7. For overcharged and undercharged minesin which the L. R. R. and crater radius differ materially in length the results deduced from the preceding equations are not applicable. For such mines the following equations, due to Gumpertz and Lebrun, are in common use, viz.:
For an overcharged mine,
For an undercharged mine,
In whichC= charge of explosive in pounds,l= L. L. R. in yards,r= crater radius in yards,C´= amount of explosive in pounds necessary to throw out one cubic yard of earth in a common mine in the same soil.
These formulæ are deduced as follows, viz.:
It was found by experiments made independently by Belidor and Marescot that 3660 lbs. of powder in a mine with L. L. R. equal to 4 yards gave a crater with a radius of 12 yards in earth requiring for a common mine 1½ lbs. of powder per cubic yard. The charge for a common mine in the same soil with L. L. R. equal 4 yards is
Representing bylthe L. L. R. for a common mine requiring a charge of 3660 lbs., since the charges of common mines are proportional to the cubes of their lines of least resistance, we have
whence
To find from these data the relations between charges for overcharged mines, construct Figs. 2 and 2a, (Pl. XI)
Fig. (2) gives mines with crater radii of 4yand 12yand a common L. L. R. of 4y.
Divide the distance betweenAandBinto four equal parts, and assume the points of division as the extremities of the crater radii of overcharged mines, each of which exceeds the one next smaller by (¼)AB, and all corresponding to a L. L. R. of 4y.
Fig. (2a) gives common mines with lines of least resistance of 4yand 11y. Divide the distanceA´B´also into four equal parts, and assume the points of division as the extremities of the crater radii of common mines each of which exceeds the one next smaller by (¼)A´B´.
Since the charges for the common mines whose lines of least resistance are respectively 4yand 11yare identical with those of the overcharged mines whose crater radii are 4yand 12yrespectively, it is assumed that the charges for the intermediate common mines are the same as would be required to produce the corresponding intermediate overcharged mines.
The increment of the crater radius and line of least resistance of any one of these common mines is equal to7/8the increment of the crater radius of the correspondingovercharged mine; consequently the charge which gives an overcharged mine whose L. L. R. and crater radius arel´ andr´, respectively, will produce a common mine whose L. L. R.lwill be given by the equation
Since the charge for a common mine is obtained from equation (4),C=C1(11/6)l3, the charge for the overcharged mine will be
as above.
For ordinary earth and gunpowder, when L. L. R. is measured in feet, eqs. (6) and (7) become, respectively:
For an overcharged mine,
For an undercharged mine,
8.Giving tolthe same value in equations (4), (6), and (7), we have
In whichC= charge forcommon minewith L. L. R. and crater radius =l.C´ = charge foroverorundercharged minewith L. L. R. =land crater radiusr. Equations (6), (7), and (8) having been deduced from the relations existing betweenC,l, andrfor mines varying from common mines up to those in whichr= 3lmay safely be used foroverchargedmines up to thislimit.9In their applications tounderchargedmines they become uncertain whenr= (½)l; and whenr= (⅜)lthe computed charge generally produces a camouflet.
These computed charges are:
A rule of the French engineers states that a charge which will produce a common mine with L. L. R. =lwill produce a camouflet if the L. L. R. is increased to7/4l. At this depthC´ = 0.187C, and the formula gives a crater radius of25/49.
As a safe “rule of thumb,” we may assume thata charge which will give a common mine with L. L. R. = lwill give a camouflet with L. L. R. = 2l(r´ from formula = (3/7)l). Conversely,a camouflet will be produced by ⅛ of the charge which will produce a common mine.
9. Radius of Rupture.—The determination of theradius of ruptureis an important consideration in underground warfare, since, when it is known, miners may so place their chambers as to break in the galleries of the enemy without injuring their own.
As different mining galleries, however, differ from each other so much in strength to resist crushing, and as the cost of an exhaustive series of experiments to determine their relative strength would be so great both in time and money, but little well-established data exist upon which to found a rule for determining the radius of rupture.
10.The rule deduced by Gumpertz and Lebrun, however, from the material available at their time corresponds very nearly with the results of later experiments and observations, and is generally admitted as sufficiently near correct for practical use.
This rule is based upon the theory that the surface of rupture is an oblate spheroid, (Pl. XI, Fig. 3), with its axis of revolution vertical and its centre at the centre of the charge; the intersection with the surface of the groundADcoinciding with the edge of the crater. The ratio between the semi-transverse axisCFand the semi-conjugate axisCHof the generating ellipse of this assumed spheroid is the same as that between the radius of explosionCDand L. L. R.,CK. The rule is, thatthe radius of rupture in any direction is equal the corresponding radius of this spheroid.
From the conditions assumed the following values of the semi-transverse and semi-conjugate axeshandv(which are the horizontal and vertical radii of rupture) are obtained, viz.:10
For common mines these formulas give:
For six-line craters,
11.The English authorities adopt the value of7/4l´for the horizontal andl´√(2) = 1.41421l´= (7/5)l´for the vertical radius of rupture of all classes of mines. In whichl´= L. L. R. of an equivalent common mine =l+ (7/8)(r-l), etc.
Some later experiments at Chatham have given
and
12.There are other good reasons for believing that Lebrun’s value for the vertical radius is too small; but as its use leads to increasing the charges designed to produce crushing effects, the error, if it exists, is in the right direction, and justifies the use of the formula until more exact data are available.
13.No military mining operations of note have been carried on since the introduction of dynamite and other high explosives; consequently our knowledge of their value for work of this kind rests entirely upon the results obtained from experimental mines. Unfortunately but few experiments seem to have been made, and the published results of these are very meagre.
14.Two mines fired at Krems in 1873 with L. L. R. of 12 ft. in earth weighing 100 lbs. per cubic foot and charged, one with 173 lbs. gunpowder, the other with 58 lbs. dynamite (kind not stated), gave crater radii, respectively, of 12.75 and 10.25 feet. Lebrun’s formulasapplied to these give to gunpowder and dynamite the ratio 1 : 1.688.
Two powder-mines and one dynamite-mine, each of 12 ft. L. L. R., were fired at Willet’s Point in 1878. The powder-mines were each charged with 200 lbs. cannon-powder and the dynamite mine with 82 lbs. dynamite No. 1.
No. 1 powder-mine gave a crater radius of 15½ ft.
No. 2 powder-mine gave a crater radius of 15¼ ft., or a mean of 15⅜ ft.
The dynamite-mine gave a crater radius of 14½ ft.
The relative values of cannon powder and dynamite resulting from the application of the same formulas to these mines is 1 : 1.997.11
15. Choice of Explosive.—From these experimental mines it may be concluded that for forming craters in ordinary earth dynamite is not quite so efficient as double its weight of good gunpowder. For breaking up hard rock, blowing up strong masonry, and especially in demolitions where tamping is usually defective, this ratio does not hold; but the relative effect of the high explosive increases continually with the lack of tamping and the intensity of the local blow desired, until a point is reached at which the effect of gunpowder is almost imperceptible, while the high explosive does efficient work. This property of the high explosives renders them extremely valuable foruse in hasty demolitions, such as blowing up palisades or barriers, destroying guns, etc., etc.
Owing to their varying values in different conditions the choice of explosive to be used in any particular case must evidently depend upon the circumstances attending it.
In underground explosions both gunpowder and high explosives give out noxious gases which penetrate the soil, and which entering a gallery in sufficient quantity would suffocate the miners. Of these gases the carbonic oxide given off by some of the high explosives is probably the most dangerous to human life, and if mixed with the proper proportion of air forms an explosive mixture, resembling in this respect the fire-damp of the coal-mines. Whether in practical mining operations it would ever be retained in the soil in such quantities as to produce this effect remains to be seen.
Some of the high explosives, on the other hand, seem to produce relatively small quantities of noxious gases. The gases produced by gunpowder, while suffocating in their nature, have the advantage of always making their presence known by their odor.
16.For use in overcharged mines designed to break in the enemy’s galleries, the high explosives, from the violent character of their explosion and from the phenomena exhibited in submarine mining, promise to give relatively greater radii of rupture than gunpowder; but sufficient data are not available to state this positively.12
17.Beside the considerations above stated, which refer to the effects produced by the explosive when fired, there are others equally important relating to the safety and facility with which the explosive may be transported, handled, and placed in the mines. The latter will frequently have greater weight than the former in determining the explosive to be used in any particular case which may arise in the practical operations of mining. Of the latter considerations some of the most important in deciding whether to use gunpowder or high explosives are the following, viz.:
Gunpowder is easily obtained, and most enlisted men are more or less familiar with its properties.
It explodes when ignited by fire.
It does not ordinarily explode when struck by a bullet.
It is injured by moisture and destroyed by thorough wetting.
It is not affected by ordinary changes of temperature.
It requires thorough tamping to produce good effects.
Many high explosives are not injured by moisture, and some are unaffected by total immersion in water.
They generally burn without detonation if ignited by flame.
Some of them do not explode when struck by a bullet. The more sensitive ones do.
The properties of some of them are materially changed by freezing.
On account of their greater strength, the same effects may be produced by smaller charges, requiring smaller chambers and cases.
By reason of the violence of their action they produce good results even if imperfectly tamped.
The last two considerations, together with the possibility of using them in wet places without protection against moisture, lessen greatly the time required to excavate, charge, and tamp a mine, and may frequently enable the one using them to anticipate an enemy using gunpowder and thus secure success, when the use of gunpowder would reverse the situation. In mining operations and in expert hands the high explosives, upon the whole, seem to cause fewer accidents than gunpowder.
18. Tools and Appliances.—The different operations of mining are carried on by the use of picks, shovels, bars, saws, axes, hammers (large and small), chisels, wheel and hand barrows, windlasses, ropes, wooden or leather buckets, gauge-sticks, mason’s levels (Pl. XI, Fig. 4), plumb-lines, candles, closed lanterns, tin pipes, rubber and canvas hose, canvas, nails, etc., etc., of the kinds in common use; and the following special tools and appliances, viz.:
TheMiner’s Pick. Smaller and lighter than the common pick. Neither its head nor its handle exceeds 2 feet in length.
TheMiner’s Shovel. Similar in shape to a common shovel, but not exceeding 2 feet in length.
ThePush Pick(Pl. XI, Fig 5), which has a lance-shaped blade about 3½ in. wide and 6 in. long attached to a handle about 2 feet long.
TheField Level(Pl. XI, Fig 6), which consists of three strips of wood about 2" × ½", arranged as shown. The stripAis 4' between centres of pins;BandCare 2', 915/16"; the angle at a = 90°. A spirit-level is inserted in pieceC, and a plumb-line attached as shown. The markings onAare used for gentle slopes, those onBfor steep ones.
The other sides ofBandCare divided into degreesof arc, the centre being at the middle point of the outside edge ofA.
TheSlope Block, which is a wooden cube used in connection with a mason’s level for fixing slopes.
Angle Templets(Pl. XI, Fig. 7), making a definite angle, used in laying out galleries.
TheMiner’s Truck or Car. A small, four-wheeled wagon with fixed axles and very short wheel-base; exterior dimensions about 20" wide, 18" to 20" high, and 30" long. Used for carrying earth through galleries, and usually hoisted up the shaft and dumped outside, replacing the buckets used in sinking the shaft.
TheMiner’s Bellows(Pl. XI, Fig. 8). A leather bag with wooden top and bottom, provided with inlet and outlet valves, from the latter of which the air is led off in pipes or hose. In using the bellows the miner stands upon the lower handles and works the bellows with his hands by the upper ones.
This is frequently replaced by a common blacksmith’s bellows or the rotary blower from a portable forge; and sometimes by an improvised air-pump, consisting of a large open cask filled with water and another smaller one, with one head removed and the other provided with outlet and inlet valves and an air-tube, inverted over the large cask, supported by a spring-pole and worked up and down by hand in the water of the lower cask.
TheMiner’s Candlestick(Pl. XI, Fig. 9), which holds a candle, and may be driven into the side or bottom of a gallery.
Miner’s Lamps(Pl. XI, Fig. 10), can be used only when the ventilation is good, as they give off more smoke than candles.
When an electric-light plant is available, incandescent lamps will be used for mining.
Earth Augers, similar to those used for boring post-holes, but of different diameters, are sometimes used for placing camouflets. Their shanks are made in short lengths, which can be joined together to allow of boring a deep hole from a narrow shaft or gallery.
19. The Dimensions of Galleries and Shaftsare determined by the use to be made of them, the necessity of ventilation, and the minimum space in which a man can work.
They are usually about as follows, viz.:
Shafts vary in size—from the smallest in which a man can work (about 2' × 4'), to any size that may be required, seldom exceeding 10' × 10'.
Great galleries are used for descent into a ditch, and when it is wished to pass cannon through them.13
Common galleries are used for descent into a ditch, and for communications. Troops can pass through them “by twos.”
Half galleries answer for general purposes of attack. They allow the miner to work freely in different positions without being cramped, but are small enough toadmit of rapid driving. Branches and small branches are driven out from the galleries to the mine-chambers, etc. They can be driven rapidly for short distances (10 to 20 feet); but when of greater length the earth is removed from them with difficulty, they are not easily ventilated, and are too small for use as communications.
20. Shaft and Gallery Linings.—In very firm soil it is sometimes practicable to drive small shafts and galleries short distances without lining them; but if these are to stand for any length of time, there is always danger of their falling in, particularly if shaken by the explosion of mines in their vicinity. When it is considered safe to use them, however, the shafts should be elliptical in plan, and the roofs of the galleries should be pointed arches. As a rule, however, both shafts and galleries should be lined. Those which are permanent in their character—as the main galleries of the countermines of a permanent work—are lined with masonry. Masonry linings may be of brick, stone, or concrete walls and arches. The smaller galleries constructed during a siege, and all the shafts and galleries of the attack, are lined with wood. Wooden linings are of two general types, known ascases, andframes and sheeting.
21.Cases(Pl. XI, Figs. 11 and 12) are made of plank, from 6" to 12" wide, each case consisting of a cap-sill, a ground-sill, and two stanchions. The cap and ground-sills are cut to a length equal to the clear width of the shaft or gallery plus twice the thickness of the stanchions; a rectangular notch is cut in each end to receive a corresponding tenon cut on the stanchion. The length of the stanchions between shoulders isequal to the clear length of the shaft or height of the gallery. The length of the tenons is generally equal to the thickness of the cap and ground sill (usually 2´´), and their width about three inches. Notches are cut in the sides of all the pieces of the case, as shown in the figure, for convenience in handling them.
In grand galleries the tenons at the top of the stanchions are usually shorter than the thickness of the cap-sill, and those at the bottom, as well as the mortises in the ground-sill, are omitted. The stanchions are kept from collapsing by blocks nailed to the ground-sills. These blocks are 2" thick, and wide enough (about 9´´) to so guide the wheels of a gun-carriage as to prevent the axle striking the stanchions.
In cases for smaller galleries also the tenons are sometimes omitted at the bottom of the stanchions, the mortises in the ground-sills cut an inch or two deeper, and the stanchions kept from collapsing by keys driven in the mortises (Pl. XI, Fig. 13).
22. Shaft and Gallery Frames(Pl. XI, Figs. 14, 15, and 16) are made of scantlings, halved together at the ends, as shown in the figures.Sheetingis made of boards or planks of the necessary thickness, sawn to proper lengths, and bevelled at the ends. When sawn lumber is not available, the frames may be made of saplings, and in some cases poles may be used for sheeting.
The middle of each cap and ground sill, both in frames and cases, is distinctly marked by a shallow saw cut or otherwise.
23.The following table gives the dimensions, in inches, usually adopted for the pieces of cases, frames, and sheeting, for galleries of different sizes, viz.:
The cases of branches and small branches are sometimes made very strong, with a view to resist rupture by the explosion of neighboring mines. For this purpose cases made of oak plank 4" thick are used, and the branch near its end is packed full of scantlings provided with rope-handles at their ends for withdrawing them after the mine is fired. This packing is, however, of doubtful utility, since a compression of the case sufficient to call the resistance of the packing into play is very apt to produce a permanent deformation of the cases, which will jam the scantlings and prevent their removal. For convenience in use the pieces of cases should be of uniform width.
24. Relative Advantages of Cases, and Frames and Sheeting.—In favorable soil, cases, when they can be obtained, allow of more rapid progress and give a lining with a smooth interior. In very bad soil they cannot be used for the larger galleries.
Frames and sheeting can be used in all soils which admit of mining operations, and can usually be improvised from materials found in the vicinity.
25. Sinking a Shaft by Frames and Sheeting.—Thesize and position of a shaft are usually determined by the character and direction of the gallery which is to start out from it. It is evident (Pl. XI, Fig. 17) that the clear width of the shaft must be enough greater than the outside width of the gallery to allow the side sheeting of the gallery to be freely inserted outside the frames of the gallery and inside those of the shaft; also, that the shaft frames must be so spaced as to leave a clear space at the bottom for the gallery. This space must be equal to the clear height of the gallery, increased by the thickness of the cap-sill, the sheeting, and one or two inches for easy working. This and the thickness of one frame being deducted from the depth of the shaft, the remainder may be divided up into a number of equal or unequal parts calledshaft intervals. In order that the sheeting may not yield under the pressure of the earth, these intervals seldom exceed 4 feet.
The length of the shaft must be great enough to allow the miners to work freely, and to insert the sheeting for the first gallery interval.
The sheeting for both shafts and galleries is cut in lengths about 1 foot greater than the interval between frame centres.
26.The size and position of the shaft having been fixed, the top frame (Pl. XI, Fig. 15) is placed in position and secured by pegs, and the direction of its axis is accurately fixed by the score marks at the middle of the end pieces. The side and end pieces of this frame are respectively about 3 feet longer than those of the other frames, and are so halved together as to make of their ends four projectinghorns, 1½' long, which keep the frame in place during the excavation of the first interval.
This frame is usually placed with its top flush with the surface of the ground. The miner proceeds to excavate the shaft with pick and shovel, making it large enough in plan to admit the sheeting outside the frame. Usually the first interval can be excavated without supporting the earth at the sides, which are vertical or slightly undercut, so that at the bottom of the interval the shaft will be large enough to admit the second frame, the sheeting of the first interval, and a system of wedges which hold this sheeting out from the second frame a distance somewhat greater than the thickness of the sheeting of the second interval. The verticality of the sides is determined by the plumb-line, and the size of the shaft by two gauge-sticks cut respectively to the outside length and width of the excavation, and distinctly marked at their centres.
To avoid the inconvenience of working under the top frame, the first interval is frequently marked out and excavated before the frame is fixed in its position.
When the shaft is deep enough the second frame is put in place and nailed together; the notches in the ends of the side pieces turned upward and those of the end pieces downward. The top and second frame are connected by nailing to them four battens of proper length (two on each side), which suspend the second frame from the top frame at the established interval. The second frame is placed vertically below the top frame by using the plumb-line and the scores in the frames.
The sheeting is inserted outside the top frame, bevelled end first, bevel outside, and pushed down until its top is flush with the top frame. The lower end of the sheeting is held out from the lower frame by suitablewedges, and the excavation of the second interval is commenced.
In ordinary soil the sides of the shaft will now require support. Sheeting is therefore introduced and pushed down as the excavation proceeds, and the wedges previously placed are removed to make room for the sheeting.
27.If the pressure of the earth becomes great enough to spring the sheeting-planks inward, anauxiliary frameis introduced. This is a frame similar to the shaft frames, but from 4 to 6 inches larger in outside dimensions.
The sheeting rests directly against the outside of this frame, and is thus held out far enough to allow the third frame to be placed and the wedges to be inserted as before.
The auxiliary frame is then removed and used in the next interval.
28.Successive frames are placed in the same manner until the one directly over the gallery is reached. Great care is taken to place this frame at exactly the right height, and the shaft is then continued to the required depth. A frame is placed at the bottom with its top at the level of the floor of the gallery, and the sheeting is allowed to rest directly against the outside of this frame. When the soil will allow it, the sheeting is omitted wholly or in part over the portion of the shaft which is to form the gallery entrance.
29. Precautions.—In sinking shafts especial care must be taken to make the excavation no larger than is required for placing the lining, since if a vacant space is left outside the lining the sides of the shaft may give way through its entire height, and fall against the lining with a blow which will crush it in.
This is often the cause of fatal accidents both in shafts and galleries.
30. Partly-lined Shafts, i.e., those in which the sheeting-planks are separated from each other by greater or less intervals, should only be used for small depths and when they are expected to stand for a very short time.
They are a constant menace to the miners, owing to the danger of their caving in, and in a much greater degree to the probability of stones, etc., falling from the unprotected parts and seriously injuring or killing the men at the bottom.
31. Driving a Gallery with Frames and Sheeting.—The direction of the gallery has already been marked by the scores on the shaft frames; but it must be verified by plumb-lines, and two small pickets be driven on the line of its axis, which is located exactly by small nails, one driven in the head of each picket.
Two gauge-rods are prepared, giving the extreme height and breadth of the excavation, i.e., the height of the frame plus two thicknesses of top sheeting, and the breadth of the frame plus four thicknesses of side sheeting. The middle of each gauge-rod is also plainly marked.
A gallery frame is set up against the side of the shaft (Pl. XI, Fig. 17), its ground-sill flush with the bottom frame of the shaft; or its stanchions may rest upon the shaft frame as a ground-sill.
The gallery frame is carefully located and fastened in position with battens and braces. The shaft sheeting is then forced down two or three inches with a bar, and the top sheeting of the gallery inserted and driven in until its end is supported by the earth. It is given theproper upward pitch by a scantling laid across it and secured to the shaft frames. The shaft sheeting is forced further down, the earth at the top excavated, and the top gallery sheeting advanced. As this work proceeds the side sheeting-planks are successively inserted and driven forward.
In this way the gallery is advanced one gallery interval, usually about 4 feet, when a second frame is placed. Its position is verified by the score marks; for direction, by a line; for grade, by a spirit, mason’s, or field level; and for verticality, by a plumb-line. It is then secured in place by nailing battens to it and the preceding frame. Wedges are inserted between the frame and the sheeting, and the gallery is continued by the same methods (Pl. XII, Fig. 19). When the sheeting is advanced only by hard driving the frames are slightly inclined to the rear at first, and are afterwards driven forward until vertical.
32.If, while advancing the sheeting, the pressure upon it becomes so great as to spring it, afalse frame(Pl. XII, Fig. 18) must be used. This consists of a cap-sill, ground-sill, and two stanchions, connected by mortises and tenons. The stanchions have tenons and the sills mortises at each end. The cap-sill is usually rounded on top and, for facility in setting up and removing, its mortises are longer than the width of the tenons. The latter are held in place by wedges when the frame is in position. The false frame is usually made of the same height as the common frames and, when side sheeting is used, wider by twice the thickness of this sheeting. When side sheeting is not used, its outside width may be equal to the clear width of the gallery.
In using the frame (Pl. XII, Fig. 19) the ground-sill isfirst placed accurately in position at a half interval in advance, the stanchions are set up, and the cap-sill placed upon them and wedged. The whole frame is then raised about 2 inches by folding wedges placed under each end of the ground-sill, and is secured by battens. The sheeting will now rest directly upon the cap-sill and stanchions, and have the proper inclination to clear the next frame by its own thickness, as is required.
The next frame is then set up, the wedges driven under the sheeting, and the false frame removed; which is easily done, owing to its construction.
33.When the soil is very bad ashield(Pl. XII, Fig. 20) is used to prevent the earth in front and above from caving into the gallery. In starting out from the shaft the following method is adopted: As soon as the top sheeting is sufficiently advanced and the shaft sheeting is forced down about 1 foot, the top plank of the side sheeting is inserted and driven forward about 2 feet, and the earth at the top of the gallery is excavated for from 6 inches to 1 foot in advance. A piece of plank a foot wide and in length equal to the width of the gallery is then placed directly under the top sheeting and against the face of the excavation, and is held in place by braces at its ends secured to the shaft lining. The shaft sheeting is then lowered another foot, the next plank of the side sheeting inserted, the earth excavated, and a second plank of the shield placed in the same way as before. This is continued until the entire face is covered. The top and side sheeting are then driven forward, and the top plank of the shield is removed and replaced in advance; after which each plank is removed and replaced in succession, as above described.
34. Inclined Galleries.—Method of fixing the slope.—Ifthe gallery instead of being horizontal isascending(Pl. XII, Fig. 21) ordescending(Fig. 22), the proper slope is obtained by using aslope-blockwhose edge is equal to the rise or fall of the gallery in one interval. This is placed upon the lower of two consecutive ground-sills, and the proper height of the other is determined by a mason’s or spirit level (Fig. 21). If a field-level or a mason’s level properly marked for the slope is used, the slope-block may be dispensed with (Fig. 22).
35. Position of Frames.—In drivingdescending galleriesbetter progress will be made and less material used if the frames are set at right angles to the axis of the gallery (Pl. XII, Fig. 22); and this is the usual custom. In drivingascending galleriesthis is impracticable, and the frames are set vertically (Fig. 21). In all other respects inclined galleries are driven in the same manner as horizontal ones.
36. Partly-lined Galleries.—In very firm soil side sheeting may be omitted entirely, and in that less firm the side planks need not be in contact. When the side sheeting is omitted the width of excavation may be reduced to the clear width of the gallery, and the stanchions be let into the side wall flush with its surface. In this case the ground-sills are frequently omitted, the stanchions resting upon wooden blocks, stones,or directly upon the earth.
To save material, the planks of the top sheeting are sometimes more or less separated also. This can only be recommended when rapid and temporary work is required with limited materials; and in these cases the earth between the planks should be supported by a packing of sticks, brush, etc., etc.
37. Change of Direction in Galleries.—In changingthe direction of a gallery, the new direction is laid off by using a carefully made angle-templet (Pl. XI, Fig. 7) or slope-block, field-level, etc., and it is prolonged in the new direction by the methods already described. When the soil is firm enough to stand safely while excavating and lining one gallery interval, even if somewhat short, no difficulty exists in changing the direction of a gallery in either a vertical or horizontal plane, since the excavation in the new direction may be made so large that the miner working in it can place the new frames and introduce the sheeting and wedges. The gallery can then be carried on without diminution in size.
When the soil is bad, however, special arrangements must be made for introducing the sheeting.
38. Change of Slope.—To pass from a horizontal to anascendinggallery (Pl. XII, Fig. 21) it is only necessary to give the top sheeting the proper angle by holding down its back end with a piece of scantling placed across the gallery for that purpose; and, to give the side sheeting the proper inclination, cutting trenches in the bottom of the gallery for the lower pieces, if necessary.
In passing from a horizontal to adescendinggallery (Pl. XII, Fig. 22) the roof may be carried forward horizontally, and the floor given the desired pitch by increasing the height of the consecutive frames, until enough head-room is obtained to allow the top sheeting for the descending gallery to be inserted at the proper height and angle. The frame at this point is made with a cap-sill (upon which the sheeting rests directly), and a second cross-piece below it, serving as a cap-sill for the descending gallery. From this point forward the frames may be set perpendicular to the axis of the gallery, as previously stated.
If the descending gallery is very steep and the horizontal pressure of the soil great, it may be necessary to strengthen the stanchions of the last two or three vertical frames by cross-pieces near their upper ends.
39. Change of Direction Horizontally.—Slight changes of direction of narrow galleries, either to right or left, may be made in a manner entirely similar to that above described for descending galleries, by widening the frames until the side sheeting can be inserted at the required angle, and strengthening the cap-sills, when necessary, with additional stanchions.
When the gallery is wide or the changes of direction abrupt, however, it is customary to drive the gallery entirely beyond the turning-point, and then break out a gallery in the new direction from the side of the original gallery.
40. Returns.—A gallery starting out from the side of another is called areturn, and isrectangularorobliqueaccording to the angle made by its axis with that of the original gallery, which is called thegallery of departure.
That the return may be broken out, the interval between the frames of the gallery of departure at this point must be such as to admit between the stanchions a frame and the side sheeting of the return (Pl. XII, Fig. 23). This part of the gallery of departure is called alanding, and its floor is made horizontal.
If the return is oblique (Fig. 24), its width measured along the gallery of departure will be determined by an oblique section, and may be so great that the strength of the lining of the gallery of departure will not allow the necessary length of landing. In this case a short rectangular return is first broken out from the side of the gallery of departure, and the new gallery is brokenout from the side of this return (Fig. 25). The latter method diminishes the length of the landing when the change of direction is less than 45°.
41.The floor of a return is started at the level of the floor of its landing. In firm soils which will stand for a short time without support the first frame may be set up entirely outside the gallery of departure (Pl. XII, Figs. 24 and 25) and may be of the same height in clear as this gallery. When the soil is bad, however, and side sheeting is required in the gallery of departure, the first frame of the return must be set up against this sheeting in the interval between the stanchions of the landing (Fig. 23). This makes the clear height of the return at this frame less than that of the gallery of departure by a little more than the thickness of the sheeting.
The first frame of an oblique return should be so made that the sides of the stanchions will be parallel to the side walls of the return, thus giving a good bearing to the side sheeting.
In very bad soil, the first few frames of a return must be firmly braced to resist the backward thrust of the earth, by battens connecting them together and by struts across the gallery of departure. The latter are removed when the return is sufficiently advanced.
42. A Complete Mapmust be made of every system of mines, showing the centres of shafts, and the axes and slopes of all galleries; giving also the references and lengths of all landings, and the locations, references, and dimensions of all mine chambers.
43. Working Drawingsmust also be made from which sheeting can be cut to proper lengths and angle-templets, etc., cut and framed.
These can be easily and accurately drawn by rememberingthat, to allow the miners to insert the sheeting, every return must have such dimensions outside its sheeting that, if it were free to move, its lining could be slid back across its landing as a drawer slides in a table. The size of the landings and dimensions of frames having thus been determined, the parts of the galleries between them may be divided into intervals, which, for convenience, should be equal, since this will allow the sheeting to be cut to a uniform length.
44. Sinking a Shaft with Cases.—(Pl. XII, Figs. 26 and 27.)—A case of the required size is put together and accurately placed upon the site of the shaft, whose dimensions are marked upon the ground outside it. The case is then removed and the earth excavated to the depth of the case, which is placed in the excavation with its top flush with the surface of the ground. Its position is carefully verified, and it is secured in position by packing earth around it. The excavation is then continued for the depth of another case, which is put in place as follows, viz.:
One end piece is placed in position, the tenons of the two side pieces are inserted in the mortises at its ends, and the side pieces are pushed back into position; a pocket-shaped excavation is made with a push-pick beyond the end of one of the side pieces and running back three or four inches into the side wall; the remaining end piece is inserted in this far enough to allow the mortise at its other end to slip over its corresponding tenon; it is then drawn back, and the tenons at both ends fitted into their mortises. The notches cut in the sides of the pieces allow them to be easily handled.
The next case is placed in the same way, care beingtaken not to excavate two consecutive pockets at the same angle.
When practicable, it is well to fill up these pockets by stuffing in sods from below before placing the next case.
Some miners prefer to place one tenoned piece first, then the two mortised pieces, leaving a wedge-shaped opening behind one of them, and insert the other tenoned piece last, drawing the mortised piece forward upon its tenon.
When the sides of the cases are tenoned at one end only and secured by wedges at the other, they are easily placed in position without cutting out behind them.
45.Upon reaching the level of the top of the gallery, the pieces on the gallery side of the shaft are omitted if the ground is firm, but if it needs support these pieces are put in place and secured by cleats or braces, but the tenons are not inserted in the mortises.
46. Driving a Gallery with Cases(Pl. XII, Fig. 27).—This is practicable only when the soil is somewhat firm. In breaking out from the side of the shaft, a frame is first placed inside the shaft to support the ends of the shaft cases resting against the pieces which are to be removed. The latter pieces are then taken out and grooves are cut in the earth for the ground-sill, stanchions, and cap-sill of the gallery, and these are put in place in a manner entirely analogous to that described for sinking a shaft. This case is set flush with the inside of the shaft and supports the side pieces, whose tenons rest upon its stanchions. The projecting earth is then cut away and grooves are cut for the next case, which is placed in position and the excavation continued as before.
47.When the earth shows a tendency to cave, which it frequently will in great galleries, the cap-sill must be put in position and supported while the miner excavates the grooves for the ground-sill and stanchions.
To support the cap-sill, two crutches are used. Acrutch(Pl. XII, Fig. 28) consists of an upright piece of timber carrying a cross-piece whose length is equal to the width of two cases. The upright piece rests upon the ground-sill of the case already placed, and is raised to the proper height by wedges. The part of the cross-piece which projects in advance is made 2 inches higher than the rear part, to support the cap-sill somewhat above its final level, so as to allow the tenons of the stanchions to be easily inserted. The rear part of the cross-piece is attached to the upright by an iron rod or short chain.
So soon as the case is set and adjusted to position the crutches are taken down by removing the wedges, and are replaced under the next cap-sill.
48.In very firm soil shafts and galleries are frequently driven with cases not in juxtaposition, but separated by greater or less intervals. Pieces of planks (which may be parts of cases) placed vertically and resting against the sides and ends of the cases in shafts, or horizontally and resting upon the cap-sills in galleries, and somewhat separated from each other, are used to support the earth between the cases.
The same remarks apply to this construction as to the similar one sometimes used when mining with frames and sheeting.
49. Change of Direction in Galleries Lined with Cases.—Slight changes in direction in a horizontal plane can be easily and gradually made by settingeach case a little obliquely to the one preceding it, and separating the stanchions on one side while they touch on the other, supporting the roof in the wedge-shaped openings, if necessary, with pieces of wood, etc. For an abrupt change, it is better to break out a rectangular return from the side of the gallery and pass from this into the required direction by gradual change.
If the return is to be of the same height as the gallery of departure, the cap-sills of the latter, for a distance equal to the width of the return, are lifted off the tenons of the stanchions by struts and wedges, and the first case of the return is set as in breaking out from a shaft; the ground-sill is, however, narrowed by the thickness of the stanchions of the gallery of departure so that the face of the case of the return is flush with the inside of the gallery of departure, and the ends of the cap-sills of the latter rest upon the cap-sill of the first case of the return.
50. Change of Slope.—In passing from a horizontal to adescending gallerythe change may be made gradually, in a manner similar to that described for a change in horizontal direction, and the cases remain normal to the axis of the gallery. (Pl. XII, Fig. 31.)
To pass to anascending galleryby the method above described would require the earth at the face of the gallery to be undercut in order to introduce the case, and this undercutting would be continued so long as the cases were normal to the axis of the gallery. This construction is, as a rule, impracticable. In ascending galleries, therefore, the cases are set with their stanchions vertical, while their cap and ground sills lie in the slope of the roof and floor of the gallery.
51.To conform with this requirement, and for convenience in setting up, the ends of the stanchions receive the proper bevel, while the sides of the tenons and mortises are made parallel to the sides of the stanchions. (Pl. XII, Fig. 12.)
52. Shafts à la Boule.—In order to place a charge of explosive directly under the ground occupied, or for other reasons, it is frequently necessary to sink a small shaft in the least possible time. For this purpose a modified form of cases is sometimes used, in which the ends are halved together instead of being tenoned and mortised. (Pl. XII, Fig. 29.) They are spaced at greater or less distances apart, according to the nature of the ground, and are connected together by battens. Stones, pieces of wood, etc., etc., are driven between them and the sides of the shaft to support the latter.
This construction is called a shaft à la Boule. It is expected to stand for a few days at most. Many other extemporized linings may be used for similar purposes.
53. Blinded Galleries.—Galleries cannot be successfully driven with less than 3 to 3½ feet of undisturbed earth over their sheeting. In making a descent into a ditch, or in pushing forward an approach in siege operations, it is often impracticable to lower the bottom of the trench of departure sufficiently to give the requisite cover for starting a gallery at once. In these casesblinded descentsorgalleriesmay be used, the tops and sides of which are supported byblindage-frames, orblinds, each of which consists of two side parts of 4" × 6" scantling 9' long, united by two cross-pieces of the same section 3' 8" long, which are mortised or halved into them, leaving horns at each end 1' long. (Pl. XII, Fig. 30.)
54.The galleries are constructed as follows (Pl. XII, Fig. 31): A double sap with a width of 8' is broken out from the side of the trench in the direction required and is driven forward in the usual manner, but with a continual increase in depth, at a slope not exceeding ¼. The side slopes are as steep as the earth will allow. Two blindage-frames are set up vertically on the sides of the sap, 7' apart in clear, with their tops at the level of the tops of the trench gabions, their bottom horns resting in holes dug for them. These frames are prevented from falling inward by another frame placed crosswise upon them, with its horns resting on their cross-pieces. The side of this top frame toward the front may be held up by a stake or crutch, and the second pair of frames be placed at such an interval that their horns will interlock with those of the top frame. Successive frames may be placed in the same manner. The covering or “roof” is formed by three or four layers of fascines placed across the trench on top of the frames, and covered with earth thrown back upon them as the work proceeds. The sides of the gallery are held up by fascines, etc., laid along outside the frames.
As soon as the bottom of the blinded gallery has reached the proper depth a mine-gallery may be started and carried forward.
55.The blindage-frames described above give to the gallery a clear width and height of 7'. For smaller galleries the blinds may all be made shorter and of lighter scantling; or, if desired, those for the sides may be of a different length from those for the top.
56. Rate of Advance of Galleries.—The following table gives an estimate of the men and tools requiredfor shafts and galleries, with the probable rate of advance in good soil:
57.The gases resulting from firing mines and from the lamps, bodies, and candles of the miners so vitiate the air in galleries that, unless means for ventilatingthem are adopted, the miners must eventually abandon them or become asphyxiated.
In ordinary circumstances, when no powder gases are present, a gallery cannot be driven safely more than 60 feet without ventilation.
The measures adopted for ventilating galleries consist, 1st, in forcing in fresh air; 2d, in drawing out foul air; and, 3d, in assisting the natural diffusion and circulation of the air through them.
58.The first is accomplished by forcing air through pipes, which may be of tin, wood, or common hose, leading to the point where ventilation is required. The air is forced in by the use of the miner’s bellows or other apparatus already described. This method is simple in its application and places the fresh air where it is needed, but drives the foul air back into the galleries occupied by other miners. It is the only practicable method of ventilating single, long, narrow galleries and branches.
59.The second method may be applied to a system consisting of a number of galleries connected by transversals, by so placing an exhausting fan as to draw the air out through one gallery, while by light wooden or canvas doors and screens the other galleries are so arranged that the fresh air, entering from the exterior, sweeps through the galleries occupied by the miners, and escapes through the unoccupied gallery leading to the fan, carrying the gases with it.
In this method a single large gallery may be ventilated by using a canvas partition placed near the top or on one side, so that the fresh air will go in on one side around the end of the partition and back by the other side to the fan.
This method has the advantage of carrying the gases away from the galleries occupied by the men, and supplying fresh air throughout those which are occupied.
The exhaust may be produced by a fire constantly burning at the foot of a shaft instead of by a fan.
The method is, however, complicated in its application, and can seldom be used for military mines.
60.The third method, or assisting natural ventilation, is carried out by cutting numerous cross-galleries connecting those which are near each other, by making air-shafts and bore-holes connecting the galleries with the surface of the ground, and, when practicable, by placing the openings of the shafts and galleries at different levels. This method will serve for a few men working leisurely in preparing countermines before an attack, but is entirely inadequate during active mining operations.
61.By the use of masks covering the face, and supplied with fresh air either through hose or from a reservoir of compressed air carried with him, a miner may work in galleries in which the air is irrespirable. The advantages which may frequently result from the time thus saved justify providing apparatus of this kind for use in mining operations.