Timbering Underground Mining Tunnels & Galleries

Timbering Underground Mining Tunnels & Galleries

This paper is not intended to be a technical discussion of square-set framing as used in mines, but merely a short description of the different kinds of framing that have been used in the Butte mines, and what has been decided upon, after years of practical experience, as being the best, taking into consideration the physical condition of the veins and country rock, and also the supply of timber available for mining purposes.

Butte is, essentially, a square-set district. Virginia City, Nev., was the first mining camp to use this method of timbering, and to Philip Deidesheimer, who went to the Ophir mine in 1860, is given the credit of solving the problem of square-set framing, where large bodies of ore were stoped and the timbers were supposed to hold the ground from caving without any filling. That he did his work well is shown by the fact that the main features of his framing are in use to-day.

In the Butte mines the conditions are such that this method of timbering is generally used. We do not, of course, depend upon the timbers any more than is necessary. The worked-out stopes are filled with waste rock as close behind the miners as possible and not interfere with mining operations. The waste rock is obtained from development work on the different levels or from the surface. In the earlier days of mining in Butte there were numerous independent companies and naturally there were different ideas as to the best way to frame the square sets, although the general features were the same.

Timber was abundant for mining purposes and the common practice was to use sawed lumber, generally 10 by 10 in. or 12 by 12 in. square. Some of the mines used for girts 6 by 10 in. or 8 by 12 in. timber, depending upon whether they used 10-in. square or 12-in. square timber for posts and caps. This avoided the necessity of framing the ends of the girts, and also required less timber for the same work. It proved very satisfactory, as the cap, which is the stronger of the horizontal members of the square set, is placed perpendicular to the strike of the ore body and resists the pressure from the hanging wall of the vein; while the main function of the girt is to resist the side swinging of the caps as the weight from the walls comes on them.

timber sets at gagnon mine

The Gagnon mine was an exception in that round timbers were used for square sets long before it became a practice in the other mines. About 1886, while the management of the mine was under C. W. Goodale, a machine was installed to frame round timbers. With slight alterations, the same framing is in use to-day at this mine, as the Gagnon still has an independent framing mill. Fig. 1 gives a perspective of the kind of framing used at this mine at the present time. The post has a horn 5 in. square and 2½ in. high on each end. The cap is similarly framed, but in addition has a shoulder taken off the bottom, 5 in. from the center, to allow the cap to fit snugly on top of the post, and on top, also 5 in. from the center, a slab is taken off the full length to allow for a level floor in the stopes. The girt is generally less than 10 in. in diameter and is framed on two sides only. The posts are 7 ft. 10 in. in length, making the sets 8 ft. 3 in. from center to center. The caps are 5 ft. in length and butt end to end. The girts are 4 ft. 7 in. long. This makes the sets 5 ft. from center to center, either cap-way or girt-way. On the main working levels of the mine practically the same framing is used, with some slight changes which can readily be seen in Fig. 2. The timbers are especially selected and much larger, and the posts, which are flat bottomed, are 1 ft. farther apart on the floor of the sill than they are at the cap. This gives additional strength to resist the side pressure which tends to push the posts into the drift, thus adding materially to the life of the drift before repairs are necessary.timbering methods at steward mine

The Steward mine also has an independent framing mill and continues the same method of framing that has been in use for a number of years. (See Fig. 3.) The posts have a horn 5 in. square by 3½ in. at the top, while the horn at the bottom of the post is 5 in. square by 1½ in. It was supposed to be easier and quicker to stand a post in the stopes with a short horn than with a long one. It necessitates the horn of the cap being out of center, which does not weaken the cap, but, generally speaking, the more symmetrical the timbers, the easier they are to put in place in the stopes. The posts and girts are round timbers, while the caps are 10 by 10 in. square. The posts are 6 ft. 7 in. in length, making the sets 7 ft. from center to center. The caps are 5 ft. 2 in. and butt end to end. The girts are 4 ft. 5 in., making the sets 4 ft. 10 in. center to center girt-way. The timber for the support of the ground in the main working levels is especially selected from the best stulls, varying in size from 14 to 23 in. in diameter. The framing is simple and inexpensive. The posts are framed on the top end only, being sized back 2½ in. and down 1½. in. to form a shoulder for the cap. Back of this shoulder both the cap and post are cut on a batter of 1½ in. to the foot, giving the post a pitch of 3 in. to 1 ft. toward the foot or hanging wall of the vein. This makes the distance between the posts on the floor of the drift 8 ft. While it might appear that the batter given the posts is too great, experience has shown such not to be the case in this mine.

The end view, Fig. 3, also shows the method of putting in the sheeting above which the stope is filled with waste rock as soon as possible after the ore has been removed. This sheeting is placed at a sufficient height above the cap of the drift set to allow for repairs when necessary. The general practice is to use small stulls for sheeting, or larger ones sawed in half. There is also shown the position of short, flat-bottomed posts and angle braces; also the chute which is used for ore or waste, and is carried up with the stopes.

Figs. 4 to 8 show the different framing in use at several of the mines before the advent of round timber.

Fig. 4 shows the framing at the High Ore mine. The post has a long and rather small horn, which is very liable to break with heavy side pressure, and with heavy pressure from the top the horn tends to crush.

With the Syndicate group (see Fig. 5) we have, perhaps, the other extreme. The horn of the post is only 1 in. in height, giving a small shoulder for both the cap and girt; besides the framing of the girt is unnecessarily complicated, which would, of course, increase the cost of timbering.

Fig. 6 shows the framing at the Anaconda group. This is, I think, the best, method of framing sawed timber for square sets. It is simple, cheap, and retains the full strength of the timbers in whatever direction the pressure may come. The horns on the posts are 6 in. square by 2 in. They are strong and give a good shoulder for the cap and girt. The caps butt end to end with a horn 6 in. square by 3 in., while the girts are framed 6 by 10 in., with a 2-in. shoulder to fit. The girts are unnecessarily large compared with the other timbers in the set, and the framing, as shown in Figs. 7 and 8, which is the same as Fig. 6 except that the girts are 4 in. less one way and require no framing at all—is just as good and less expensive.

Fig. 7 shows the framing for 10-in. square timber. The girt is 6 by 10 in., and 4 ft. 6 in. long. The sets are 5 ft. from center to center, either cap-way or girt-way, and are 7 ft. 6 in. from center to center in height.


Fig. 8 shows framing for 12-in. square timber, where the girts are 8 by 12 in. This size of timber was not used except in stopes where the ground was unusually heavy.

Timber suitable for the saw mill and available for the Butte mines became rapidly less, owing to the great and ever increasing demand from the mines. There were extensive forests of pine and fir trees growing in the mountains surrounding Butte which were large enough for mine timbering if round timbers could be used for square sets; besides, the expense of timbering, which is a large item in mining costs, would be materially decreased.

The first machine that was installed in the Rocker framing mill, which is located at Rocker, about 3 miles from Butte, where most of the timber for the Butte mines is framed, was made to cut a strictly mitered set. The post had a flat top 8 in. square, with no horn, and a miter cut to the outside of the timber. The cap had a 5-in. square horn, 4 in. on the top and bottom and 24 in. on the sides to the beginning of the miter. The girt had a horn which was 14 in. on top and bottom, while on the sides the miter extended to the end of the girt.

framing methods before advent of round timbers

This style of framing was used for some time in many of the mines, but it was not easy to set the timbers in place and it was found difficult to block the timbers so that the sets would resist the pressure of the ground satisfactorily. Also the concussion caused by blasting gave trouble.

An improvement was made in what was known as the combination square and bevel framing, shown in Fig. 10. The posts have a short horn 8 in. square by 11 in., then the miter cut to the outside of the timber. The cap framing is somewhat complicated—the horn for a length of 1½ in. is 8 by 5 in., then for 2½ in. it is 5 in. square. At the base of the horn is a 1½-in. shoulder from which the miter begins.



The girt has a horn 5 in. square by 11 in., a shoulder of 11 in., then it is mitered the same as the other members of the set. The posts are 7 ft. 5 in., making the sets 7 ft. 10 in. center to center. The caps are 5 ft. 10 in. and butt end to end. The girts are 5 ft. 5 in. These lengths make the sets 5 ft. 10 in. either cap way or girt-way. These dimensions were the same in the mitered framing.

This framing, although an improvement, was not altogether satisfactory and a further improvement was made by eliminating the miter entirely, and having nothing but square shoulders, locally known as step-down timber framing. (See Fig. 11.) The post starts with a horn 4 in. square, then a step down of 2 in., and another horn 8 in. square, another step down, and, if the post is large enough, another horn 12 in. square. The cap is framed exactly like the post, and in addition one side is slabbed off 5 in. from the center of the stick to allow for a level floor for the stopes. The girt starts with a horn 4 by 8 in., then a step down and a horn 8 by 12 in., but the girt is the smallest member of the set and seldom reaches 10 in. in diameter. The size of the square set was changed as well as the style of the framing. The posts are 7 ft. 5 in., making the sets 7 ft. 9 in. from center to center. The caps are 5 ft. 4 in. and butt end to end. The girts are 5 ft. These lengths make the sets 5 ft. 4 in. from center to center either cap-way or girt-way. This change was made for two reasons : First, 5 ft. 4 in. square is large enough for most of our stopes, as the ground is frequently heavy; second, the common length for pole lagging, stulls and 2-in. plank, which is sometimes used for lagging as well as for flooring in the stopes, is 16 ft. and three pieces 5 ft. 4 in. long can be cut from a 16-ft. stick without waste.

This is, I think, the best framing for round timbers of various sizes. We obtain, as nearly as possible, the full strength of each member of the set regardless of the size of the stull. If the stulls were all of the same diameter we could have a more simple framing, somewhat similar to that shown in Figs. 6 and 7 for square timber. For the main working levels the timbers are especially selected, generally from 12 to 18 in. in diameter for posts and caps, the girts being smaller. The same framing is used, except the posts are framed on one end only. It is the intention ultimately to use this method of framing in all of the Anaconda Copper Mining Co.’s mines.

The framer used, shown in Figs. 12 and 13, is a new type made by Greenlee Bros. & Co., Rockford, III., from specifications and drawings prepared by the Anaconda Copper Mining Co. The first machine has been in operation only a few months, while the second one has just been installed. The framing is done by saws with insert teeth mounted to form a cutter head on a horizontal arbor. The round timbers are stepped down by a series of 2-in. shoulders. The cutter heads are made up to form a series of faces 2 in. wide with saws stepping down 4 in. in diameter each time. Each of these faces of the cutter head is made up of three saws equipped with insert teeth 0.75 in. wide, staggered 0.5 in. ahead of one another, and with the teeth overlapped so that they cut a face on the timbers just 2 in. wide. There are two arbors carrying the cutter heads at each end of the machine, one above the other. As the stull is fed through the machine it is completely framed on the top and bottom sides. The stick is turned

machine for framing mine timbers

90° and again fed through the machine to complete the framing. Over the saws are hoods connected with suction fans which take the saw-dust and shavings to the fire boxes of the boilers.

back filling system of mines

It might be interesting to describe a system of mining known locally as the “back-filling system,” although it does not come under the head of timbering, but rather of mining without timbers. It is made use of whenever the physical conditions of the vein and wall rock are such that it is safe to do so. Both the ore and country rock must be hard and free from faulting planes. Fig. 14 gives a general view of this system of mining.

After a sill has been driven on the vein, a raise is put through to the level above. The ore on the first floor is then mined, and chutes started about every 25 ft. along the drift. Between the chutes, sheeting, composed of small stulls cut in half and resting on timbers a few feet above the drift sets, is placed from the foot to the hanging wall of the vein.

Starting from the raise the miners begin to break the ore. After the ore has been broken from foot to hanging for a short distance, the miners return, set up their machine on the broken ore, and again work their way forward. This is done two or three times until the broken ore is 15 or 20 ft. high, then the miners start again farther ahead and the shovelers begin to shovel the broken ore, first into the raise chute, until they reach the next ore chute. As soon as they have passed the ore chute, a temporary opening is made in the raise chute and waste is dumped into the chute from the level above and allowed to run on to the sheeting as long as it will; then a track is started about 6 ft. from the top of the slope, the waste is taken from the chute in a car and the filling of the stope continued to the first ore chute. The shovelers being now out of the way, the timbermen build up the ore chute one compartment wide and two in length, using small round stulls, 6 to 8 in. in diameter, to the level of the waste track. The waste-men then continue tilling the stope, following the shovelers.

When the miners have gone as far as the slope is to be worked, they return and start another floor on top of the waste filling, which has been covered with a floor of 2-in. plank to keep the ore clean.

If at any time the ground changes and gets heavy, square setting can be started on the waste filling, and this is generally done when the stope has been worked out to within a floor or two of the level above.

For particulars regarding square-set timbering reference is here made to a series of articles by Claude T. Rice. These articles were largely made up from data furnished by the author of this paper.

timbering in underground mining tunnels & galleries