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.
Fig. 58.
Fig. 59.
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.
| Name of Tunnel. | Length in feet. | Time in days. | Average daily advance, in feet. |
|---|---|---|---|
| *Penn Railroad, | 3,612 | 697 | 5.18 |
| *Kingwood B. & O. R. R. | 4,100 | 750 | 5.47 |
| Board Tree B. & O. R. R. | 2,250 | 675 | 3.32 |
| *Welling, B. & O. R. R. | 1,240 | 524 | 2.37 |
| Pacific Railroad, | 700 | 210 | 3.33 |
| Pittsburgh and Connelsville, (estimated) | 4,500 | 810 | 5.56 |
| General average daily advance, in feet, | 4.205 | ||
| Those marked * being for a double track. | |||
The following table also gives the time and cost of other tunnels in different parts of the world.
| Name and location of tunnel. | Material. | Length in feet. | Time in days. | Daily average in feet. | Section. | Cost per foot. |
|---|---|---|---|---|---|---|
| $ | ||||||
| Nerthe, France, | Hard limestone | 15,153 | 29½ × 26¼ | |||
| Riqueral, France, | Chalk | 18,623 | 2,139 | 8.7 | 26¼ × 26¼ | 39.89 |
| Pouilly, France, | Chalk & clay | 10,928 | 2,504 | 4.4 | 20⅓ × 20⅓ | 113.96 |
| Arscherville, France, | 7,348 | 1,878 | 3.9 | 26¼ × 26¼ | 68.38 | |
| Maurage, France, | 15,752 | 2,085 | 7.5 | 25½ × 25½ | 94.43 | |
| Rolleboise, France, | Chalk | 8,670 | 626 | 13.9 | 25 × 25 | 62.98 |
| Roule, France, | 5,645 | 522 | 10.8 | 25 × 25 | 62.98 | |
| Lioran, France, | 4,548 | 2,087 | 2.2 | 21⅓ × 21⅓ | 56.98 | |
| Kilsby, England, | Clay and sand | 7,233 | 1,252 | 5.8 | 27 × 23½ | 194.31 |
| Belchingly, England, | Blue clay | 3,972 | 626 | 6.3 | 24 × 25 | 102.86 |
| Thames & Medway, Eng’d, | Chalk | 11,880 | 939 | 12.6 | 30 × 38⅔ | 45.59 |
| Box, England, | Marble, freestone and marl | 9,680 | 1,252 | 7.7 | 35 × 39 | 148.15 |
| Harecastle, England, | Rock and sand | 8,778 | 939 | 9.3 | 14 × 16 | 57.05 |
| Nochistongo, Mexico, | Clay and marl | 21,659 | 287 | 75.4 | 13¾ × 11½ | |
| Blisworth, England, | Rock and clay | 9,240 | 2,191 | 4.2 | 16½ × 18 | 23.18 |
| Sapperton, England, | Rock | 12,900 | 1,878 | 6.9 | 15 × 15 | 12.44 |
| Black Rock, U. S. | Greywacke slate | 1,932 | 19 × 17¼ | 77.18 | ||
| Blaisy, France, | Chalk and clay | 13,455 | 1,043 | 12.9 | 26¼ × 26¼ | 136.06 |
| Edge Hill, England, | Clay & freestone | 6,600 | 22 × 16 | 30.15 | ||
| Littlebourg, England, | 8,607 | 590 | 14.6 | 27½ × 24 | 129.61 | |
| Woodhead, England, | Millstone | 15,840 | 1,800 | 8.8 |
The cost per cubic yard for excavating tunnels in some places has been as follows:—
| Name. | Material. | Cost per cubic yard. |
|---|---|---|
| Blackrock, U. S. | hard greywacke slate, | $6.60 |
| Lehigh, U. S. | very hard granite, | 4.36 |
| Schuylkill, U. S. | slate, | 2.00 |
| Union, U. S. | slate, | 2.08½ |
| Blue Ridge, U. S. | ——, | 4.00 |
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.
CHAPTER VIII.
WOODEN BRIDGES.
139. Wooden bridging, owing to its cheapness and fitness for universal application, has been and is being adopted in all parts of the country. Almost any variety of form may be seen upon our railroads, and though less durable than stone or iron, it may with proper precaution be made to last a long time.