Two-piece sets. In highly pitching beds where both the coal rib and the top rock require more support than is afforded by the one-piece set, a two-piece set (see fig. 12) may be used; this is commonly called a

"post-and-bar." One end of the collar, or bar, is placed in a suitable notch or hitch cut into the rib, while the other end, which is properly framed, is supported by a leg or post. The post or leg is set adjacent to the top rock and at the same inclination as the bed, unless the bed is over 4 inches per foot in height. Lagging is required sometimes behind the legs and generally over the bars to prevent small pieces of loose coal or rock from falling out. The spacing of the sets is usually on 5-foot centers.

Three-piece (double) timbers.-The three-piece set (see fig. 13), comprising the two legs and a collar together with the necessary wedges and lagging, is the most common type of gangway and tunnel timbering.

It is applicable to any opening where top and sides require support or where the top alone requires support and the ribs are not strong enough to carry one end of a bar, but the bottom is firm. The legs are placed in suitable hitches cut into the bottom and are braced

securely and held with "lash bars," after which the collar is placed on top of them and is wedged securely over the legs. If the roof and ribs are friable, lagging is placed behind the legs and over the collars to prevent loose material from falling out.

Four-piece (drift) timbering. The four-piece set (see fig. 14) is used for essentially the same purposes as the three-piece set but where the bottom is not firm enough to support the legs in hitches. In such

case, a mud sill is added to the three-piece set. Such sets are used commonly in drifts, slopes, and airways that cut across the strata, but only where the floor or bottom is relatively soft or likely to become soft on exposure to air or water.

When four-piece sets are placed, a trench is first dug across the opening at a point where the timber is to be set; the sill is then placed in this trench and is leveled by wedging and filling. Next, the legs are set on the sill and the collar is placed on these, as described under threepiece sets. Sills must be framed properly for the ends of the legs. 13. Q. What is the purpose of a batter on the legs of gangway timber? A. In placing gangway timber in anthracite mines, it is customary to give the legs a batter of about 22 to 4 inches per foot of height. This setting requires a shorter and therefore stronger collar, and the legs offer greater support for the ribs. As a matter of fact, often there is more pressure on the "high side" leg than on the collar of gangway timber in pitching beds.

14. Q. Where should tight wedges be placed on three- and four-piece sets! A. Wedges should be driven between the roof and the collar directly over the legs. Generally, wedging should not be done over the collar between the legs. 15. Q. Where should braces or "sprags" be placed in three- and four-piece sets! A. Braces or "sprags" are placed between timber sets to give collective longitudinal support and to prevent collapse of timber from the force of shots or derailed cars. Collar braces should be placed directly over the notch, while leg braces should be placed about 1 foot down from the collar. In all four-piece sets and in any three-piece sets where it is difficult to make hitches, a bottom sprag should be placed about 12 inches above the top of the bottom hitch or 12 inches above the sill in four-piece sets.

16. Q. What type of collar and leg joint or framing gives the best service?

A. Several types of framing joints are commonly used in anthracite mines (see fig. 15); these are rated from A (good) to C (poor).

17. Q. What should be the size of the legs in a timber set where a 12-inch collar is used?

A. Although there is no theoretical justification for it, the common practice in anthracite mines is to use legs of the same size as the collar. This is a serious waste of timber and can be justified only by the appearance of the sets and by the fact that a full bearing of the collar on the leg

is obtained without the use of a plate. In steeply pitching beds and in swelling ground with considerable side pressure where the legs take a heavy lateral pressure, the excessive size of legs can be justified. In general, 6-inch legs can withstand more pressure than a 12-inch collar, hence, ordinarily the collar should be larger and stronger than the legs. 18. Q. Is it good practice to support the ends of bars or collars in hitches cut into the ribs?

A. Yes, provided the ribs are strong enough to insure a firm support (see fig. 11); this practice provides adequate roof support without excessive cost for timber and prevents timbers from being knocked out by derailed mine cars and locomotives. Although it is somewhat expensive to install, the hitching or recessing of timber legs also into the rib of haulage roads is likely to be justified in the long run, not only because of greater safety but also for greater efficiency.

19. Q. What is good practice in gangway timbering to protect the miners ahead of the regular gangway sets?

A. A system of forepoling, using stout poles, ahead of the regular gangway sets provides safety for the miners in that part of the mine where many of the roof-fall accidents occur.

20. Q. What method of timbering is practicable where openings are driven through loose or broken ground?

A. The method shown in figure 16 is a more or less standard forepoling system used in driving drifts through broken ground with a soft bottom, while figure 17 shows the more commonly used system of forepoling. 21. Q. What method of timbering is practicable in large openings where a squeeze is anticipated?

A. Cribs consisting of layers of timber laid transversely, as in building a log house, are used where massive supports are required. They may be filled with rock or refuse, in which instance they are called filled

cribs or solid cribs. Cribs or cogs are used also to strengthen corners of pillars along haulageways, to aid in controlling the break line in pillar-extraction and longwall work, and to support a roof cavity over gangway timber sets. Figures 18 and 19 show two uses for cribs. 22. Q. What is the purpose of lagging? Is lagging intended to hold the roof in place between timber sets?

A. Lagging over collars and between legs is intended to prevent small pieces from falling out. Lagging is not effective in holding the roof in place but may give warning of roof movement between timbers.

23. Q. What precautions are advisable to prevent legs from being knocked out by derailed mine cars and locomotives?

A. Wherever ribs are strong and firm, legs should be omitted and collars or bars hitched into the ribs. Wherever legs must be used, they should be hitched preferably back into the ribs so that they do not project beyond the line of the ribs. If the ribs are weak and require lagging, neither of these methods is applicable, and the only safeguards practicable are strong sprags between sets and in some instances, espe cially on fast haulage, planks or skids spiked to the legs.