The American Cyclopædia (1879)/Tunnel

TUNNEL, a subterranean or subaqueous way, constructed for purposes of passage. In mining, the term is often applied to horizontal excavations, especially to such as are known by the designations gangway, heading, drift, and adit, used as underground roads or for the passage of water. (See .) Herodotus mentions a tunnel in the island of Samos, cut through a mountain 150 orygia (900 ft.) high. Its length was seven stadia (4,247 ft.), and its cross section 8 ft. high by 8 ft. wide. In Bœotia a tunnel was constructed for the drainage of Lake Copais. When Cæsar arrived at Alexandria, he found the city almost hollow underneath from the numerous aqueducts; every private dwelling had its reservoir, supplied by subterranean conduits from the Nile. The aqueducts of the ancient Romans, and of the Peruvians and Mexicans, included remarkable tunnels. (See .) Among the many Roman aqueducts on which tunnels were built were the Aqua Claudia, of which 36½ m. passed underground; the Aqua Appia, built in 312 B. C., 11,190 Roman paces in length, 11,130 being underground and arched; and the Aqua Virgo, 14,105 paces long, 12,865 underground. A tunnel was begun in 398 B. C. to tap Lake Albanus, at the instance, Livy tells us, of the oracle of Delphi. It was 6,000 ft. long, 6 ft. high, and 3½ ft. wide. Fifty shafts were sunk on its line, and the work was finished within one year, though it was driven through the hardest lava. A similar work of greater magnitude was undertaken to connect Lake Fucinus (now Celano) with the river Liris (now Garigliano); 30,000 men were employed on it for ten years, and it was finished at a vast expense A. D. 52. A minute account of the modern clearing out of this work by the Neapolitan government may be found in “Blackwood's Edinburgh Magazine,” vol. xxxviii., p. 657. The accuracy of the surveying in these works is astonishing when we consider the rudeness of the instruments. Among those used in levelling by the Romans were the libra aquaria and dioptra, of which we have no clear

description. The chorobates seems to have been preferred. It consisted simply of a rod or plank about 20 ft. long, mounted on two legs, at its extremities, of equal length. The rods or legs were secured by diagonal braces, on which were marked correctly vertical lines. A plumb line attached at each extremity, and passing over these diagonal braces, indicated whether the instrument was level. When the wind prevented the plumb bobs from remaining stationary, a channel in the upper edge of the horizontal rod was filled with water, and if the water touched equally both extremities the level was supposed to be correct; and then the observation of the descent or elevation of the ground was made with accuracy.

&mdash;Tunnelling might be classed under four general heads: 1, ancient tunnelling, to which we have just referred; 2, modern tunnelling through soft ground (clay deposit, &amp;c.) and loose rock, requiring arching; 3, modern tunnelling through solid rock before the introduction of machinery; 4, modern tunnelling through solid rock with the aid of machinery. The art of tunnelling at the present day constitutes a profession in itself, now developments succeeding each other with great rapidity. Figs. 1 and 2 show cross sections that may bo adopted in tunnelling: fig. 1 through rock tenacious enough to require no artificial support; fig. 2 where arching may be found necessary. These examples are from plans adopted in the construction of the Musconetcong tunnel, New Jersey, on the Lehigh Valley railroad extension, finished in 1875.—Tunnelling through Soft Ground. Under the designation “soft ground,” technically so called, the miner includes all such material as clay, earth deposit, &amp;c., which, if tunnelled through, requires a temporary timber arch to

hold it in place, until the permanent brick or stone arching is built. Loose rock, as its name indicates, is rock either so seamy and broken by folding or compression, or so disintegrated, as to require an arch, generally much lighter than those necessary in soft ground. According to the method generally adopted in driving a tunnel through soft ground, the first step is, if practicable, to open out a small bottom heading or adit, for the double purpose of draining the ground above and making an opening through which to carry away the material subsequently excavated; this heading also is required for passing in the materials used in arching. Often, however, owing to long and heavy cuttings necessary in the outside approaches to a tunnel, it is deemed advisable to begin with a top heading before the bottom bench of the open cut is brought up to the face of the proposed work. If a bottom heading has been driven (and it is generally best to do so when practicable in soft ground, while the opposite rule holds in tunnelling through hard rock), one of the methods of subsequent enlarging that may be used is shown in figs. 3, 4, and 5. These represent the English plan, so called, it being the one generally adopted in England. For a full description of this method of enlarging, see the “Engineering and Mining Journal,” vol. xix., p. 392; also Simms's “Treatise on the Blechingly and Saltwood Tunnels.”

Fig. 3 shows the bottom heading driven, with a section excavated and ready for arching. The enlarging and arching of a tunnel to its full size is generally done in lengths or sections. If there is no top heading previously driven, 15 or 20 ft. of an advanced heading is excavated at the top of the proposed work (shown in figs. 3 and 4). Heavy longitudinal bars of timber are then successively put in, beginning with those numbered 3, 6, and 7. The miners gradually work down, putting in a temporary arch of timber. When this is done, and foundations have been dug for the succeeding masonry, the masons take the place of the miners, and run up an

arch under the timber, which is then withdrawn during the excavation of the next section, and the spaces left are securely blocked up with pieces of timber or stone. In some methods of tunnelling, it is deemed more secure to brick the timber in and leave it in place, though at a considerable cost, especially when it is necessary to bring all the heavy timber down a shaft or slope, and through a long distance underground. Shafts are often sunk, and sometimes slopes, so that the work may be attacked from several points at once.



5.

Fig. 5 shows the arch built, and is divided into two portions: that on the left shows the completed tunnel, with the ballast in place and the track laid; that on the right shows the arch in place, and the supporting timbers struck, but still undrawn. Where the ground is very treacherous, and much water is encountered, an inverted arch is often put in across the bottom of the tunnel, to withstand the pressure from below. Other methods are in vogue on the continent of Europe. A description of a new system of tunnelling by the use of iron centres, in place of timber, devised by himself, may be found in Ržiha's work, cited below.—Tunnelling through Rock. One of the methods of

through loose rock, with subsequent timbering and arching, is shown in figs. 6 and 7; it is the one most used in America, and is expeditious, though probably more expensive than the European systems.

The timbers 1 and 2 are put in to support the roof and sides when the top heading (which is generally preferred through rock) is driven; the “legs” (2) are occasionally braced by a bar (3), which is supported by a raker (4), while the sides are being dressed down when the tunnel is enlarged and arched. The apace between the timber and the rock above, and between the masonry and the timber (which latter in this work should be left in place), is packed tight with fragments of stone, to prevent a sudden fall or stress being brought to bear on the masonry.—Tunnelling through solid rock by hand labor is still, in many cases, held to be more economical than by machinery. It is certainly so, as yet, in the case of small tunnels through a comparatively soft rock, where the necessary cost of a plant of air drills and compressors would be in excess of the economy in time gained by their use. In driving a tunnel through rock, an advanced heading is first driven either at bottom or top; and this may either be of the full width of the

proposed excavation, or narrower. The heading is always the most difficult and expensive part of the work; for whether it be driven at top or bottom, the miner, in removing the remaining portion of rock, of course has much less resistance to contend against in blasting. Removing the top rock or the lower “bench” is more like open-air quarrying. Longer holes can be drilled, and heavier charges of powder used. At the present day, however, most heavy tunnel work is carried on with the aid of machine drills, driven by compressed air, which, on being liberated after acting as a motor, serves to ventilate the work. Since the introduction of machinery, the rate of driving attained in tunnelling has been greatly increased. Machine drilling was born of the necessity for some more rapid method of executing certain works, deemed almost too heavy to be accomplished by ordinary means. These were, in Europe, the Mont Cenis tunnel (see ), and in America, the Hoosac tunnel in Massachusetts. Various types of drills have been invented and tried abroad; among them the Sommeiller, Dubois-François, Sachs, Osterkamp, Brydon Davidson and Warrington, Azolino dell' Acqua, Ferroux, McKean, and others. Among compressors that of M. Colladon of Geneva may be particularly noted. At Mont Cenis the air pumps were worked by hydraulic power. The perforators used there were built partly from designs already presented, but improved with original modifications made by the engineers in charge, Messrs. Sommeiller, Grandis, and Grattoni. A description of the Sommeiller machines may be found in the Portefeuille économique des machines (1863). The Mont Cenis tunnel was begun by hand labor in 1857, and finished in 1871, at a total cost of about $15,000,000. The following table, from M. Opperman's Nouvelles annales de la construction (1869), shows the rate of advance in that work by hand, and the increased rate attained immediately after the first introduction of machinery down to 1865, working throughout with two headings:

The St. Gothard tunnel, also through the Alps, is now (1876) in progress. From a late paper on the subject by Daniel K. Clark, M. Inst. C. E., London, we obtain the following general facts concerning it. The length of the tunnel is to be 16,295 yards or 9¼ m. The contract prices sum up to a total estimated cost of £1,896,945. Construction was begun in the

autumn of 1872, and the total progress attained (two headings) up to Aug. 31, 1875, was as follows:

The heading is driven at the top, about 8 ft. square, dynamite being used as an explosive. Dubois-François perforators were first used, making an average advance of 6.63 lineal feet a day. They were succeeded by Ferroux's, the daily advance being raised to 10.11 ft. Subsequently the machines of two or three inventors, Dubois-François, McKean, and Ferroux, were placed and worked together on the same carriage; and it is said by M. Louis Sautter, in an official report published in the Revue industrielle, Aug. 18, 1875, that the improved McKean drill has proved to be decidedly superior to any of its competitors; its best work on competition, with 6½ atmospheres of pressure, was a penetration of 12 in. a minute. While actually at work, its rate will vary from 3 to 8 in. a minute, with about 800 strokes. The power is derived from water through the agency of turbines. The cylinders or air pumps of the compressors are 18.1 in. in diameter, and the stroke is limited to 17¾ in., in order that the mean speed of piston may not exceed 266 ft., or 90 revolutions a minute, the turbine making 390 turns. The compressed air is cooled on Dr. Colladon's system; every piece that is in contact with the air when undergoing compression being cooled by currents of cold water, passed through air-tight envelopes. It is calculated that at the present rates of advance the St. Gothard tunnel may be finished during the summer of 1879, or within seven years from the date of M. Favre's contract.—In America, both North and South, many tunnels have been built, the modern ones being mostly driven since the introduction of railroads. Until the building of the Hoosac tunnel in Massachusetts, all tunnelling through rock in the United States was done by hand labor, by the methods above described. The project of tunnelling the Hoosac mountain was broached as early as 1825. In that year a board of commissioners, with Loammi Baldwin as engineer, was appointed to ascertain the practicability of making a canal from Boston to the Hudson, in the vicinity of the junction of the Erie canal with that river. Their report (“Massachusetts Commissioners' Report,” 1826, p. 141) declares that “there was no hesitation in deciding in favor of the Deerfield and Hoosac river route,” and that “there is no hesitation therefore in deciding in favor of a tunnel; but even if its expense should exceed the other mode of passing the mountain, a tunnel is preferable.” Railways being shortly after

the canal project was dropped. In 1828 surveys were made for three routes to afford Massachusetts railway connection with the west, viz., by Greenfield, by Northampton, and by Springfield. The last or southern route was chosen. The work was not begun immediately, but Massachusetts never lost sight of the advantage of a direct route to the Hudson river. This was finally accomplished in 1842, by the completion of the Western railroad to Albany. In 1848 application was made for a charter for a railroad from the terminus of the Vermont and Massachusetts line, at or near Greenfield, through the valley of the Deerfield and Hoosac, to the state line, there to unite with a railroad leading to Troy. The location was filed in the clerk's office of Franklin and Berkshire counties in November, 1850. In 1854 an act was passed “to enable the Troy and Greenfield railroad company to construct the Hoosac tunnel,” by which the state, on certain conditions, lent its credit to the amount of $2,000,000. The estimated cost of the proposed double-track tunnel was $1,948,557, and of the road and equipment $1,401,443; total, $3,350,000. Still the company were unable to raise the funds necessary, in addition to the state loan. In 1855 a contract was made with E. W. Serrel and co., under which some work was done; and another was made with them in 1856 for the construction of the road and tunnel for $3,500,000, they subscribing $440,000. This contract also fell through, as did one made with H. Haupt and co. in the same year, by which the railroad company agreed to pay $3,880,000 for the completion of the road and tunnel. In 1858 a contract was again made with H. Haupt and co., by which the contractors themselves agreed “to assume the labor of collecting subscriptions and of carrying on and completing the Troy and Greenfield railroad and the Hoosac tunnel.” Under this contract H. Haupt and co. were to receive $2,000,000 in bonds of the state of Massachusetts, to be exclusively appropriated to work done on the tunnel; $900,000 in mortgage bonds of the company; and $1,100,000 in cash, through cash subscriptions and capital stock of the company. Under this contract the work was vigorously prosecuted up to July, 1861, when, a difference arising between the contractors and the state engineer, a certificate for the amount claimed by the former on a payment was refused, and the work was thereupon abandoned by them. In 1862 an act passed the Massachusetts legislature, providing “for the more speedy completion of the Troy and Greenfield railroad and Hoosac tunnel.” Under this act a board of commissioners was appointed to examine into the matter on the part of the state. At the request of these commissioners, the Troy and Greenfield railroad company, acting under the authority of certain provisions of the act, surrendered to the commonwealth of Massachusetts, under the several mortgages held by said

the road and property of the company; such surrender having been authorized by the board of directors, by a vote passed on Aug. 18, 1862. This action was ratified by a vote of the stockholders, and on Sept. 4, 1862, the commissioners took possession of the road and its property. The commission after a full examination made a thorough report (dated Feb. 28, 1863), embracing the three following most valuable sub-reports: 1, a report of Charles E. Storrow on European tunnels; 2, a report by Benjamin H. Latrobe on the Hoosac tunnel; 3, a report by James Laurie on the Hoosac tunnel and the Troy and Greenfield railroad. In conclusion the commissioners recommended that the work should be undertaken by the commonwealth. At this point the cost and estimates were as follows:

At this time, according to the report of James Laurie above noted, the condition of the work proper was as follows:

The shaft here referred to was on the western slope of the mountain, 325 ft. in depth. Mr. Laurie estimated that by sinking a central shaft about 1,000 ft. deep and working therefrom (which was afterward done) the tunnel, advancing at the rates respectively of 55 ft. a month from the two end portals, and 40 ft. each way from the shaft, would be completed in 11 years from date, i. e., in 1874; this estimate being based on the supposition that the central shaft would reach bottom in four years from its commencement. Work was resumed on the tunnel under the auspices of the state in October, 1863, under the control of the same board of commissioners, who appointed Thomas Doane chief engineer in charge. The governor at the same time appointed Benjamin H. Latrobe of Baltimore state consulting engineer of Hoosac tunnel.—Mr. Laurie in his report to the commissioners says that shortly after the Troy and Greenfield railroad was chartered, the attention of inventors was turned to the subject of tunnelling machines. One was constructed at South Boston in 1851, especially for the Hoosac tunnel, which weighed about 70 tons, and was designed to cut out a groove around the circumference of the tunnel 13 in. wide and 24 ft. in diameter, by means of revolving cutters; the central core left was

to be subsequently blasted out with gunpowder. It is reported to have cut, on a trial made March 16, 1853, on a vertical face of rock near the proposed entrance of the tunnel, at the rate of 16½ in. an hour, and under more favorable conditions at a previous trial 20 in. an hour. Various trials were made with this machine, the total distance cut by it amounting to about 10 ft., but it did not prove successful. A second machine constructed at Hartford, and known as the “Talbot tunnelling machine,” also working on the principle of revolving cutters, and adapted to cut out a core 17 ft. in diameter, was tried about this time near Harlem, but proved a failure. A third machine was constructed in New York, adapted to cut a core of 8 ft.; this was adopted by Mr. Haupt during the continuance of his contract, in the early days of the tunnel, but also proved a failure. Experiments were instituted by Mr. Haupt himself, while engaged with his contract at Hoosac, toward the elaboration of a percussion drill; but in 1861 the termination of his contract for a time put an end to them. Afterward he again took up the subject, and in 1867 published a description of the Haupt drill. By the time this invention had been perfected, the Burleigh drills, which have since attained so great a reputation (see ), had been adopted and were in full use at Hoosac. They were first tried in June, 1866, under the direction of the commissioners, and even in their crude and unimproved condition were favorably noticed in Chief Engineer Doane's report. In January, 1867, the office of chief engineer was abolished, and the engineer corps reduced to one resident engineer, W. P. Granger; Mr. Latrobe still supervising as consulting engineer. In October, 1867, owing to the accidental lighting of some naphtha at the central shaft, the head house, shaft buildings, &amp;c., were consumed, and 13 lives were lost. Previous to this time portions of the work had been let out by contract, Messrs. Dull, Gowan, and White having the east and central shaft headings, through rock, and Mr. B. N. Farren the west end, through soft ground, including the arching of the same. Owing to the above mentioned accident, Messrs. Dull, Gowan, and White voluntarily surrendered their contract, received their pay, and abandoned the work, returning it to the hands of the commissioners. Benjamin D. Frost was appointed superintending engineer in May, 1868, and on Dec. 24 of that year a contract was effected between Messrs. Shanly brothers of Montreal and the commonwealth of Massachusetts for the final completion in full of Hoosac tunnel. The dimensions were to be: “in rock, unarched, 24 ft. wide and 20 ft. high, in the clear; where arching required, 26 ft. wide and 24½ ft. high (above the rail), in the clear.” The prices bid in the contract varied in the different portions of the work, and also according to whether the work was “already begun,” “to be finished,” or for

“extension of full-sized tunnel.” The bids accepted for the latter item were as follows: east end section, per cubic yard, $11; central section from shaft, $14; west end section (part soft ground), $12; for arching part of the tunnel with brick, per thousand of bricks laid, $22. The total price agreed on for the work specified by the contract was $4,594,268, the whole to be done by March 1, 1874. At this time Mr. Latrobe resigned as consulting engineer; and that post, after the successive resignations of James Laurie and Edward S. Philbrick of Boston, is now (1876) held by Thomas Doane. The work was vigorously attacked by the Messrs. Shanly at all points. The Burleigh drills and compressors were used throughout their contract with excellent results. Under their patronage, the manufacture of nitro-glycerine (previously used in the tunnel) was carried on and improved by George M. Mowbray of North Adams. The east heading met the one driven east from the central shaft on Dec. 12, 1872; the west heading met the one driven west from the shaft on Nov. 27, 1873; the errors in alignment and levels were astonishingly small, especially as the former meeting was at a distance of 1,563 ft., the latter of 2,056 ft., from the shaft, down which the plumb lines had to be carried over 1,000 ft. The Messrs. Shanly concluded their contract and effected a final settlement Dec. 22, 1874. Independently of the contract taken by them, an agreement was entered into between the state and B. N. Farren, on Nov. 19, 1874, to do certain arching and enlarging at the eastern portal of the tunnel. By authority of an act passed by the legislature in 1874, a commission of experts, comprising Prof. T. Sterry Hunt of Boston and Prof. James Hall of Albany as geologists, and Thomas Doane, Josiah Brown, and Daniel L. Harris as civil engineers, was appointed to examine and report on the amount of arching that would be still necessary. Their reports are embodied in that of the commission of 1875, as is also a report from Edward S. Philbrick, consulting engineer, recommending an additional amount of 1,600 ft. of arching, besides that included in the Shanly contract. Work on this arching is still (March, 1876) in progress. Under a law of 1874 a board of corporators of the Boston, Hoosac Tunnel, and Western railroad was created, who reported that the tunnel had up to that time cost the state about $14,000,000. By a subsequent act of 1874 the corporators were superseded by five directors, to whom the interest of the state in the tunnel and railroad was transferred.—The next tunnel in the United States in which machine drills were introduced with effect, after their practicability had been demonstrated at Hoosac, was the Nesquehoning tunnel in Pennsylvania, constructed under the direction of J. Dutton Steele as chief engineer. (See paper by J. Dutton Steele in “Transactions of the American Society of Civil Engineers,” 1871.) Here the Burleigh drill and

ordinary black powder were used. The Musconetcong tunnel, on the Lehigh Valley railroad extension through New Jersey, was the next heavy piece of work in the eastern states on which machine drilling was adopted. This tunnel was begun in April, 1872, and finished in June, 1875, under the charge of Robert H. Sayre, chief engineer and general superintendent of the Lehigh Valley railroad company. Charles McFadden of Philadelphia took the contract, and completed what has been conceded to be one of the heaviest pieces of tunnel work ever attempted in America, and yet one of the most rapidly built. Every modern appliance was used. The Ingersoll drill was adopted, about 26 being kept on hand, and from 16 to 18 in constant use. Four Burleigh compressors supplied the air required at the west end, and four Rand and Waring compressors at the east. Dynamite was used throughout as an explosive, and gave entire satisfaction. Very heavy difficulties were encountered in the prosecution of the work, owing to the large bodies of water met with. The total length of the tunnel was a little less than one mile. It was begun by sinking a slope to grade on the western side of the mountain, about one third of the distance through, virtually dividing the tunnel into one third of soft ground working at the west, and two thirds of very hard ground at the east. The headings were started east and west from the bottom of this slope in November, 1872. The east heading had been started in July, 1872. Owing to the heavy cutting necessary at the west end, the heading could not be connected with those from the slope, and from a shaft subsequently sunk, until November, 1873. In May, 1873, so heavy a body of water was struck in the slope heading going east, that it could not be controlled. The miners were driven out, and the slope half filled. The water undermining the props and backing of the timbering in the slope, part of the roof fell in, and the work at that point had to be abandoned temporarily. A shaft was then sunk west of the slope, and headings were driven east and west to tap and draw off this water. Here again new and even heavier bodies of water were encountered, resulting in great expense and much loss of time. Finally the difficulties were overcome, the water tapped, and work resumed on the original slope heading going east, which met the east heading coming west in December, 1874, the errors in alignment and level being less than half an inch. (For further details on the construction of this tunnel see a paper by Henry S. Drinker in the “Transactions of the American Institute of Mining Engineers,” vol. iii.) With the admirable and delicate instruments now so readily obtainable, it would require a positive effort of carelessness on the part of the engineer to entail any serious error in tunnel surveys. Especially noticeable among instruments are those recently perfected by Messrs. Heller and

of Philadelphia, who have made a specialty of tunnel transits.—The above described three tunnels have been taken as particular examples, because they are the latest driven at the present time (March, 1876), and are the best examples of the present stage of the art of tunnelling in the United States. A large tunnel in Nevada, known as the Sutro tunnel, has been in process of construction with machinery for some years. (See .) It is intended to serve as an adit to the Comstock lode. (See “Report of United States Sutro Tunnel Commission,” Washington, Jan. 6, 1872.)—One of the first tunnels in the United States was on the Alleghany Portage railroad in Pennsylvania. It was built in 1831, double track, 900 ft. long; contract price, $1 47 per cubic yard; total cost, 14,857 cubic yards, $21,840. Another early work was the Black Rock tunnel, on the Reading railroad, built in 1836. This was 1,932 ft. long, and the excavation proper of the tunnel cost $125,935. According to data furnished by Mr. B. H. Latrobe of Baltimore, there are 44 tunnels on the line of the Baltimore and Ohio railroad and its branches, with an aggregate length of 37,861 ft., or 7 m. 901 ft., the tunnels varying from 80 to 4,100 ft. in length. The Sand Patch tunnel, on the Pittsburgh and Connellsville branch, was begun in 1854 and finished in 1871. The work during this time was intermitted for a total period of nine years, owing chiefly to the financial embarrassments of 1858. It was driven through the old red sandstone, and cost nearly $500,000. The Kingwood tunnel, 4,100 ft. long, was begun in September, 1849, and finished in May, 1852, at a total cost, including excavation and arching, of $724,000. The Broadtree tunnel, 2,350 ft. long, on the same road, begun in the spring of 1851, was completed in April, 1853, at a total cost (excavation and arching) of $503,000. The Chesapeake and Ohio railroad is 423 m. long, and has 7 m. of tunnelling; the Big Bend tunnel, on the Greenbrier division, is 6,400 ft. long.—Of the rates of progress attainable by machine drilling, a fair average can be deduced from three large tunnels driven through different kinds of rock. At the Hoosac tunnel, through mica schist and micaceous gneiss, with nitro-glycerine, the progress attained by Shanly brothers at the east end in 1869 averaged 139½ ft. a month, and in 1870, 126½ ft.; at the west end in 1870, 100¼ ft. In sinking the central shaft 1,080 ft. in depth, through rock, the average total progress per working month was 21 ft., but the 230 ft. sunk by Shanly brothers was driven in 7½ working months, or at the rate of 30.7 ft. a month. At Nesquehoning, through conglomerate, the average attained in 12 months' driving was 100 ft. a month; while through red shale an experience of two months gave an average of 160 ft. a month. Common black powder was used, the consumption in the conglomerate being about 6 lbs., and in red shale 3½ lbs. per cubic yard of rock broken. At

the Musconetcong tunnel the average monthly advance through a very hard syenitic gneiss, pronounced harder by experts familiar with both than any body of rock met in the Hoosac tunnel, was in 1874: east heading, average of 12 months, 115.8 ft.; west heading, average of last 6½ months, when steady work was attained, 136.8 ft. At this tunnel a shaft was also driven 110 ft. in depth through soft ground, with timbering, at an average rate of 24¼ ft. a month. The prices bid at the present day for tunnel excavation vary from $4 to $7 and $8 per cubic yard. But the contract prices are not always a sure criterion as to the final cost; $6 per cubic yard is a medium bid. Very heavy and expensive tunnel work is often done in constructing underground railways through cities. In these the plan generally adopted is first to make an open-air excavation through the streets, then build the arches and fill in the ground again. A very heavy tunnel was lately finished under the London docks, passing also under some large warehouses, and needing very careful work. The quantity of water pumped was enormous. The final cost was at the rate of £390,000 a mile.—Subaqueous Tunnels. Among these should be particularly noted the first one built under the Thames at London. Except however in view of its vast expense, and the fact that it was the forerunner of modern subaqueous tunnelling, its record at the present day, since the system has been further developed, has no very practical interest. It was begun in 1807, intermitted, and resumed in 1825, under Sir M. I. Brunel, intermitted again, and at last completed and opened for foot passengers in 1843. Its total length is 1,200ft.; final cost nearly £1,200 per lineal yard advanced. (See, vol. x., pp. 616-617.)—A tunnel that has attracted much attention throughout both Europe and this country is the one at Chicago, driven out under Lake Michigan, for the purpose of obtaining pure water for the city. This tunnel, begun in March, 1864, and completed in March, 1867, was entirely original in plan; the engineer was Mr. E. S. Chesbrough. A crib was first sunk in Lake Michigan, about two miles from the shore, 58 ft. in horizontal outside measurement on each of the five sides, and 40 ft. high. The inner portion or well has sides parallel with the outer ones, 22 ft. long each, leaving the distance between the inner and outer faces of the crib, or thickness of the breakwater, 25 ft. This breakwater was built on a flooring of 12-inch white pine timber, laid close together. The outer and inner vertical faces, and the middle wall between them, were all of solid 12-inch white pine timber, except the upper 10 ft. of the outside, which was of white oak, to withstand better the action of the ice. The outer and inner walls were strengthened and connected with brace walls and cross ties of 12-inch timbers, all securely bolted. The crib was built on land, launched, towed into place, filled with atone, and sunk. An iron cylinder,

cast in 9-foot sections, of 9 ft. internal diameter and 2¼ in. thick, was then lowered within the crib to the bottom of the lake; and this cylinder was connected with the land two miles distant by a tunnel under the lake bottom. Gate wells were constructed in the sides of the crib, and after the completion of the tunnel the top section of the cylinder, extending above water level, was removed, and the water admitted through a screen. The tunnel, of circular cross section, was driven through a stiff blue clay; diameter of excavation 5 ft., subsequently lined with two rings of brick. The final cost in full to the city was $457,844. According to the statements and books of the contractors, the items were: crib and outer shaft, $117,500; land shaft, $12,000; tunnel proper, $195,000; total, $324,000. The balance of the expenditure was used in necessary contingencies. For full details of this work see “Eighth Annual Eeport of the Board of Public Works” (Chicago, 1869); also a report of Prof. W. P. Blake, commissioner of California to the Paris exposition (1867). A second tunnel, 7 ft. in diameter, extending to the same crib, was completed in July, 1874, at a total cost of $411,510; and two tunnels for traffic have been constructed under Chicago river. A tunnel under Lake Erie, at Cleveland, Ohio, begun in August, 1869, finished in March, 1874, is similar in plan, purpose, and construction to the one first driven under the lake at Chicago, except that much greater difficulties were encountered in its construction, from meeting several bodies of very soft ground. It is 6,606 ft. in length, and the total cost amounted to $320,352.—It was estimated by Capt. Tyler in 1873 that between 300,000 and 400,000 persons yearly crossed the English channel at Dover, that the number was constantly increasing, and that if a tunnel were built it would probably be doubled. The idea of a tunnel under the channel was first broached by M. Mathieu, a French engineer, who laid plans for one before Bonaparte in 1802. Owing to the subsequent disturbances the projector and his plans were lost sight of. Subsequently plans were proposed by M. Thomé de Gamond, Dr. Payerne, Messrs. Franchot and Tessier, Favre, Mayer, Dunn, Austin, Sankey, Boutet, Hawkins Simpson, Low, Boydon, Brunlees, Waenmaker, and others. To M. Thomé de Gamond is conceded the credit of pushing the project to its present advancement. In 1872 the present channel company was incorporated, Sir John Hawkshaw, Mr. James Brunlees, and M. Thomé de Gamond being appointed the engineers. The route finally adopted places the tunnel on a line drawn from St. Margaret's bay near the South Foreland, on the English side, to a point between Sangatte and Calais in France. The total proposed length of the tunnel is 31 m., of which 22 m. will be under the channel. Should the preliminary tests prove favorable, it is proposed to begin the actual construction by sinking shafts on either shore to the depth

of 450 ft. below high-water mark. Driftways will be driven from the bottom of these for the drainage of the subsequent tunnel proper. The tunnel, if constructed, is to begin 200 ft. above the driftway, and will be driven from both ends. It is to be through the chalk, and in no part of it will there be less than 200 ft. of ground between the crown of the arch and the bed of the channel. It will be on a down grade of one foot in 80 to the junction of the drainage driftway, and then on an up grade of one in 2,640 to the middle of the strait. It is proposed to drive the driftway or heading with Dickinson Brunton's machine for tunnelling through chalk, which works like an auger boring wood. It is believed, from actual work done, that this machine will advance at the rate of from a yard to a yard and a quarter an hour. At this rate it would require two years to construct the driftway, driving from either end, at an estimated cost of £800,000. After the heading has been driven through, it has been estimated that four years' time and an outlay of £4,000,000 will finish the work, including arching; but Sir John Hawkshaw and his associates consider it best, before beginning the work, to double this figure as an estimate. The preliminary works to be undertaken are the sinking of two shafts at either extremity of the tunnel, from which an ordinary mining driftway is to be driven about half a mile out under the sea, the cost of which is estimated at £160,000. This done, the engineers will be better able to judge of the ultimate practicability of the work.—See Lehrbuch der gesammten Tunnelbaukunst, by F. Ržiha (6 vols., Berlin, 1865-'72); and Der Tunnelbau, by J. G. Schön (4to, Vienna, 1866). There is no complete work in English on modern tunnelling. The facts in this article are largely drawn from a practical treatise on American and European tunnelling, now (1876) in course of preparation by Henry S. Drinker, E. M., of Philadelphia.