The monthly advance by hand-labor was twenty-two and a-half yards. The progress is doubled by machinery, and during the past year has averaged 330 feet per month. Air, compressed by water power, is conveyed inside to give motion to chisels, which form cavities for blasting by gunpowder. The average progress per day in 1865, with the machinery, was about 9 feet.
The estimated cost was $550 per running foot, but the rate was increased to $640; the entire cost of the tunnel being estimated at $9,200,000. The use of machinery at Mt. Cenis was found to expedite the work, but at an increase of expense.
The trial of machinery at the [Hoosac tunnel], upon the Troy and Greenfield Railroad, has not been favorable to its employment. This tunnel will be four and three-quarter miles long. Originally projected with a width of 24 feet, and a height of 20 feet, it has been contracted to 14 feet wide, and 18 feet high. The estimated cost was $2,696,229. The rate first assumed was $137 per running foot. The rate per cubic yard varies from $5 to $22, and $30, for the excavation of shafts.
The contract prices for the [Hoosac tunnel], in 1869, were as follows:
| Tunnel enlargement, per yard | $ 16.00 |
| Heading enlargement, east end, per yard | 9.00 |
| Heading enlargement, west end, per yard | 9.75 |
| Full size tunnel extension, east end, per yard | 11.00 |
| ““ west end, per yard | 12.00 |
| ““ central section, per yard | 14.00 |
| Central drain, with air and water pipes complete, per lineal foot | 13.00 |
| Sinking shaft (27 × 15), per foot, depth | 395.00 |
| Pipes (10 inch), set in shaft | 6.00 |
| Arching (in brick at $9 per M), per M | 22.00 |
| Excavating and constructing 50 lineal feet of stone arch, and filling | 23,000.00 |
Although more than two hundred railroad tunnels have been constructed in the United States, and an unknown number of canal tunnels, facts in regard to them are difficult of access. Recent bids for tunnel work upon United States railroads have been offered at $5.40 per cubic yard for excavations. Canal tunnels, of the ordinary dimensions of 297 square feet area, would cost $113.20 per running foot.
The uncertainty of the nature of tunnel excavation, the unexpected difficulties to be overcome, baffle all anticipatory estimate. The variable rates in the preceding tables establish this fact. The average cost per running yard upon French canals is about $152, which sum probably includes arching. Rates of labor in the United States would increase the cost about four times this amount.
Comparing the contract price of American tunnels, as given above, with the table of English tunnels, and bearing in mind that the cost of arching is included in the latter, we find in Nos. 3, 6, and 9, the cost of English tunnels is in excess; number 3 being nearly double, and number 9 one-tenth more, while, in every other case, the cost at American rates is greater, varying from one-third to five and one-half times more.
The shale, schist, and trachyte of the Isthmean ridge is of variable consistence. Many places exhibit friable, seamy strata, disintegrating upon exposure to the atmosphere. A tunnel of the dimensions to admit the passage of ships, when carried through rock of this character, will require a lining of masonry to prevent falling material from obstructing the way.
To pass ships with the topmast struck, the intrados of the arch should be 100 feet above the surface of the water. A semi-ellipse with semi-transverse, and conjugate diameters of 100 feet, added to the canal prism of thirty feet in depth, will give an area of tunnel equal to 10,104 superficial feet, or to 1,976,263 cubic yards per mile.