LIGHTING.

The object and necessity of a perfect lighting of the tunnel-workings during construction are so obvious that they need not be enlarged upon. Comparatively few tunnels require lighting after completion; and these are generally tunnels for passenger traffic under city streets, of which the Boston Subway is a representative American example. Considering the methods of lighting tunnels during construction, we may, for sake of convenience, chiefly, divide the means of supplying light into (1) lamps and lanterns usually burning oil; (2) coal-gas lighting; (3) acetylene gas lighting; and (4) electric lighting.

Lamps and Lanterns.

—Lamps and lanterns are commonly employed by engineers for making surveys inside the tunnel, and to light the instrument. For ranging in the center line, a convenient form of lamp consists of an oil light inclosed in glass chimney covered with sheet metal, except for a slit at the front and back through which the light shines, and on which the observer sights his instrument. To direct the operations of his rodmen the engineer usually employs a lantern, either with white or colored glass, much like the ordinary railway trainman’s lantern, which he swings according to some prearranged code of signals.

Lamps and lanterns are used by the workmen both for signaling and for lighting the workings. For signaling purposes red lanterns are usually placed to denote the presence of unexploded blasts or other points of possible danger; and colored or white lights are usually placed on the front and rear of spoil and material trains. For lighting purposes, two forms of lamps are employed, which may be somewhat crudely designated as lamps for individual use and lamps for general lighting. Individual lamps are usually of small size, and burn oil; they may be carried in front of the miner’s helmet, or be fixed to standards, which can be set up close to the work being done by each man. Miners’ safety lamps should be employed where there is danger from gas. A great variety of lamps for mining and tunneling purposes are on the market, for descriptions of which the reader is referred to the catalogues of their manufacturers.

Lamps for general lighting are always of larger size than lamps for individual use. A common form consists of a cylinder ten or twelve inches in diameter, provided with a hook or bail for suspension, and filled with benzine, gasolene, or other similar oil. Connected with this cylinder is a pipe of considerable length and small diameter through which the benzine or gasolene vapor runs, and burns when lighted with a brilliant flame. Lamps of this type burning gasolene were extensively employed in building the Croton Aqueduct tunnel. Various patented forms of lamps for burning coal-oil products are on the market, for descriptions of which the manufacturers’ catalogues may be consulted.

Coal-gas Lighting.

—A common method of lighting tunnel workings is by piping coal-gas into the headings and drifts from some nearby permanent gas plant, or from a special gas works constructed especially for the work. Gas lighting has the great advantage over lamps and lanterns of giving a light which is more brilliant and steady. Its great objection is the danger of explosion caused by leaks in the pipes, by breaks caused by flying fragments of rock, and by the carelessness of workmen who neglect to turn off completely the burners when they extinguish the lights. In nearly every tunnel where gas has been used for lighting, the records of the work show the occurrence of accidents which have sometimes been very serious, particularly when fire has been communicated to the tunnel timbering.

Acetylene Gas Lighting.

—The comparatively recent development of acetylene gas manufactured from carbide of calcium has given little opportunity for its use in tunnel lighting, and the only instance of its use in the United States, so far as the author knows, is the water-works tunnel conduit for the city of Washington, D. C. Col. A. M. Miller, U. S. Engineer Corps, who is in charge of this work, describes the method adopted in his annual report for 1899 as follows:—

“It had been the practice to do all work underground by the light of miners’ lamps and torches. This means of illumination is very poor for mechanical work. The fumes and smoke from blasting, added to the smoke from torches and lamps, render the atmosphere underground, especially when the barometer conditions were unfavorable to ventilation, very offensive and discomforting to the workmen. An investigation of the subject of lighting the tunnel by other means, more especially at the locality where the mechanics were at work,—brick and stone masons, and the workmen on the iron lining,—resulted in the selection of acetylene gas as the most available and economical in this special emergency. Accordingly, an acetylene gas plant for 300 burners was erected at Champlain-Avenue shaft, and one for 60 lights at Foundry Branch. The engine-houses at the shafts, the head-houses, and localities in the tunnel, when required, are lighted by these plants.

“Gas pipes were carried down the Champlain-Avenue shaft and along the tunnel both in an easterly and westerly direction, with cocks for burners at proper intervals every 30 feet; and this system sufficed for illumination from Hock Creek to Harvard University, a distance of over two miles. The plant erected at Foundry Branch was in like manner utilized for the illumination from that point in both directions.

“By connecting with the stopcocks by means of a rubber hose, a movable light, chandelier, or ‘Christmas-tree’ of any required number of burners is used, thus concentrating the light in the immediate vicinity of the work, and also enabling the illumination to be carried into the cavities or ‘crow-nests,’ so called, behind the defective old lining.

“This method of illuminating has proved very satisfactory and quite economical. It is especially valuable as enabling good work to be done, and facilitating a thorough inspection of the same.”

Electric Lighting.

—By far the most perfect, and at present the most commonly employed means for lighting tunnel workings, is electricity. The light furnished by electric lamps is steady and brilliant, and does not consume oxygen or give off offensive gases. The wires are easily removed and extended, and the lamps are easily put in place and removed. About the only objection to the method is the fragility of the lamps, which are easily broken by the flying stones and the concussion produced by blasting.

CHAPTER XXV.
THE COST OF TUNNEL EXCAVATION AND THE TIME REQUIRED FOR THE WORK.

Cost.

—The cost of a tunnel will depend upon the cost of the two principal operations required in its construction, viz., the excavation of the cross section and the lining of the excavation with masonry, metal, or timber. These two operations may in turn be subdivided, in respect to expense, into cost of labor and cost of materials. It is a comparatively simple matter to calculate the cost of the building materials required to construct a tunnel; but it is very difficult to estimate with accuracy what the cost of labor will be. The reason for this is that it is impossible to foresee exactly what the conditions will be; the character of the material may change greatly as the work proceeds, increasing or decreasing the cost of excavation; water may be encountered in quantities which will materially increase the difficulties of the work, etc. Nevertheless, while accurate preliminary estimates of cost are not practicable, it is always desirable to attempt to obtain some idea of the probable expense of the work before beginning it, and the more usual means of getting at this point will be discussed here.

Two methods of estimating the cost of tunnel work are employed. The first is to calculate the probable expense of the various items of work, based upon the available data, per unit of length, and then add to this a margin of at least 10% to allow for contingencies; the second is to apply to the new work the unit cost of some previous tunnel built under substantially the same conditions. In the first method it is usual to consider the strutting and hauling as constituting a part of the work of excavation. To estimate the cost of excavation involves the consideration of three general items, viz., the excavation proper, the strutting of the walls of the excavation, and the hauling of the excavated materials and the materials of construction.

The cost of excavating the preliminary headings or drifts is greater per unit of material removed than that of excavating the enlargement of the section. The cost of bottom drifts is also always greater than that of top headings, the material penetrated remaining the same. Mr. Rziha gives the comparative unit costs of excavating drifts, headings, and enlargement of the profile as follows:—

Bottom drifts	$9.20	per	cu.	yd.
Top headings	4.80	„	„	„
Enlargement of profile	2.84	„	„	„

The cost of hauling increases with the length of the tunnel. This fact and amount of this increase are indicated by the following actual prices for the Arlberg tunnel:—

Top heading	$6.76	per	cu.	yd.,	increasing	37	cts.	per	mile
Bottom drift	7.40	„	„	„	„	26	„	„	„
Enlargement of profile	2.70	„	„	„	„	10	„	„	„

In all the prices given above, the cost of strutting and hauling is included in the cost of excavation.

The cost of excavation is not always the same for the same character of materials in different tunnels. The following figures show the prices paid for the excavation of calcareous rock in four different German tunnels:—

Berliner Nordhausen Wetzler R.R.	$1.24	per	cu.	yd.
Ofen	1.30	„	„	„
Stafflach	2.76	„	„	„
Gries	1.92	„	„	„

The method of tunneling has little influence upon the cost of the work, as shown by the following figures from tunnels excavated through calcareous rock by different methods:—

Ofen tunnel	Austrian method	$93.19	per	lin.	ft.
Dorremberg tunnel	Belgian method	86.08	„	„	„
Stafflach tunnel	English method	91.69	„	„	„

The Martha and Merten tunnels, excavated through soft ground by the Austrian and German methods respectively, cost $87.95 and $87.55 per lin. ft. respectively. In the excavation of the various sections of the tunnel for the new Croton Aqueduct in America, the following prices were paid:—

Excavation of heading	$8	to	$10.00	per	cu.	yd.
Tunnel in soft ground	8	to	9.00	„	„	„
Tunnel in rock	7	to	8.50	„	„	„
Brick masonry	10.00			„	„	„
Timber in place	$40 per M. ft. B. M.

It is the practice in America to include the work of hauling under excavation, but not to include the strutting, which is paid for separately. In some cases only the market price of the timber is paid for separately, the cost of setting up being included in the price of excavation. The writer prefers the European practice of including the total cost of timbering under excavation, since the two operations are so closely connected, and since the contractor employs the same timber over and over again. Knowing the dimensions of the several members of the strutting, it is a simple, although somewhat tedious, process to calculate the total quantity required. An idea of the quantity of timber required for strutting in soft ground may be had from the data given on [page 55]. The quantity will decrease as the cohesion of the material penetrated increases, until it becomes so small in hard rock-tunnels as to cut very little figure in the total cost.

The cost of hoisting excavated materials through shafts depends upon the depth from which it is hoisted, and upon the character of hoisting apparatus employed. The following table, showing the cost of hoisting for different lifts and by different methods, is given by Rziha, the cost being in francs per cubic meter:—

Height in Metres.	Windlass.	Horse Gins.		Steam Hoists.
Height in Metres.	Francs per Cu. M.	One Horse. Francs per Cu. M.	Two Horses. Francs per Cu. M.	Francs per Cu. M.
15	0.172	0.077	0.062	0.035
30	0.212	0.087	0.070	0.045
45	0.257	0.100	0.080	0.050
60	0.305	0.112	0.092	0.082
90	0.410	0.152	0.110	0.087
120	0.535	0.195	0.135	0.092
150	0.722	0.240	0.157	0.112

Mr. Séjourné, a French engineer, who has been connected with the construction of numerous tunnels by the Belgian method where he was in position to secure comparative figures, has given the following rules for calculating the cost of tunnels. Assuming A to represent the cost of excavating a cu. yd. in the open air, the cost of excavating the same quantity underground in driving headings will be from 9 A to 11 A, and in enlarging the profile it will be about 5 A. The cost of constructing single-track tunnels varies with the thickness of the lining, and may be calculated by the following formulas:

Without lining,	C = 5.5 A.
With roof arch only,	C = 6.4 + 6.4 A.
With lining 18 in. thick,	C = 9.4 + 7 A.
With lining 2 ft. thick,	C = 11 + 8 A.

In these formulas C is the cost per cu. yd. of excavation, including the masonry. For double-track tunnels the amounts given by the above formulas may be used by reducing them about 7¹⁄₂% or 8%.

The second method of estimating the cost of tunnel work consists in assuming as a unit the unit cost of tunnels previously excavated under similar conditions. Mr. La Dame gives the following unit prices for a number of tunnels driven through different materials:

Nature of Soil.	Tunnels, No. of	Excav. per Cu. Yd.			Cost per Lin. Ft.	Max. and Min. per Lin. Ft.
Granite-gneiss	56	$3.07	@	$3.85	$100.	$61.46	@	$190.40
Schist	39	1.38	@	1.53	75.42	43.11	@	70.68
Triassic	3	...			90.85	84.75	@	93.33
Jurassic	69	1.23	@	1.38	77.86	35.24	@	157.2
Cretaceous	34	0.61	@	0.77	59.60	27.37	@	92.25
Tertiary and modern	39	0.33	@	0.61	105.80	51.52	@	188.36

In the following table is given a list of tunnels excavated through different soils, from the most compact to very loose materials, and driven according to the various methods which have been illustrated.

DOUBLE-TRACK TUNNELS.

Name of Tunnels.	Quality of Soil.	Cost per Lin. Ft.	Method of Tunneling.
Mt. Cenis	Granitic,	$273.73	Drift.
St. Gothard	...	193.63	Heading.
Stammerich	Granitic,	157.90	English.
Stalle	Broken schist,	290.58	Austrian.
Bothenfels	Dolomite,	115.64	English.
Dorremberg	Calcareous,	86.08	Belgian.
Stafflach	Calcareous,	91.69	English.
Ofen	Calcareous,	93.19	Austrian.
Wartha	Grewack,	87.95	Austrian.
Mertin	Grewack,	87.55	German.
Schloss Matrei	Clay schist,	94.25	English.
Trietbitte	Clay and sand,	229.0	German.
Canaan	Clay-slate,	69.50	Wide heading.
Church-Hill	Clay with shells,	178.0	...
Bergen No. 1	Trap rock,	182.31	...

SINGLE-TRACK TUNNELS.

Name of Tunnels.	Quality of Soil.	Cost per Lin. Ft.		Method of Tunneling.
Mt. Cenis	Gneiss,	$82.27		Heading.
Stalletti	Granite and quartz,	62.75		Austrian.
Marein	Clay schist,	64.36		English.
Welsberg	Gravel,	165.07		Austrian.
Sancina	Clay of 1st variety,	129.40		Belgian.
Starre	Clay of 2d variety,	191.61		Belgian.
Cristina	Clay of 3d variety,	307.42		Italian.
Burk	...	83.90		Wide heading.
Brafford Ridge	...	85.33		Wide heading.
Dunbeithe	Limestone,	70.47		Wide heading.
Fergusson	Sandstone,	37.46	[16]	Wide heading.
Port Henry	Limestone,	80.00	[17]	Wide heading.
Points	Granite,	72.00	[16]	Wide heading.

[16] Are unlined.

[17] Lined with timber.

The Habas tunnel through quicksand, between Dax and Ramoux, France, cost $118.50 per lin. ft. The cost of the Boston subway was $342.40 per lin. ft. The Severn and Mersey tunnels, constructed through rock under water, cost respectively $208.38 and $263 per lin. ft. The First Thames Tunnel, driven by Brunel’s shield, cost $1661.66 per lin. ft. The Hudson River and St. Clair River tunnels, excavated through soft ground by means of shields and compressed air, cost respectively $305 and $315 per lin. ft. The Blackwall double-track tunnel under the River Thames, which is the largest tunnel ever built by the shield system, cost $600 per lin. ft.

In making estimates of the cost of projected tunnel work based on the cost of tunnels previously constructed through similar materials, it is important to keep in mind the date and location of the work used as the basis for calculations. For example, a tunnel excavated in Italy, where labor is very cheap, will cost less than one excavated in America, where labor is dear, all other conditions being the same. Other reasons for variation in cost due to difference of date and location of construction will suggest themselves, and should be taken into full consideration in estimating the cost of the new work.

Time.

—The time required to excavate a tunnel depends upon the character of the material penetrated and upon the method of work adopted. Tunnels driven through soft ground by hand require about the same time to construct as tunnels driven through hard rock by the aid of machinery. Tunnels can be driven through hard rock at about as great a speed as through soft or fissured rock, chiefly because the work of blasting is more efficient in hard rock, and because no time is required in timbering. The following table shows the average rate of progress in different parts of the tunnel excavation through both hard and soft materials in feet per month:—

Quality of Soil.	Heading.						Excavation of Shafts.						Enlargement of Profile.
Quality of Soil.	By hand.			By machine.			By hand.			By machine.			By hand.
Very loose soil	16.7	-	26.8	...			6.6	-	16.7	...			6.6	-	16.7
Loose soil	33.4	-	100	...			16.7	-	33.4	...			16.7	-	33.4
Soft rock	66.8			233.8	-	334	33.4	-	66.8	66.8	-	132.6	33.4	-	50
Hard rock	50	-	66.8	233.8	-	334	33.4	-	50	66.8	-	132.6	66.8	-	100
Very hard rock	33.4			233.8	-	334	16.7	-	33.4	66.8	-	132.6	66.8	-	100

The following tables showing the average rate of progress have been compiled from the actual records made in the tunnels named:

Name of Tunnel.	Dimensions in Feet.	Monthly Progress in Feet.	Character of Material.	Observations.
Excavation of headings by hand:
Mount Cenis	10 × 10	65.8	Schist,	Bottom drift.
Sutro	6.7 × 5.7	70.14	Quartzose,	...
St. Gothard	8.4 × 8.7	70.14	Granite,	Top heading.
Excavation of headings by machine:
Mount Cenis	10 × 10	188.7	Calcareous schist,	Bottom drift.
Sutro	8.15 × 10	227.45	Quartzose,	...
St. Gothard	8.4 × 8.7	339.45	Granite,	Top heading.
Trari	8 × 9.35	167	Gneiss,	Top heading.
Arlberg	8.35 × 9.35	474.2	Mica schist,	Bottom drift.
Palisades	16 × 7	160	Trap rock,	Top heading.
Busk	15 × 7	126	Granite,	Top heading.
Cascade	16 × 8	180	Basaltic rock,	Top heading.
Franklin	15 × 7	240	...	Top heading.

The following table shows the monthly progress of completed tunnel in feet excavated through rock:

Name of Tunnel.	Progress in Feet.	Material.	Method.
Cascade	207	Basalt,	Top heading.
Palisades	186	Trap rock,	Top heading.
Busk	190	Granite,	Top heading.
Tennessee Pass	169.5	Granite,	Top heading.

The average monthly progress in feet of excavating tunnels through treacherous ground may be quite generally assumed to be for: (1) clay of the first variety from 43.4 ft. to 60 ft.; for clay of the second variety from 33.4 ft. to 43.4 ft.; for clay of the third variety from 23.3 ft. to 33.4 ft., and for quicksand from 30 ft. to 50 ft. The monthly progress in feet made in sinking the shafts of the Hoosac and Musconetcong tunnels in America was as follows:—

Name of Tunnel.	Dimensions in Feet.	Depth in Feet.	Progress in Feet.	Character of Material.
Hoosac:
East shaft	15.4 × 27.7	1035	21.7	Mica schist.
West shaft	8 × 16	267	16.7	Gneiss.
Musconetcong:
Vertical shaft	8.35 × 16.7	113.5	100	Loose rock.
Inclined shaft	8.35 × 26	304.	32	Loose rock.

The average monthly progress of sinking shafts in treacherous soils may be assumed to be as follows: clay of first variety, 50 ft. to 75 ft; clay of second variety, 36.75 to 50 ft; clay of third variety, 23.4 ft. to 36.75 ft; quicksand, 16.7 ft. to 33.4 ft.

For the reason that the details change with the various conditions encountered in every work, all the tunnel operations have been treated in a general way, purposely avoiding to give any detail. Also the rate of progress and items of cost of tunnels have been given in a broad manner because they greatly vary in the different works. This information, however, can be easily obtained by consulting the Engineering Magazines, where are reported all the tunnel works of America and Europe, and where are given so many details which are very valuable to expert engineers in charge of similar works, but not to students and people who are looking only for general knowledge.