Handbook of Railroad Construction; For the use of American engineers. / Containing the necessary rules, tables, and formulæ for the location, construction, equipment, and management of railroads, as built in the United States.

ROCK EXCAVATION.

125. The sides of rock excavation are sometimes cut to a small slope, as one fourth or one fifth horizontal to one vertical, and sometimes cut quite perpendicularly. The earth, when it occurs, which covers the rock, is first taken out at the proper slope; a berm of one or two feet being left between the foot of the earth and the crest of the rock.

126. Rock is taken out one or two feet below grade, as well as earth, to allow the introduction of the necessary ballast.

BLASTING AND QUARRYING.

127. The most common mode of removing rock is by blasting; for this holes are drilled by steel-edged jumpers, worked either by hand or by steam. The first object in cutting a passage through rock, is to open a working face, so as to get the necessary lines of least resistance, (this line is that by which the powder finds the least opposition to a vent at right angles to the length of the drill); these lines should, if possible, be at right angles to the beds of stratification; the holes should be drilled parallel to the seams of the rock, as the powder will then lift off the strata. In working a vertical face, it may be best to blast out the lower part first, and so undermine the overhanging mass.

128. The amount of powder in different charges to produce proportional results should be as the cube of the line of least resistance; for example:—

or

and generally,

whence

129. The following charges corresponding to lines of least resistance are from the works of Sir John Burgoyne.

130. After the powdered stone is removed, the powder is placed in the lower part of the hole; after which a wad of turf, or some other light material, follows; next the tamping of powdered brick, dried clay, or something similar, and finally a stopper of wet clay, or some other firm substance. A hole is left through all, communicating with the powder by ramming the tamping around a wire; through this hole a fuse is inserted by which to light the charge. The most perfect tamping would offer a resistance as great as that by the natural rock. A great improvement upon the above method is the sand blast; the powder is put in, and the hole filled with loose, dry sand, simply poured in and settled by a gentle stirring, but not at all rammed; the explosion of the powder spreads the sand as a wedge, and causes the power of the blast to be exerted sideways. In some cases a small cone of wood has been placed (base down) in the hole with the sand, which aids very much in stopping the exit of the blast through the drill.

131. Of late years an admirable method of lighting large charges simultaneously has been employed, namely, voltaic electricity.

132. A gigantic example of the application of this method has been furnished by the English engineers in overthrowing a portion of Round Down Cliff, about two miles from Dover, (England). Two chambers, 13 × 5½ × 4½, and one 10 × 5½ × 4 feet were cut in the rock. Within these were placed fifty bags of powder, amounting in all to eight and one half tons. The charges were lighted by the voltaic system, by which operation a mass of rock (chalk) 380 × 360 × 80 feet, amounting to 400,000 cubic yards, was thrown into the sea, and by which there was estimated to have been saved nearly $40,000.

133. The following table from Colonel Pasley’s memoranda on mining, shows the capacity of different drills for powder, by weight, and also the depth of holes of different diameters, to contain one pound of powder.

134. Blasting under water has been practised to some extent, and with great success by Messrs. Maillefert and Raasloff, both in New York harbor and in the St. Lawrence River. The method is merely to explode bodies of powder upon the surface of the rock, the water itself being a sufficient source of reaction to the blast.

TUNNELLING.

135. Tunnels are driven through hills to avoid very deep cutting. When in rock of a solid nature, the roof supports itself; but when in earth or in loose rock, an artificial arched lining becomes necessary. Figs. 58 and 59 show sections in both rock and earth; the invert b b is placed in a bed of concrete. In excavating earth, a temporary roof is made use of while the work is in progress, which is afterwards replaced by an arch of brick or stone. The back of the arch must be closely wedged, grouted, and the earth well rammed in.

The great disadvantages attending the construction of tunnels are want of air, light, room, and drainage. To facilitate the latter requirement, a very light grade may be introduced; this may easily be done, as they generally occur on summits, or on the approach to summits; 1
1000 or five feet per mile is sufficient.

In working a tunnel which is upon a grade, one end naturally drains itself if the approach is taken out; the other drains the wrong way, to meet which obstacle we must resort to pumps which follow the work, keeping always in the lowest place, or by sinking a well at the shaft through which the water is raised to the surface.

The ventilation of tunnels is effected by drawing off the bad air when a fresh supply must enter.

136. In taking out the rock, the expense will depend much upon the nature and stratification of the rock encountered.

SHAFTS.

137. In tunnels of considerable length, a long time would be consumed in working from the ends only. In such cases it is customary to sink shafts at the most convenient places (the shallowest when at the proper distance,) and to commence at the bottom of these to work both ways. This operation involves considerable expense, as all draining, ventilating, and removal of excavated materials must be effected through the shaft.

In leaving openings for the exit of smoke and for admission of light in artificial arches, regard must be had to their position. They should be at the springing rather than at the crown of the arch, as they will thus less affect the strength of the masonry.

The approaches of tunnels in cities and in other places where appearance is of importance, are finished with face coping and wings.

138. Tunnels, when conducted in the most expeditious manner, require for their completion a long time. The following table shows the rate of progress upon some of the most important tunnels of America.

The following table also gives the time and cost of other tunnels in different parts of the world.

The cost per cubic yard for excavating tunnels in some places has been as follows:—

The Blue Ridge tunnel on the Virginia Central Railroad is 4,280 feet long, made for a single track, 21 × 15 feet. Lining about four feet thick. Excavation where lining is used is 26 × 23.

The Hoosac tunnel (Massachusetts) is proposed to be four and one half miles long, 23 × 22 feet section. To have two shafts eight hundred and fifty and seven hundred and fifty feet deep, and ten feet in diameter.

Artificial ventilation becomes necessary in headings over four hundred and fifty or five hundred feet in length.

The cost of the shafts of the Blechingly tunnel, (England,) ninety-seven feet deep, and ten and one half feet in diameter, cut through blue clay, and lined, was $68.44 per yard down.

The shafts of the Blaisy tunnel average five hundred feet deep, through clay and chalk and loose earth, (being lined,) cost $139.11 per yard down.

The shafts of the Black Rock tunnel, one hundred and thirty-nine feet deep, in hard slate, cost $18.72 per cubic yard.