QUESTION 73. Where are short plumb posts permissible to support collars in entries?

Where the coal is firm and strong and shows no tendency to spawl; also where strong bands appear in the coal. Short posts should never be used where coal is under the influence of a squeeze as it may cause the coal and bands to spawl off. The principal advantage is one of economy; moreover, the short posts are not liable to be knocked out by derailed cars. (See fig. 12 for illustration.)

Short posts

FIGURE 12.-Crossbars supported by short plumb posts recessed into coal.
recessed into and resting on coal should be used only where the coal is firm and solid

QUESTION 74. Are lagging bars supported by crossbars effective in holding the roof in place?

Where the roof material is scaly and tends to become detached from the roof material above it and fall in relatively large pieces lagging bars or stringers are used in narrow rooms and in entries which have only a temporary life. For permanent use it is not good practice to bring excess weight in the center of a crossbar as the latter is liable to break and cause a fall of the roof material. (See fig. 13 for illustration.)

FIGURE 13.-Roof stringers supported by crossbars and props for weak or scaly roof in rooms

QUESTION 75. Where the roof is very bad and many props at the working face would interfere with getting the coal, what system of timbering is suitable?

A system of timbering called forepoling would meet this condition. (See fig. 14 for illustration.)

QUESTION 76. When a cavity occurs in the roof from a fall of material and the exposed surface of the cavity continues to drop pieces and thus enlarge the cavity, what method of treatment is practicable?

The cavity left by a fall of material may gradually become enlarged and affect the roof for a long distance, and the object

sought is to catch up the roof. This may be accomplished by erecting crossbars under the cavity and building on them a structure or cog of timber to support lagging bars, which may be props or mine ties. (See fig. 15 for illustration of one method of timbering and fig. 16 for another method.)

QUESTION 77. For a more permanent method of timbering a large cavity what method of construction is preferable?

Large cavities are sources of danger and prove expensive if not properly protected when discovered. For permanence steel H bars and posts should be preferable; the superstructure should be built of timber and the surface of the exposed cavity lagged.

The weight should be brought as near the end of the collar as possible. (See fig. 17 for illustration.)

FIGURE 17.-Timbering large cavity in roof, using props and ties for lagging. (Space over the lagging boards should be inspected at frequent intervals)

QUESTION 78. When should a place be timbered where the roof material appears to swell when exposed to the air?

Bad roof conditions often result when timbering of such roof has been deferred until it has begun to sag. Any effort to timber at that time only defers a larger job of removing the timber and taking down the loose material. Figure 18 illustrates the effect of timbering after the roof material has begun to sag. If the roof had been adequately timbered at the proper time, this material might have remained up as long as the place was in service. Guniting done promptly before weathering begins may retard or prevent swelling of the roof material.

QUESTION 79. How should props be set in place on a slope?

The props on a slope should be set a little out of a right angle with the floor, so the angle on the lower side of the prop at the floor is greater than 90°.

FIGURE 18.-Timber broken in attempt to hold roof material sprung before timber was

placed

QUESTION 80. What should be done to prevent the props on a slope from being knocked out by falling or sliding material?

A safe plan to prevent the props from being knocked out on a slope is to dig a recess in the floor in which the prop is set. (See fig. 19, which shows props set in a recess in the floor.) On mild

slopes where material is not liable to slide the props may be placed without a recess, as shown in Figure 20.

FIGURE 20.-Props set on floor without recess

QUESTION 81. What are some of the usual methods used in placing timber in rooms?

Rooms vary in width. Some are made narrow-14 to 18 feet and some are made wide-20 to 40 feet. The nature of the immediate roof in general governs the width most economical for operation, and the quantity and method of timbering needed. The immediate roof over some coal beds can be supported only by crossbars and props set close together; where this method is required in rooms, the expense of the timbering often makes it necessary to change the method of mining.

QUESTION 82. What is meant by rows of props?

When single props are set in line along the rib of the coal or along the tracks, they constitute a row of props. When the props are set along two lines parallel to the rib or track they are called a double row.

QUESTION 83. What are some of the methods of placing props in rows in a room?

The methods of placing props in rows in rooms can best be shown by diagrams, and some of the methods used are indicated in Figures 21, 22, 23, and 24. Where the track is in the center of the room, four rows of props set staggered can be used (see fig. 21). Figure 22 shows a room with the track at one side of the center and five rows of props. Figure 23 shows a room with the track near the side of the room and four rows of props.