may be seen. I may here state that the original name of this place was Willyama, a name which has recently been also given to a new mineral found in the Consols mine. The name was given by Mr. PITTMAN, Government Geologist of N.S.W., as Willyamite.
No doubt all of our members have read of the finding of silver by the stockman on Mount Gipps Station and of Rasp’s shaft (Rasp himself being a station hand at the time), and of the almost romantic mining of earlier years, when only the oxidised ores of the upper levels of the lode were worked down to 300 or 400 feet and, as on all new fields, the methods adopted were of the crudest description until, with great advance of output and rush of population, more modern and advanced systems were adopted; and the arrival of American mining managers and engineers with the “square set ” system of timbering, as carried out in ore mines of America and elsewhere. I think it would not be out of place for me to make this system the first to which I will draw your attention.
Square Set System
If you refer to Fig. I. you will there see an isometric drawing of the square set system showing sizes of timbers and all joints. All timbers are 10 in. x 10 in., the vertical ones called legs and the horizontal ones caps and struts ; they are all cut to the required scantlings in the sawmills. Some of the mines have their own sawmills, and buy their timbers in long lengths straight from the Port, and cut all timbers to templates. They are then sent underground quite ready for the miners and timbermen to frame up in the stopes. When the ore is hard and compact (and it is hard sometimes, especially in the class of ore containing rhodonite) I have frequently seen five drills blunted to bore a hole less than one inch; but when the ore is friable, it is then timbered close up to the working face (as shown in Plate I.) on the upper floors. You will notice that, in this system, the miners are always working close to the face or back, which they can easily examine to make sure of its safety. One disadvantage of keeping the timbers so close to the face is that frequently a heavy shot will “ throw,” and thus knock down several sets and shake others, thereby causing delay and rendering the working face unsafe until the timbers are re-erected. You can readily imagine the difficulty in securing them all as firmly as they were originally. Laths of 10 in. x 2 in. Oregon are laid on each floor as the stope rises upwards to the next level, and chutes for conveying the broken ore to the sills and thence to the trucks are provided at convenient intervals, and slides placed to run the ore to the chutes from the working face. These may be seen on the plan and section showing the stope on the square set system (Plate I.). It will also be noticed that the end sets of each floor are wedged firmly to the foot and hanging walls of the ore body, and frequently notches or hitches cut to secure a solid bed. In theory, as a system by itself, these seem admirable, but in practice (without being filled with mullock as they now are) they fail lamentably, for after the ore has been extracted, any movement or pressure of the walls of the lode causes an entire collapse of the sets, like a toy brick house. These have gained the weird name of “ creeps,” and a more complete state of chaos can hardly be imagined than a creep, i.e., broken and splintered timbers, and masses of ore and mullock in one almost unapproachable mass, often rendering the further working of that portion of the lode almost impossible, and thereby losing large quantities of ore in the debris, but it is a noticeable fact that in those mines where the managers did not rely on the timbers alone, but judiciously filled in the sets with mullock from wall to wall (leaving only
the necessary openings for shutes and gangways, etc.,) when any movement came in the walls of the lode the timbers and surrounding filling stood the burden, and the mines were singularly free from “ creeps.” Some idea of the immense strain or pressure imposed on these timbers may be imagined, when I state that I have seen a piece of 10 in. x 10 in. Oregon timber compressed to barely 3 in., have also seen a 10 in. x 10 in. vertical leg driven 4½- in. into the horizontal cap and sill at its ends, without bending the leg, and have frequently noticed, when there has been any lateral pressure, the huge 10 in. piece of Oregon splintered like a piece of willow on the convex side, and on the concave side, though bent one foot, was still unbroken.
Life of Timbers
The life of timbers underground depends a great deal on location. In some mines I have noticed Oregon that had been in for approximately 10 years almost sound, whilst in others the same class of timbers, if put in a badly ventilated stope, in about three or four years had completely decayed by a kind of mouldy dry rot. In the upper portions of one mine I noticed a lot of joggled logs of blue gum, from six inches to nine inches diameter (brought from the river in the early days), and they were worm-eaten and quite rotten, whilst the mulga and black oak, both hard native local timbers, were quite sound ; but as these latter are usually only about 4 in. to 8 in. diameter they are almost useless for underground timbering, except as laths, or for latticing the sides of square sets and enclosing the mullock fillings, for which are sometimes used. I omitted to mention when describing the mullock filling of the square sets that 10 in. by 2 in. laths are used by some mines, but other mines, having their own sawmills, rip the 10 in. by
2 in. lath in halves and use the 5 in. by 2 in. lath, thus effecting a small saying in the amount of timber used; but I have noticed also that these lighter laths frequently give way when any great pressure from the mullock is thrown on them.
Other great defractions to, the square set system are the great cost of timber and the liability to fires.
Cost of Timber
When one comes to consider the great amount of timber required, to timber up a lode the total of super feet is enormous, and at, approximately, 16/- per-100 super feet, the cost greatly reduces the profits. We have in the “ Central ” a width of over 270 ft. from footwall to hanging well at the 600 ft. level, the greatest width on the Barrier, taken out in blocks about 50ft. wide right across from one level to another, viz., 100 ft.)
Then again there is the liability to fires, of which the fire in Block 11, of about seven years ago, and the fire in Block 12, of four years, both of which are still burning, are examples. These cause great expense in extinguishing and greatly hamper the working of the upper portions of the lode. It will be readily seen that so much timber of an inflammable nature is a great menace, and especially when the stopes are well ventilated by winzes from the upper workings. These winzes serve as vents or chimneys, and spread the deadly gases throughout the mine, and the result is loss of life, as it was on both occasions just referred to, when men went below to locate and attempt to overcome the fire before it gained, too strong a hold.
I have endeavoured to explain at some length the advantages and disadvantages of this square set system, and I hope, with the aid of the accompanying drawings, I have been able to make, them dear to all. It will be seen that the deduction to be drawn is that this system is an admirable one when combined with mullock filling; but it is also an expensive one. I
desire to acknowledge my indebtedness to the Mines Department of N.S.W. for data, etc., in reference to the square set system of timbering.
Owing to the sulphide ore requiring more costly and tedious treatment and preparation before being suitable for smelting than oxidised ores, and the many and varied expanses incidental to the mining, due to the hardness and the work of extraction and handling, it was necessary that managers and others interested should devise some safer and cheaper methods, which would also be more suitable to the class of ore to be mined and treated, and thus, born of actual experiment and necessity, these later methods have gradually evolved into their present forms; and their almost universal adoption on several mines on the line of lode—with sundry modification to suit individual cases—proves their efficiency. I must mention that the square set system is far from annihilated, as it is still used where applicable, especially on the sill floors where gangways are required, also shutes, outlets, etc. Here the sets are on solid bottoms; being well wedged against the hanging and foot walls or the sides of the stopes, they are firm and permanent, and with the 10 in. by 3 in. planking to carry the mullock filling they form most convenient passages about the workings. So you will see it is still used in conjunction with these later systems, as a very valuable and necessary adjunct.
Underground Open Cut System
To many this term at first may seem a little erroneous or misleading, as the term open cut is generally applied to excavations from the surface downwards, but the above name is that generally given by the miners to the large stopes which are worked under this system. I will now draw your attention to Plate II.—by comparison with Plate I.—the great difference is soon evident,. The drives are first put along the foot and hanging walls, and then through the ore body from the upper levels winzes are sunk, and crosscuts are driven at convenient intervals. The winzes serve several important purposes. Firstly, they ensure a complete and lasting ventilation to the stopes during their upward way, by carrying off all noxious gases as they form on the lower workings, or are given off by the sulphide ore, and thus enable the miners to work with a greater degree of comfort, and also to do a fair shift’s work, which could hardly be expected in a hot stope with a constant atmosphere of about 90°, where a singlet and pants seem almost too much. Secondly, the winzes serve as passes or shutes through which the mullock filling is conveyed from the upper levels and, by a succession of shutes and winzes from the surface, is deposited where desired. This system is entirely successful in the Central Mine, for the mullock is broken in a large open cut on the surface, and conveyed in side-tipping trucks of capacity of one cubic yard, drawn by horses through a tunnel, then discharged into a shute, from, which, by a series of winzes shutes, etc., it is distributed throughout the mine where required. Thirdly, the winze is used as a starting place or face from which to work the stope, and, after the ore is extracted, say the first 10 or 20 feet, it is timbered up closely into two compartments, as shown on Plate II. One compartment serves as a shute or pass for the ore to the sill floor, as the stope works upwards, and the other compartment as a ladder way and means of ingress and exit for the miners and others to the upper workings of the stope. The sides of the initial drives on the sill floor are extended to the desired width along the lode, and thus the stope is formed on the sill floor, the sill timbers placed in position, and then mullocked up. On top of these timbers the bedding for the filling is placed, as will be noticed on the section, being 10 in. by 10 in. and 10 in. by 3 in. timbers, arranged as best to carry the great burden imposed. Above these “sollars” (as they are called) the only timbering is that of the shute and ladder way, all other spaces being filled in with mullock from wall to wall as indi-
cated, which, it will be readily seen, is placed in layers of 7 ft. to 12 ft. As the broken ore falls and the traffic also is all on the mullock filling, each succeeding layer gets well rammed, and solidifies before the next one is placed on it. In the large open stopes in the Central Mine almost all the boring is done by machine rock drills driven by compressed air. These bring down the ore in large pieces, frequently from 7 ft. to 8 ft. by about 2 ft. wide. These pieces are then bored by hammer and drill and popped into smaller sizes, then spawled into suitable sizes (generally less than about 1 ft. long) for throwing down the shutes and removal in the trucks, which are all end-tipping and hold about 16 cwt. of broken ore. When the “ back ” or top portion of the stope is “ heavy,” or seems dangerous and likely to come away, “ bulkheads ” are built under it. These consist of 10 in. by 10 in. timbers, placed at right angles to each other, one above the other, and tightly wedged, as shown on Plate II. When bulkheads are built on the mullock filling, a bed of 10 in. by 4 in. sollars is first laid on the mullock to distribute the pressure over as large an area as possible; then the first 10 in. by 10 in. timbers forming the bulkhead are laid transversely across the sollars. These timbers are afterwards removed, the burden shot down, and the same timbers used over and over again.
Sloping Stope System
I think it would be advisable before drawing your attention directly to the advantages and disadvantages of this particular system of stoping to describe a somewhat similar modification of the same system called the “ Sloping Stope System” as shown on Plate III. This method is extensively used on the Broken Hill Proprietary Mine, and I must here express my indebtedness to E. J. HORWOOD Esq., C.E., mining manager, for his kindness and courtesy in granting me permission to copy his drawings, showing this system. In many instances the same description will apply as in the foregoing notes on underground open cut stoping, viz., the levels on the sill floors are first formed, taking notice that the width of the stope depends on the nature of the ore to be mined, or its ability to support itself by leaving the back in the form of an arch, the whole stope when formed is somewhat in shape like an isosceles triangle, of which the level or sill floor forms the base and the winze the apex ; also the winze (as before) is stink from the level above, and the stope is started from the winze as in the other open cut system, whilst the winze serves the same purpose for ventilating, and as a pass for the mullock filling into the stope, also shute for the ore to the sill floor. The great difference is that the stope slopes laterally to each side instead of going up with a level or even floor, and as these sides rise with the stope, provision must be made for preventing the mullock filling from running into the adjoining stope when it rises ; this is done by placing vertically at the sides of the stope, about five feet apart, 10 in. x 4 in. stringers, which overlap at the ends, and are then covered with 10 in. x 2 in. laths placed horizontally against the face of the ore. These may be removed and used over and over again. When the adjoining stope is afterwards being worked, the stoping advances forward from 5 ft. to 8 ft. at a time, and from 8 ft. to 12 ft. upwards. The advantage of these sloping sides is that the broken ore falls on 10 in. x 4 in. sollar boards placed on the incline plane of the mullock, and thus rolls to the shutes at the sides without further handling excepting of course the large pieces, which require hand boring, popping and spawling as before mentioned. It will be noticed on referring to Drawing A, Plate III., that the stope is filled in with mullock to within two feet or three feet of the back, and the stope is always worked downwards, starting from the winze ; but if the back is heavy or faulty it is secured by 10 in. x 10 in. legs resting on the sollars (or on bed logs left in the mullock), or at right angles to the sollars and also to the back, as they are nearly parallel; these are tightly wedged and blocked and only removed as the ground is taken out. When the stope is about nine feet high the sollars are taken up and stored aside
for further use. The stope is mullocked up again to within two or three feet of the back, when the shutes are again built up a proportional height and the sollars replaced on the mullock, the sloping process as before taking another slice from the back, also starting from the winze downwards. The Broken Hill Proprietary Company has of late years been adopting a modification of the square set system in working these “sloping stopes” by timbering up the middle of the stopes with square sets which are filled with mullock as the work proceeds ; but as the back (or working face in this instance) is sloping as in the last mentioned method, each successive floor of sets stands back one or in such a manner that the outside faces of the sets follow as nearly as possible the same angle of inclination or inclined plane as the face of the ore body. In this way the miners are always within a safe distance of working and examining the face and back of the workings, and all the favourable points of the other adaptation of the sloping stope system apply to this system, with the advantage that the miners have a good footing on the set timbers, and the great convenience offered for the despatch of the ore through the shutes constructed in the square sets.
I will now briefly describe another modification of this system as adopted in the Central Mine (where I have the honour of being engaged) and a method which, up to the present, is not used in the other mines as it is in the Central with the most satisfactory results. I refer to the “block system,” which you will notice portrayed on Plate IV. The lode for its entire length through M.L. No. 9 has been surveyed into parallel blocks each 50 ft. in width (i.e., 10 sets, each 5 feet wide); each alternate division is a block and the next a stope; the whole level is gradually developed by a drive along the footwall and by crosscuts to the hanging wall, thereby determining the width of the lode along its entire length, and the stopes are then carried from the footwall to the hanging wall on the sill floor, and the whole space filled with square sets, leaving every facility for forming the necessary gangways, shutes, etc. These are then filled in with mullock (as before mentioned in the paragraph on square set timbering) and the stope starts on its course upwards, being exactly 50 feet wide by the entire width of the lode at that point, thus leaving a pillar of ore 50 feet wide on each side of it from wall to wall, which will carry all pressure during the mining of this stope. A run of square sets is put in each side of the stope as it goes upwards, forming a gangway and ladderway, the sides of which are lathed or paddocked off, thereby confining the mullock filling in the centre of the stope. The ore is broken by machine drills (as formerly, driven by compressed air), and in the same lifts and proportions as in the before mentioned open stope system; the ore falling on the mullock filling in the centre of the stope is popped and spawled into suitable size for handling and trucking to the shaft for haulage to the surface. One great difference in this system from the others is that the winzes (6 ft. x 5 ft.) are always sunk 100 feet apart at the side of each alternate stope (being half in the stope and half in the adjoining block), thereby saving a second winze when the block is being taken out at any future time. The ore from the adjoining stopes having been all extracted, and the space filled with the mullock, this winze will then be available, and serve the same purpose for the remaining block. The same advantages re ventilation, mullocking, and stoping, all apply to these stopes, as in the foregoing open cut and sloping stope systems, and they are mullocked up in the same manner, excepting that the shutes for conveying the ore from the working faces to the sill floor are placed in the runs of sets, placed on the sides of the stopes for that purpose, and the shutes can be placed at any suitable intervals for the workings.
Having thus briefly described the various methods of underground stoping, I will now venture to draw your
attention to the advantages and disadvantages of them. Firstly, the whole of the ore body is, or at least eventually can be, extracted, and after extraction of lode material, comparatively few, if any, large voids or openings are left, which also leaves the surface areas for works, mills, and machinery, etc., almost free from risk of subsidence. Secondly, the great advantage evident from the presence of mullock filling—in lieu of a forest of timber—is the immunity from risk of fire. Thirdly, the miner is always in reach of the “ back,” i.e., from 3 ft. to 9 ft., and can readily sound and examine the back of the workings, and thereby make sure they are safe; and this ensures a great freedom from accidents caused by masses of ore falling on men whilst at work immediately under them, though unfortunately recent experiences have shown that, as far as human knowledge or judgment can avail, a place may be sounded and examined by miners with a life time experience and reported as safe, when a few hours afterwards the back falls in and reveals a fault or crack which the sounding did not make known, serious or fatal accidents consequently resulting ; but even then this cannot in any way be compared to the great risk incurred by men when re-erecting square sets that have been knocked down by a heavy shot, when sometimes a charge will bring away more ground than anticipated, for then a dozen or more sets will come down, and the men will have to work under, probably, a dangerous back in re-erecting the sets or staging, before they can actually examine it and assure themselves of its safety. Fourthly, another advantage is the saving in the expense of timber. Of course against this must be placed the cost of quarrying the mullock filling on the surface and conveying it to the stopes, which, would however, be required in any case in filling the square sets in the other systems. Fifthly, the great advantage of good air, as the mullock fills in all spaces except the winzes and stope itself where it is being worked, this ensuring always at the working face a current of air which also carries off the smoke after firing and adds to the miners’ health and comfort, removing much that in former days made the miner’s life a hazardous and unhealthy one.
Conclusion of Underground Stoping
Having at this length described and compared the various systems of underground stoping and ore extraction, I am afraid I would weary you if I were to attempt to describe as fully—and go into detail—the one other important method employed, and by which such a large amount of oxidised ore has been removed. I refer to the “ open-cut system ” and “surface extraction,” but will briefly describe the operations, and I hope you will form some idea of its methods and the extent of its workings, which really have to be seen to be appreciated. In reference to the open-cut system, which I purpose making the last one to describe in this paper, I shall endeavour to be brief, yet explicit, and hope I have not already tired your interest. I would here beg to tender my best thanks to J. J. SHAW, Esq., engineer in charge and manager for Baxter and Saddler’s contracts in working their open excavations, for his courtesy in supplying me with the necessary data, drawings, etc.
Certainly the large excavations are one of the chief sights of Broken Hill, and though descriptions may give a slight idea of their extent I really think they must be seen to be understood or appreciated. Imagine a huge open cavern, three-quarters of a mile long—for they extend from the north boundary of Block 10 to the south boundary of Block 14—thus traversing the whole of the Broken Hill Proprietary Company’s Blocks 11, 12 and 13, each of which is 20 chains along the line of lode and 20 chains wide. The widths of these cuttings vary from 120 feet near the viaduct in Block II (in what is known as Smith’s cut) to about 350 feet wide across Baxter and Sadler’s immense cutting in Block 12. There is also a great width of 300 feet opposite McGregor’s shaft in the centre of Block II. The cuts are down about 250 feet, and they are recovering a large amount of timber that was used underground in the square set system of stoping in the old 200 feet level workings, the ore from which was then hauled up the various shafts before the open-cuts reached their present depths. The great mass of over-burden, mullock, country-rock, etc., that has been removed in the extraction of the ore, can be imagined by the heaps on both sides of the line of lode, reaching (near the outer portions) a height of 60 feet or more (the angle of repose being between 30° and 40°). The areas covered by the base of these heaps (following the area described by the toe of the various dumps) will almost equal the area of the adjoining bases of the lode; in fact, supplementary leases have been taken up on which the mullock heaps and debris have been deposited.
One of the most interesting features of the works is the system of “ incline tramways,” by which the waste is removed from the cuts. The grade is 2 ft. to 1 ft. approximately; they are always double lines, and two end-tipping trucks, called “dobbins,” are used, being hauled by a ¾-inch wire rope attached to a hauling engine or winch having a loose drum and 9 in. cylinders, approximately 15 horse-power. The empty dobbin runs down the incline, while the full one is being drawn up, thus aiding the traction of the loaded one. On arrival at the surface, near the engine room, the full dobbin is stopped by placing a “ sprag ” on the wheel (this is a piece of hardwood about 18 in. or 2 ft. long and 4 in. in diameter) ; the hauling rope is detached, and a horse hitched to the dobbin, which is then drawn along a surface line of 3 ft. 6 in. gauge, and being an end-tipping truck is run to the end of the bank, where a “ cradle ” or tipping-trough of old sleepers is formed ; the sudden impact of the front wheels in this cradle causes the dobbin to discharge its load over the end of the dump ; of course, as the dump grows the cradle or tipping- trough is removed to the outer edge, the invert being generally about 6 inches below the level of the line.
It is remarkable the instinct the horses used in running out these dobbins possess. The driver runs alongside and on nearing the end of the dump pulls a strap fastened near the breeching, which releases the fastening attaching the horse to the dobbin. The horse immediately runs off the line and allows the dobbin to pass him, and then trots up behind and turns round, ready to be re-hitched to the empty dobbin to draw it back to the cutting again, often needing nothing more than a word from the driver. The horses used in this class of work are a splendid type of draught animal; unfortunately those working in the bottom of the cuts (where the oxidised ore is mined and dust frequently blows about) sometimes get leaded, and then become the same type of wreck that the men do when they suffer from the same malady, which, since the stopping of the smelters on the Barrier, I am pleased to say, is not such a menace, or nearly so prevalent as in the early days. This, I think, is largely due to the greater cleanliness of the workmen and the increased facilities given by all the mining companies—for each and every mine has its own changing house, where the men can indulge in a hot or cold plunge or shower bath. Yet it is remarkable how many miners go home daily covered with black sulphide dust, looking more like aboriginals than white men.
The next most important method of extraction and that by which the greater part of the ore from the open cuts is raised to the surface is the “ Flying Fox,” which is the name given to a large skip which is hauled up and conveyed along an aerial ropeway, and thence discharged into large ore bins at the sides of the railway lines on the surface, from which it is conveyed to the mill or else to the smelters. To the new arrival (or one who has never seen an aerial ropeway before), these are certainly one of the most notable and interesting features of the place. A mast is erected on each side of the cut, and a cable stretched over an iron saddle near the top; the cable is anchored securely on either side, while on the surface is located the hauling engines, of the same type as used on the incline, having a loose pulley and reversing gear ; an attachment called a bicycle runs along the main cable across the cut, having on it four pulleys—the upper two travel along the cable, whilst the lower two are used in hoisting the skip vertically from the cut, and the same rope (called the “ travelling rope ”) then draws the bicycle, and, of course, with it the skip along the cable; when it is over the bin on the surface a self-acting catch holds it steady, whilst it is lowered and discharges its load into the bin. The skip is again hoisted and is run out along the cable and again lowered into the cutting; in the meantime, a second skip has been filled and it is attached and hoisted and discharged as before.
Capacity of Skips
The skips are about 1 ft. 4 in. deep, 4 ft. wide and 5 ft. long, and hold about one cubic yard ; they are suspended by four chains, one at each corner, the back two being fixed to the skip, whilst the front ones are fastened by hooks, which are undone to release the load. These skips are used for many purposes besides hauling ore, for I have seen a workman with a broken leg hauled to the surface and deposited safely, thus saving a steep climb up the banks; this was spoken of appreciatively by the surgeon who attended the case. The sides of the cutting are designed with a batter of a half to one and three-quarters to one and would meet at a depth of about 250 ft.; but owing to the frequent slips of the sides, they are at present very irregular. Some very heavy firing is done in these cuttings, which shakes the ground for a great distance, and sends large clouds of dust up in the air, but immense burdens and masses of the rock are removed.
The ore body in the centre of the cut is about 25 ft. to 50 ft. in width, and of a distinctive colour from the surrounding country rock. The open cutting in Baxter and Sadler’s contract, Block 12, and also in Smith’s Cutting, Block II, are now working immediately above the fire area, and frequently pieces of burnt timber and molten rock are found. These cuts were of great use in coping with fires underground by pouring large quantities of water down the shafts right above the fire area.
There is just one other feature, in closing, to which I will draw your attention, viz., the massive “ sets ” as used in the double track drives, where horses are used underground. The application of these will be self-evident from the drawing, Plate V. They are specially designed to withstand heavy vertical and lateral pressure, and are placed at intervals according to the nature of the back or country they have to hold, but generally about four feet or five feet centre to centre.
It may be of interest to know that the horses used in underground traction are stabled underground and seem to thrive well, for they are all in good condition and take kindly to their work. The stables are lighted with electric light, and are quite as sweet and clean as the average stable on top.
I am afraid that I have trespassed on your patience rather more than I at first intended, but I hope I have been able to make my subject sufficiently interesting, and have not wearied you by too exhaustive a paper. However, I have purposely
left this last portion, re open cuts, till the end, so that it could readily be excised should the paper be found too long or take up more time than is usually allowed. I will conclude by hoping that it may have given our members as much pleasure to listen to, as it as given me to write this, I hope interesting, description of mining in “ Australia’s Silver City ” in this remote part of the State, which, being so far distant, is I fear, frequently forgotten as really being a portion of the parent State of Australia.
Mr. JOHN WARREN said that to residents of the Barrier any further explanation of this system was not necessary, but to visitors no doubt there were several points which required explanation. No doubt other gentlemen would speak to the paper. The reason why square sets were necessary in some stopes, and not in others, was that where there were bands of hard ore mixed with friable it was almost impossible to work the open-cut system, and he thought that square sets were necessary. Where they had firm walls, as they happily had got in the deeper levels, there was no difficulty in taking it out from wall to wall. The sloping stope was a very old fashioned affair ; it was invented before he was at any rate. They all knew the principle. There was plenty of room for explanations, and visitors would, no doubt, be very pleased to be further enlightened in the matter.
Mr. HORWOOD said that there were one or two things he would like to refer to. As to the square set system, he entirely agreed that it was an admirable one in the ground where it was suitable. In hard ground it was almost impossible to use it for where heavy shooting was required it was worse than useless, as it was no protection to the men, but rather gave them a false sense of security as it was necessary to keep the square sets so far from the working face when heavy blasting was being done. Where the difficulty of the problem was in the keeping of a large mass of friable ore in place rather than in the mere mining or breaking of the ore the system was an admirable one. He did not agree with Mr. BEAUMONT that it was an expensive system. Although the timber cost was heavy, the great convenience in having stages to work on, and the additional security given, enabled the men to do more work, and the extra cost of timber was more than made up by these advantages. With regard to the system of taking out the ore in widths of 50 feet, as adopted in the Central Mine, he quite recognised that it was a system which was calculated to keep the surface intact for a longer period than with other systems, but he was inclined to think that the widths were unnecessarily small, and that they might easily be twice the width, thereby saving largely on the whole of the ring of timber around, and giving much freer work to the miners. In connection with pillar work he thought they gave the idea of greater security in the meantime only; for when these pillars had to be taken out, the number of difficult places to be worked in was then increased unnecessarily, and in making unnecessary pillars the work was rendered much more costly in the future than need be. In regard to the Proprietary mine open cuts, in case the visitors might think that the open cuts were very dangerous, he would say that this was not borne out by figures. The usual batters were from half to one to three- quarters to one, and in no case a quarter to one. Referring to stables underground, the method described by Mr. BEAUMONT was in use up to a short time ago, but now they changed the horses every eight hours, so that they had as much daylight as the men, and it was very beneficial to the horses.
Mr. JOHN WARREN said that he thought in all cases there was never an advantage without a disadvantage. In taking out the first cuts you had something to watch, but when you came to take out the pillars you would have to watch against the ring of sets, and when the pillar was once undercut at the bottom, the whole weight of the pillars was on the mullock.
You were safe in taking out the first cuts, and as it had been stated that they had ore there to last them as long as they lived, well, that was an easy position to be in.
Mr. HEBBARD said that as a representative of the Central he would like to defend this system. The president, Mr. J. WARREN, was quite right in thinking that they did not care much about the difficulties attending the taking out of the second blocks or pillars, as by the time that became necessary most of them hoped to have been gathered to Abraham’s bosom ; but, as far as their experience went, with the pillar and block system, they found that the pillars even now could be taken out just as readily and safely as the original stope, for the system adopted in connection with this stope and block system was to put the filling in in horizontal layers. Then the entire mass of the ore as it was broken down on the filling helped to compact it, and after a time, with the help of a little water which leaked in, the mullock became so hard that they could work through it as well as they could through the original ore body. He thought experience had proved that the system was a good one and that the pillars left would be just as readily worked as the original bodies of ore.
Mr. JOHN WARREN said that there was a gentleman named ARTHUR on the Barrier some years ago who suggested taking out the ore in 10 feet blocks; but these would be very awkward to handle. There were, however, advantages in this block system. You must have something to support the overhanging bulk. It is a great improvement on the former system of stoping.
Mr. E. K. BEAUMONT said that he was aware that the horses in the Proprietary came to the surface each shift, but he was referring more particularly to the Central Mine, where the system described by him was still in force. With reference to the cost of timber, he never for a moment suggested that the block system should be adopted in toto for the whole.