Dron's Rule.-Draw a line enclosing all surface buildings that should be protected by the shaft pillar. Make the pillar of such size that solid coal will be left in all around this line for a distance equal to one-third of the depth of the shaft.

D=s+

2d 3

(7)

in which s=diameter of circle, or side of square, in yards, at the surface. Hughes's Rule.-For the diameter of a circular pillar, or the side of a square pillar, allow 1 yd. for each yard in depth.

D=d

(8)

Central Coal Basin Rule.-In the Central Coal Basin of the United States, for shaft mines worked on the room-and-pillar method, the rule is: Leave 100 sq. ft. of coal for each foot that the shaft is deep, it being understood that a main entry of average width is driven through this pillar. If the bottom is soft, the result given by this rule is increased by one-half.

SIZE OF SHAFT PILLAR OBTAINED BY USE OF SEVERAL FORMULAS

*The seam is assumed to be 2 yd. (6 ft.) in thickness. †An allowance of 100 ft. has been made for the diameter of the circle, or side of the square, enclosing the buildings on the surface.

When using formulas 2, 3, and 4, negative results in the fractional part must be rejected, as the diameter of pillar cannot be less than the minimum diameter or side allowed by the rule. For example, it is useless to apply Andre's rule to depths less than 150 yd., Wardle's rule to depths less than 60 fath. (120 yd.), or Pamely's rule to depths less than 100 yd.

The foregoing table shows clearly that no hard-and-fast rule can be given for determining the size of shaft pillar required in any particular case. The rules stated, however, determine the size of pillar required within certain practical limits, and suited to different conditions of roof strata, and are, therefore, useful and desirable. The presence of faults or slips in the roof makes larger pillars necessary.

Pillars in Inclined Seams.-The inclination of the seam increases the uncertainty in respect to the draw in the strata overlying the seam, making it more difficult to give any rule of universal application. The general practice in regard to the size of pillar required when the seam is inclined, is to increase the pillar on the rise side of the shaft, while that on the dip side of the shaft is often made the same as for a flat seam. To what extent it is necessary to increase the pillar on the rise side is largely a matter of experience and judgment in particular localities, and this is always the most reliable guide.

One method is to calculate the extent of the pillar on the dip side of the shaft by the rules given for flat seams, choosing for this purpose the rule that seems best suited to the conditions with respect to the character of the seam and overlying strata. The diameter of the circular pillar, or the side of a square pillar, thus obtained, will give the width of the pillar measured on the strike

of the seam, and half of this width will give the extent of the pillar measured below the shaft on the dip of the seam. Then, calling the width of the pillar D, the depth of the shaft d, and the inclination of the seam a, the extent of

D

the pillar measured on the pitch of the seam may be taken as +d sin a. This rule is arbitrary, but approximates to a certain extent the condition relative to the inclination of the seam. All the rules and formulas given for determining the sizes of pillars, both in flat and inclined seams, are only suggestive of what is required, and must always be modified according to the experience and judgment of the person in charge of the work.

PILLARS FOR MISCELLANEOUS PURPOSES

Pillars for Supporting Buildings, Etc.-Dron's rule for shaft pillars is probably the most practicable, as it provides for a given pillar of coal all around the buildings, etc., to be supported.

Reserve Pillars.-Extra large pillars of coal are often left at regular intervals in the workings; their purpose is to divide the mine into sections or districts so as to localize the effect of any squeeze that may start in one of these districts by breaking the roof at the reserve pillar. These pillars are usually equal to the width of one room and two pillars, and are formed by not driving one room as called for by the plan of the mine. They are taken out before the entry or gangway is abandoned.

Chain Pillar.-A chain pillar is usually left across the ends of a group of rooms to protect the gangway, or entry, toward which the rooms are being driven. The miners frequently drive their rooms too far and hole through into the next gangway in spite of the precautions that are taken to prevent this occurrence. To avoid the possibility of rooms being driven too far and holing through the chain pillar, a cut-off room is sometimes driven parallel to the entries or gangways. This place is driven wide enough to avoid the expense of yardage, and rooms driven from the next gangway are allowed to hole into it, thus avoiding the necessity of accurately measuring the length of the rooms and of carefully watching the miners to see that they do not exceed the limit allowed. The method also possesses the advantage of giving a regular width to the entry pillar and thereby avoiding the loss of a considerable amount of pillar coal when these entries are abandoned and their pillars drawn. When drawing back an ordinary chain pillar, any irregularity in the width of the pillar may cause a loss of some of the coal, which cannot occur when a cut-off room is driven as described.

Barrier Pillars.-The laws of some states require a pillar of coal to be left in each bed of coal worked along the line of adjoining properties, of such width, that, taken in connection with the pillar to be left by the adjacent property owner, it will be a sufficient barrier for the safety of the employes of mines on either property in case one should be abandoned and allowed to fill with water. These pillars are known as barrier pillars. The width of such pillars is determined by the engineers of the adjoining property owners and the mine inspector in whose district the properties are located.

An arbitrary rule for the width of barrier pillars, adopted by a number of coal companies and by the state mine inspectors of the anthracite districts of eastern Pennsylvania, is as follows:

Rule.-Multiply the thickness of the deposit, in feet, by 1% of the depth below drainage level, and add to this five times the thickness of the bed.

Thus, for a bed of coal 6 ft. thick and 400 ft. below drainage level, the barrier pillar will, according to this rule, be (6X400X.01)+(6X5) = 54 ft. wide.

The Bituminous Mine Law of Pennsylvania requires a thickness of 1 ft. of pillar for each 11 ft. of water head if, in the judgment of the engineer of the property and of the district mine inspector, this thickness is necessary for the safety of the persons working in the mine. The same law makes it lawful for any operator whose mine is endangered by an accumulation of water in abandoned workings located on an adjoining property, to drive a drift or entry protected by bore holes, across the barrier line, for the purpose of tapping and draining such water, and makes it unlawful for any person to attempt to, or in any way to obstruct the flow of such water to a point of drainage. The law also provides that no coal shall be mined within 50 ft. of any abandoned workings containing a dangerous accumulation of water, until such danger has been removed as described above.

Thickness Mined From Seam, in Feet

0

50

100

150

200

250

300

350

400

450

500

650

SIZE OF BARRIER PILLARS TO BE LEFT BETWEEN ADJOINING PROPERTIES*

Depth Below Water Level, in Feet

34567

15 17 18 20 21 23 24 26 27 29

30 32 33 35 36 38 39 41 42

44

45

47

48 50 51 53 54 56 57 59 60 78 80

20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76
25 28 30 33 35 38 40 43 45 48 50 53 55 58 60 63 65 68 70 73 75 78 80 83 85 88 90 93 95 98 100
30 33 36 39 42 45 48 51 54 57 60 63 66 69 72 75 78 81 84 87 90 93 96 99 102 105 108 111 114 117 120
35 39 42 46 49 53 56 60 63 67 70 74 77 81 84 88 91 95 98 102 105 109 112 116 119 123 126 130 133 137 140
8.... 40 44 48 52 56 60 64 68 72 76 80 84 88 92 96 100 104 108 112 116 120 124 128 132 136 140 144 148 152 156 160

6....

7.

45 50 54 59 63 68 72 77 81 86 90 95 99 104 108 113 117 122 126 131 135 140 144 149 153 158 162 167 171 176 180
50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 185 190 195 200
55 61 66 72 77 83 88 94 99 105 110 116 121 127 132 138 143 149 154 160 165 171 176 182 187 193 198 204 209 215 220
60 66 72 78 84 90 96 102 108 114 120 126 132 138 144 150 156 162 168 174 180 186 192 198 204 210 216 222 228 234 240
65 72 78 85 91 98 104 111 117 124 130 137 143 150 156 163 169 176 182 189 195 202 208 215 221 228 234 241 247 254 260
70 77 84 91 98 105 112 119 126 133 140 147 154 161 168 175 182 189 196 203 210 217 224 231 238 245 252 259 266 273 280
75 83 90 98 105 113 120 128 135 143 150 158 165 173 180 188 195 203 210 218 225 233 240 248 255 263 270 278 285 293 300
80 88 96 104 112 120 128 136 144 152 160 168 176 184 192 200 208 216 224 232 240 248 256 264 272 280 288 296|304|312 320
85 94 102 111 119 128 136 145 153 162 170 179 187 196 204 213 221 230 238 247 255 264 272 281 289 298 306 315 323 332 340
90 99 108 117 126 135 144 153 162 171 180 189 198 207 216 225 234 243 252 261 270 279 288 297 306 315 324 333 342 351 360
95 105 114 124 133 143 152 162 171 181 190 200 209 219 228 238 247 257 266 276 285 295 304 314 323 333 342 352 361 371 380
100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320 330 340 350 360 370 380 390 400
105 116 126 137 147 158 168 179 189 200 210 221 231 242 252 263 273 284 294 305 315 326 330 347 357 368 378 389 399 410 420
110 121 132 143 154 165 176 187 198 209 220 231 242 253 264 275 286 297 308 319 330 341 352 363 374 385 396 407 418 429 440
115 127 138 150 161 173 184 196 207 219 230 242 253 265 276 288 299 311 322 334 345 357 368 380 391 403 414 426 437 449 460
120 132 144 156 168 180 192 204 216 228 240 252 264 276 288 300 312 324 336 348 360 372 384 396 408 420 432 444 456 468 480
125 138 150 163 175 188 200 213 225 238 250 263 275 288 300 313 325 338 350 363 375 388 400 413 425 438 450 463 475 488 500

*Each adjoining owner is to leave one-half of the pillar thickness required. The formula used in this case is: (Thickness of workings X1% of depth below drainage level)+(thickness of workings X5) = width of barrier pillar

850

006

950

1,000

1,050

1,100

1,150

1,200

1,250

1,300

1,350.

.1,400

1,450

SQUEEZE AND CREEP

When the roof and floor are strong and unyielding and the pillars are insufficient to withstand the pressure thrown on them, they are filled with breaks and cracks, large pieces split off, and the pillars are finally crushed into small coal and the roof comes down. This is known as a squeeze, thrust, or crush.

When the material composing the floor or roof, or both, is soft and weak and the pillars left are too small, the weight on them causes the roof to sag or the floor to bulge upwards, or both. This result is known as a creep. The soft character of the floor or roof permits the pillars under the enormous roof pressure either to sink into the floor or to be forced into the roof, pressing out the softer material, which fills the openings. Fireclay is particularly susceptible to creep, and many of the fireclays that are hard when dry become extremely soft and plastic when moist; it is important to keep such a clay bottom dry.

The terms squeeze, thrust, crush, and creep are often incorrectly used synonymously. A squeeze and a creep may be going on at the same time. A squeeze or a creep does not generally come suddenly, but the pillars and timbers usually give evidence of the too great roof pressure by cracking and by pieces flaking off at the sides. The chipping or nicking of the pillar coal, indicating that the pillars are too small, should not be mistaken for the gradual spalling or chipping due to weathering alone. When pillars or timbers thus give evidence of increased pressure, they are sometimes said to be taking the weight. The coming of a squeeze is often first told by the departure of the rats from the affected area, as their sense of hearing is more acute than man's; next the coal begins to crack; and then the timbers split and crush.

Stopping a Squeeze.-When any sign of a squeeze appears, the pillars should be reenforced as much as possible by wooden chocks, or cribs, as here shown, and by supports of any kind that can be put up just outside of the part affected. If the action of the squeeze

is slow, some of the pillars may be removed rapidly, which will allow the top to break and thus relieve the standing pillars of part of the weight.

The treatment of a squeeze should be determined by the inspection of an accurate and complete map of the workings. If the disturbed region cannot be isolated by timbering and building strong stoppings in all the roads round about it, the trouble may often be stopped with little expense by draw

ing out some of the timber already in place, and throwing the weight on some small outlying patches of coal that can, with advantage, be sacrificed to save the roads and pillars of the district affected. In many cases, such trouble can more quickly be arrested by helping it than by trying to prevent it. When once the roof becomes unsteady and the timbers are breaking and the floor is lifting, a force is operating that cannot be stopped by artificial means; it can, however, be directed by assisting it to find relief where the least damage will be done. If the roof does not break readily, dynamite should be used at different points to start the fall. By this means, the power of the squeeze may be broken and the danger of its spreading to adjacent workings lessened. The building of large cribs to avoid the disastrous results of squeeze often acts to increase the evil rather than to diminish it, especially if the cribs are placed at points where complete settlement is desired. The cribs are not easily removed, and serve as fulcrums by which the weight is carried forwards to other points. As permanent supports for the roof, cribs are of great advantage, but care should be taken to break the roof back of them when the weight comes on, in the same manner as over entry pillars, by the use of shots placed in the roof near them. Confining a squeeze to a certain limit is a difficult, expensive, and dangerous operation, requiring the utmost skill and care in every individual engaged in the work.

The creep will continue until the excavations are filled, and the whole becomes compact enough to resist the weight. This sometimes takes many months, but it is a sure result, whether the action is fast or slow. A creep cannot be resisted unless the space from which the coal has been removed is filled with other material like culm.

Reopening a District Closed by Squeeze.-Time should be given for the complete settlement of the roof before any attempt is made to reopen a district closed by a squeeze, for if work is begun before the action of the squeeze

has wholly ceased, the movement will begin again and may extend to other parts of the mine. The work of reopening is expensive and seldom pays in thin seams unless the coal is very valuable. Where the entries are wholly closed, it is often possible to drive a new entry in the old pillars, or even across the pillars.

It is not usually economical to attempt to reopen old entries closed, or partially closed, by squeeze, as a larger amount of material must be handled, and more timber will be required than when a new opening is driven.

In the treatment of creep, it is usually better to excavate in the roof and leave the bottom undisturbed as the bottom often keeps working and fills up about as rapidly as it can be taken out.

Whenever practicable, the work of reopening can be done to better advantage by driving a pair of entries beyond the affected district, and coming back on the coal. By this means, the least affected portion of the district will be reached first and as much of the coal recovered as is found desirable; the demand for coal, however, will not always permit the adoption of this method.

FLUSHING OF CULM

In the anthracite regions of Pennsylvania, in Europe, and in South Africa, worked-out portions of the mine are now commonly filled with refuse material brought in on streams of water. This is done not only to support the roof over the workings but also to permit the recovery of the coal or the ore which would otherwise have to be left in the pillars.

Abroad, the material used for filling is very generally sand, but in Pennsylvania, the culm, or fine refuse from the breakers, washeries, etc., is commonly employed. In addition to culm, ashes from the boiler house, crushed slate, and the like are employed, either alone or, preferably, mixed with culm.

The plants for handling culm are more or less elaborate. The Dodson plant cost $7,473.42 with a capacity of flushing 119 T. a da. and the Black Diamond plant, with a daily capacity of 287 T., cost $6,280.12. They usually consist of crushers (where needed), troughs, conveyers, settling or mixing tanks, and in some instances storage tanks. Where culm alone is employed. it is usually brought from the breaker or culm bank by means of a scraper conveyer to a mixing tank, which may be anything from a simple oil barrel at a small operation to extensive wooden and concrete tanks at the larger ones, If a number of openings are being flushed at the same colliery, the mixing tank is generally set on a hill and is made of large size, pipes radiating from it to the various bore holes through which flushing is going on. Where coal is wet screened, the screenings from the breaker are generally caught in settling tanks so that the excess water flows away, the dewatered culm alone being elevated to the central mixing or distributing tank. At the Shenandoah City colliery, Shenandoah, Pa., all the waste material from the breaker is sent through the mine. The slate and screenings are brought out on separate conveyers. The screenings are dumped directly into the first of two flights of conveyers, which carry them to a distributing tower where enough water is added to flush them into the mine. The slate is carried first to a No. 3 Williams crusher where everything over 2 in. is broken and then conveyed back to be mixed with the breaker screenings. The ashes from the boiler house are run into the slate conveyer so that the lumps of clinker may be broken by the crusher and are thus mixed intimately with the culm and slate. When the breaker is not running the ashes pass to a concrete storage bin holding 1 wk.'s supply, from which they may subsequently be flushed when the breaker is running. The composition of the slush or sludge, the material used for flushing, is 50% screenings or culm, 44% slate, and 6% ashes.

The amount of water required for flushing depends on the material being flushed, the pitch of the seam and that of the pipe, and the distance to which the sludge has to be carried. At West Shenandoah, the proportion between water and screenings is made as nearly 2 to 1 as can be estimated. At this mine, the seam pitches 45° and less water is required than if the seam was flatter. At the Kohinoor colliery, 565 cu. yd. of culm were flushed daily with an expenditure of water of from 67 to 334 gal. per cu. yd. of culm; an average of 200 gal. of water per yd. of dry material. Experience has shown that from 1 to 1 lb. of water is required to flush 1 lb. of culm to level and down-hill places; 3 to 6 lb. of water to 1 lb. of culm to flush up-hill for heights varying from 10 to 100 ft. above the level of the shaft bottom. Any elevation of the pipe very materially increases the amount of water necessary. Mr. James B.