So many emergencies that crop up in mining necessitate a departure from ordinary methods that the writer has been led to illustrate a few of the examples that have come under his notice during a somewhat lengthy career; and as most of the methods illustrated are certainly outside the ordinary, it may be hoped that while to many they may convey little that is new, to the larger number some acceptable information may be gleaned whenever the occasion calls for it.
Experience has shown that there can be no hard and fast rule in mine timbering that will fit the varying necessities of different localities, and the engineer must divest his mind of all prejudices that his practice of any particular system or systems may have induced, and must direct his attention to the character of the local timber and its fitness or otherwise for the purpose he has in view. Fortunate indeed are the man and mine that can claim an abundance of good and cheap mining timber; it simplifies, oftentimes, what would be under unfavourable timber conditions, a difficult problem. In many parts of the States there is either none or at best a scrub; if the latter, he has to either pigstye, balk, or bulk-head, if his ground will permit; if, not, and the necessary timber has to be imported, then some system of recovering his timber and using it over and over again may be possible.
As an illustration of this, Figs. Nos. 1 and 2 show a method applied to the copper deposits of the Lyell Tharsis, Tasmania. The workings were wide, varying from 30 ft. to 50 ft., and the deposit a talcose schist of fairly good
standing character. Some occasional styes were used, but the main bulk of the ground was taken up by the timber shown. It will be seen in the transverse section that the vertical timber, A, has a tenon on both ends and a corresponding mortice in the cap and sill, the latter set on a number of round or rough split pieces, about four feet long and bedded on the filling ; the leg is then stepped into the sill, the cap adjusted on the leg, and the whole reared up under the ground, which should not reach to within, say, 4 in. to 8 in. of the back. There is absolutely no necessity to trim the ground, split filling pieces filling the inequalities and assisting to keep the timber against the shots ; wedges are then driven firmly wherever possible, until the leg rings solid when struck with the hammer.
A stope is shown in the longitudinal section with the mullock filling brought forward until the centre leg, A, is nearly buried ; it can then be drawn with the loss of the bottom sill and the rough flooring below.
It will be seen that to apply this method the ground must have some standing character, good enough for 8 or 10 ft. at the least, while the filling must be kept up as the work progresses. Under favourable conditions its economy is beyond question, and in many instances that have come under my notice its application would have been satisfactory, where the very much more costly system of square setting has been employed.
Figs. 3 and 4 exhibit plan and transverse section of a method employed by the Harrietville Gold Mines, Victoria. The lode had an underlie to the east of about 6o°, varied from 10 ft. to 23 ft. wide, and was an exceedingly hard quartzite, requiring a large amount of explosives, flying, when broken, with great force. The footwall was a dense sandstone; the hanging wall totally different—slate, alternating in bars from 10 ft. to 12 ft. wide. The slate was very treacherous where it came into contact with the lode channel. A glance at the section will show that the main difficulty occurred in taking
out the last stope approaching the level. It left a great stretch of wall that had to be maintained at any cost. Level, G ; stull timber, H ; filling overhead, I; last stope in lode, J ; previous stope, K; with stulling piece and filling below.
Timber was plentiful and good, consequently cheap, so that it became more a question of keeping the wall against the violence of the explosive than the timber cost, hence the plan adopted. 15-in. to 18-in. timber was split in two to form the strap, L, the flat side against the hanging wall, and stepped on to the stull piece below. The headboard of the stull and a lath towards the top of the strap kept the latter sufficiently away from the wall to allow starting the laths. The strap was then temporarily tommed, flat pieces dressed off at the points where the horizontal pieces, M, would sit, and a mortice cut from nothing at the head to 2½ inches deep at the bottom of the piece when driven home. A tenon was then cut on the piece by allowing the difference between the square of the stick and the level required, as shown in the dotted lines at the bead of each piece. The centre stull was left out as long as possible, the upper and lower ones driven at once and tommed up permanently (see plan). Once in position they could only be shifted by smashing them, and this was avoided by employing careful, competent men.
Fig. 5 shows a system of cross-timbering immediately below a faulting, in the same mine. Although no actual dislocation of the lode took place, it was contorted out of position and pinched at the fault to such an extent as to make two hanging walls as shown in the section. The country, although similar to that previously described, was, with the lode, very much softer, and at the same time much broken. The main feature lies in footing the stulls on the headboards, as shown, instead of on the pieces themselves. The method is one that is not often required, and men are apt to get hazy over what is called cross-timbering, and are, in consequence, likely to make a mess of it.
Figs. 6, 7 and 8 represent what is called a horn set, a form of timbering that can be applied in several ways, and takes its name from the fourth of the leg (see plan, Fig. 7) being carried through the cap and either set on to a headboard (as shown in both section and elevation, Figs. 6 and 8) or cut off flush with the back of the cap piece and utilised as an ordinary set against blasting. As shown in the section, it is being used in a very low underlay shaft, instances of which occur from time to time, and which necessitate some form of timbering sufficiently rigid to withstand blasting with the sets close into the face. The toms, N, are protected from the shots by the cap piece, which in its turn is kept in position by the toms, the whole, with the assistance of the horn, making a perfectly rigid set and one that will stand all the stress the timber is capable of.
Figs. 9 and 10 exhibit an easily and quickly made ore or mullock pass. Very little explanation is required, as the drawings in both elevation and section are to scale. The only piece that is let in is the 4-in. x 4-in., diagonally into the front hangers and bolted ; this forms the lip of the pass and carries the floor; the rest of the sawn timber is simply hung on bolts.
The frame is generally made on the surface, taken down again, and again put together underground.
Fig. 11 shows a system of timbering an underground chamber used in the United Brothers’ Gold Mine, Mount Wills (Vic.) It will be observed that the collar set over the shaft has been extended the full width of the chamber and the studs, which are from 12-in. to 15-in. round timber, are morticed into the set. Sawn timber, 12-in. x 6-in., was used to carry the centring and form the skeleton shaft, and the only bolts used in the whole structure were employed drawing the 12-in. x 6-in. stuff up the centres so as to prevent warping.
Figs. 12 and 13—Poppet heads employed at the Mount Lyell Blocks Copper Corporation’s main shaft. I do not propose to advise the use of these poppet heads for any elevation over from 50 ft. to 60 ft., but to the engineer opening out for shaft sinking in rough country they are all that need be desired, both in the way of rigidity and the saving in time and labour. The small amount also of floor space needed is a feature that will be appreciated by those who have had to lay off in hard rock for the usual pyramidal system. The elevated brace is shown at the back of the section, and can be made large or small as occasion requires, while the ordinary safety requirements, though not shown, are in use.