Timbering in stopes ranges from occasional light props or stubs to temporarily support patches of unsafe ground to regular timbering with square sets and large bulkheads or “cribs” of solid timbers.
Regular stull timbering has been mentioned. Figure 94 shows this type of timbering in a narrow, steep-dipping vein. Figure 116 shows regular stub timbering in the Conglomerate mine, Calumet, Mich., where mining operations were conducted at great depths without filling in a flat-dipping lode. A retreat system of mining was employed, whereby stopes 100 feet long were worked up quickly from one level to the next, using clusters of heavy stulls for temporary support and then allowing the hanging wall to crush down on the timbers and cave. Standard practice in ore up to 20 feet thick was to use four rows of stubs in clusters of two 18-inch to 24-inch-diameter timbers alongside each other, set 7 feet apart between clusters, and in rows on 7-foot centers (fig. 116). Above the four rows, single stulls were used since the upper part of the stope did not have to remain open very long. A double row of “breaker” stulls was set along the end of each 100-foot section to confine the breakage of the hanging wall to the worked out section inby of the section being worked. The heavy timbers used in this mine had to be lifted into position by block and tackle.
Stulls are used incidentally in shrinkage stopes and either incidentally or in a systematic manner in cut-and-fill stopes, as shown in
figure 92 (stringer-set-and-fill system). In the latter instance vertical posts generally are set between the stulls, and girts or ties are used for horizontal bracing. To support weak spots in the back, occasional props are often set on the fill, with a headboard on top and under the stope back and with sill boards underneath on top of the fill. In wide stopes with a slabby back, temporary stringers supported on posts may be employed, as shown in figure 93, in which case most of the timber may be recovered and used again. Larger areas of heavy back may be supported on cribs as shown in figure 117, which shows inclined cut-and-fill stoping between lines of square-set timbers at the Campbell mine, Bisbee, Ariz. At B in this figure is also shown an umbrella stull with several laggings laid across the headboard to cover more hanging-wall area.
In square-set stoping, the timbers may be either round or sawed but in either case are framed at the ends to fit together so that, as pressure develops, the joints tighten (fig. 95, A). The framing at different mines varies considerably in detail, but there are two general types of joints—the post-butting and the cap-butting type. Figure 118 shows several typical methods of framing. At A and B are shown, respectively, sawed timbers framed with post- and cap-butting joints; at C and D are shown methods of framing round timbers; and E illustrates the Moore diagonal set, in which the posts are replaced by diagonal members, a useful method where, for any reason, a vertical post cannot be set. Gardner and Vanderburg have compared the post-butting and cap-butting types as follows:
Although the post-butting type has an advantage under some conditions and continues to be used, present practice tends toward the adoption of the cap-butting type of framing. Where both side and top pressures are encountered the cap
butting type is preferable, as the long horn on a post of a post-butting set is a point of weakness. The strength of a post is proportional to the area of cross section over which the load is distributed. If a post has an area of 12 by 12 inches and the bearing on its end is confined to a 6- by 6-inch area, the timber will begin to fail as soon by crushing as though its full cross section were only 6 by 6 inches; consequently, the post-butting joint is not as strong as the other. The pressure coming on the horn of a post has a tendency to split and spread it and in so doing may force the caps and girts from their seats on the post. Furthermore, the bearing area of the horn is small laterally, and if the horn is crushed when pressure comes upon it the sets may be forced out of alignment. These objections are largely eliminated with the cap-butting joint, because where the caps butt against each other the horn or tenon on the post is short and has a relatively large cross-sectional area; consequently, when the caps are compressed the weight is carried over the greatest cross section of the post instead of on the long, narrow horn. A joint with the post-butting also generally has the disadvantage of a relatively deep mortise at the top; this mortise often becomes filled with fine ore from blasting, so that if it is not entirely cleaned out an imperfect joint results when the next post above is placed. Moreover, miners are disposed to cut off the horn at the bottom of the post instead of cleaning out the hole to fit the horn.
In framing square-set joints a relatively larger proportion of the original timber is cut away in making the horns for a post-butting set than in forming the tenons on a cap-butting set. Consequently a little more timber is required for a set of the former type. However, as shown in table 35, fewer cuts with the saw are required in framing a typical post-butting set; this extra framing for a cap-butting set about offsets the saving in timber over the other type of set.
(Table 35 also gives other details covering types and sizes of square-set timbering employed at a number of individual mines.)
In the post-butting type of framing, the horns usually are the same length on each end of the post and both ends are framed symmetrically.
Slice timber used in top slicing and sublevel caving is required to stand only a short time and is then blasted out or allowed to crush. Therefore, timber that would not be suitable for drift sets or square-set timbers may be acceptable for slicing. Slice timber, however, should be tough and strong, yielding or crushing gradually without breaking short, and green timber is, therefore, best. Specifications of the Cleveland-Cliffs Iron Co. some 15 years ago were as follows:
Timber Specifications, Cleveland-Cliffs Iron Co.
Stull timber (posts and caps).—Hemlock, hard maple, soft maple, yellow birch, tamarack, and Norway pine. Must be sound, straight, and green. Ash, white birch, poplar, and balm of gilead not accepted. Tamarack in top diameters of 8 and 9 inches containing not more than ¼ inch sap rot in depth accepted; in diameters of 10, 11, 12, and 13 inches containing not more than ½ inch sap rot in depth. Lengths 8 and 16 feet.
Lagging.—Straight sound cedar, tamarack, and spruce (10 percent jack pine permitted). Round, 3 to 4½-inch top, larger than 4½ inches to be split. Split, not less than 2½ by 4 nor greater than 3 by 6 inches. In 5-foot lengths, 160 cubic feet per cord. In 7- and 8-foot lengths, 128 cubic feet per cord, not less than 125 pieces.
Covering-down boards.—No. 3 maple and birch. To be resawed from 2-inch maple and birch hearts to 5/8 inch. To be sound green lumber. Widths 6 inches and wider. Lengths not under 6 feet and not over 9 feet.
Since these specifications were written, exhaustion of some kinds of green timber has made it necessary to use varying proportions of dry timber. Tamarack and pine have become relatively scarce in the region.
Specifications for slice timber at three mines on the Mesabi range in 1930 were as follows:
At one mine on the Cuyuna range, specifications were as follows: Posts, 9- to 12-inch tops; caps, 12- to 15-inch tops; lagging, 5- to 7-inch tops; poles (for flooring), 3- to 5-inch tops.
Flooring may consist of mining boards (a grade below the lowest standard commercial grade) of 1-inch pine or 3/8- to 5/8-inch hardwood boards, round poles and boards, or round poles covered with wire fencing. Where heavy scrapers are used in the slices, the bottom may be covered with long poles or plank laid lengthwise. Wire fencing is used extensively instead of boards for covering the floor and sides of slices on the Lake Superior iron ranges, and the material cost is about the same as for boards per square foot covered but the cost of handling and laying is only about a quarter as much.
Slice timbers are simply drift sets either with almost vertical posts for use under shallow depths and light gob, or with a batter for use where the gob is heavy and there is considerable side pressure as well as top pressure; in sublevel caving, posts generally have a decided batter.
Data on timber consumption in stopes per ton of ore mined are given in table 36.