TUNNELLING. The process of making a more or less horizontal underground passage, or tunnel, without removing the top soil is known as tunnelling. In former times any long tube-like passage, however constructed, was called a tunnel. At the present day the word is sometimes popularly applied to an underground passage constructed by trenching down from the surface to build the arching and then refilling with the top soil; but a passage so constructed, although indistinguishable from a tunnel when completed, is more correctly termed a " covered way," and the operations " cutting and covering," instead of tunnelling. Making a small tunnel, afterwards to be converted into a larger one, is called " driving a heading," and in mining operations small tunnels are termed "galleries," " driftways," and " adits." If the underground passage is vertical it is a shaft; if the shaft is commenced at the surface the operations are known as "sinking," and it is called a " rising " if worked upwards from a previously constructed heading or gallery.
Tunnelling has been effected by "natural forces to a far greater extent than by man. In limestone districts innumerable swallow-holes, or shafts, have been sunk by the rain water following joints and dissolving the rock, and from the bottom of these shafts tunnels have been excavated to the sides of hills in a manner strictly analogous to the ordinary method of executing a tunnel by sinking shafts at intervals and driving headings therefrom. Many rivers find thus a course underground. In Asia Minor one of the rivers on the route of the Mersina Railway extension pierces a hill by means of a natural tunnel, whilst a little south at Seleucia another river flows through a tunnel, 20 feet wide and 23 feet high, cut 1600 years ago through rock so hard that the chisel marks are still discernible. The Mammoth cave of Kentucky and the Peak caves of Derbyshire are examples of natural tunnelling. Mineral springs bring up vast quantities of matter in solution. It has been estimated that the Old Well Spring at Bath has discharged since the commencement of the 19th century solids equivalent to the excavation of a 6 feet by 3 feet heading 7 miles long; and yet the water is perfectly clear and the daily flow is only the 150th part of that pumped out of the great railway tunnel under the Severn. Tunnel-ling is also carried on to an enormous extent by the action of the sea. Where the Atlantic rollers break on the west coast of Ireland, on the seaboard of the western Highlands of Scotland, and elsewhere, numberless caves and tunnels have been formed in the cliffs, beside which artificial tunnelling operations appear insignificant. The most gigantic subaqueous demolition hitherto carried out by man was the blowing up in 1885 of Flood Bock, a mass about 9 acres in extent, near Long Island Sound, New York. To effect this gigantic work by a single instantaneous blast a shaft was sunk 64 feet below sea level, from the bottom of which four miles of tunnels or galleries were driven so as to completely honeycomb the rock. The roof rock ranged from 10 feet to 24 feet in thickness, and was supported by 467 pillars 15 feet square; 13,286 holes, averaging 9 feet in length and 3 inches in diameter, were drilled in the pillars and roof. About 80,000 cubic yards of rock were excavated in the galleries and 275,000 re-mained to be blasted away. The holes were charged with 110 tons of "rackarock," a more powerful explosive than gunpowder, which was fired by electricity, when the sea was lifted 100 feet over the whole area of the rock. Where natural forces effect analogous results, the holes are bored and the headings driven by the chemical and mechanical action of the rain and sea, and the explosive force is ob-tained by the expansive action of air locked up in the fissures of the rock and compressed to many tons per square foot by impact from the waves. Artificial breakwaters have often been thus tunnelled into by the sea, the com-pressed air blowing out the blocks and the waves carrying away the debris.
With so many examples of natural caves and tunnels in existence it is not to be wondered at that tunnelling was one of the earliest works undertaken by man, first for dwellings and tombs, then for quarrying and mining, and finally for water supply, drainage, and other requirements of civilization. A Theban king on ascending the throne began at once to drive the tunnel which was to form his final resting place, and persevered with the work until death. The tomb of Menptah at Thebes was driven at a slope for a distance of 350 feet into the hill, when a shaft was sunk and the tunnel projected a further length of about 300 feet, and enlarged into a chamber for the sarcophagus. Tunnelling on a large scale was also carried on at the rock temples of Nubia and of India, and the architectural features of the entrances to some of these temples might be studied with advantage by the designers of modern tunnel fronts. Petrie has traced the method of underground quarrying followed by the Egyptians opposite the Pyramids. Parallel galleries about 20 feet square were driven into the rock and cross galleries cut, so that a hall 300 to 400 feet wide was formed, with a roof supported by rows of pillars 20 feet square and 20 feet apart. Blocks of stone were removed by the workmen cutting grooves all round them, and, where the stone was not required for use, but merely had to be removed to form a gallery, the grooves were wide enough for a man to stand up in.

Where granite, diorite, and other hard stone had to be cut, the work was done by tube drills and by saws supplied with corundum, or other hard gritty material, and water, —the drills leaving a core of rock exactly like that of the modern diamond drill. As instances of ancient tunnels through soft ground and requiring masonry arching, re-ference may be made to the vaulted drain under the south-east palace of Nimriid and to the brick arched tunnel, 12 feet high and 15 feet wide, under the Euphrates. In Algeria, Switzerland, and wherever the Romans went, re-mains of tunnels for roads, drains, and water-supply are found. Pliny refers to the tunnel constructed for the drainage of Lake Fucino as the greatest public work of the time. It was by far the longest tunnel in the world, being more than 3J miles in length, and was driven under Monte Salviano, which necessitated shafts no less than 400 feet in depth. Forty shafts and a number of "cuniculi" or inclined galleries were sunk, and the excavated material was drawn up in copper pails, of about ten gallons capacity, by windlasses. The tunnel was designed to be 10 feet high by 6 feet wide, but its actual cross section varied. It is stated that 30,000 labourers were occupied eleven years in its construction. With modern appliances such a tunnel could be driven from the two ends without intermediate shafts in eleven months.
No practical advance was made on the tunnelling methods of the Romans until gunpowder came into use. Old engravings of mining operations early in the 17th century show that excavation was still accomplished by pickaxes or hammer and chisel, and that wood fires were lighted at the ends of the headings to split and soften the rock in advance (see fig. 1). Crude methods of ventilation by

FIG. 1.—Method of mining, 1621. (Prom De Me Metdttica, Basel, 1621.) shaking cloths in the headings and by placing inclined boards at the top of the shafts are also on record. In 1766 a tunnel 9 feet wide, 12 feet high, and 2880 yards long was commenced on the Grand Trunk Canal, England, and completed eleven years later; and this was followed by many others. On the introduction of railways tunnel-ling became one of the ordinary incidents of a contractor's work; probably upwards of 4000 railway tunnels have been executed.
Subaqueous Tunnelling. — In 1825 Brunei commenced and in 1843 completed the Thames tunnel, which was driven at points through liquid mud by the aid of a "shield" at a cost of about £1300 per lineal yard. It is now used by the East London Kailway. In 1872 Chesborough began tunnelling under the Detroit river, between Canada and Michigan, U.S., but the work was abandoned owing to continued irruptions of water after some 600 yards of headings had been driven.
The most important subaqueous work yet accomplished—the Severn tunnel, 4^ miles in length—was commenced in 1873 and finished in 1886, Messrs Hawkshaw, Son, Hayter, and Richardson being the engineers and Mr T. A. Walker the contractor. The bed of the Severn is formed principally of marls, sandstones, and conglomerates in neaidy horizontal strata, overlying highly inclined coal measures, shales, and sandstones, which are also exposed in the bed of the river. The tunnel is made almost wholly in the Trias and Coal Measure formations, but for a short distance at its eastern end it passes through gravel. The lowest part of the line is below the "Shoots," wdiere the depth is 60 feet at low water and 100 feet at high water, and the thickness of Pennant sandstone over the brickwork of the tunnel is 45 feet. Under the Salmon Pool, a de-pression in the bed of the river on the English side, there is a cover of only 30 feet of Trias marl. Much water was met with through-out. In 1879 the works were flooded for some months by a large land spring on the Welsh side of the river. The water which sup-plied the spring came from fissures in the carboniferous limestone, which was met with only at this place, and it is now conveyed by a side heading parallel to the tunnel to a shaft 29 feet in diameter, in which are fixed pumps of adequate power. On another occasion the works were flooded by water which burst through a hole in the river bed at the Salmon Pool. This hole, which was in the Trias marl and had an area of 16 feet by 10 feet, was subsequently filled with clay and the works were completed beneath it. The tunnel is for a double line of railway and is lined throughout wdth vitrified bricks set in Portland cement mortar. A heading was first driven entirely across the river to test the ground and sub-sequently another heading at a lower level. "Breakups" were made at intervals of two to five chains and the arching was carried on at each of these points. All parts of the excavation were timbered, and the greatest amount excavated in any one week was 6000 cubic yards. Owing to the inrush of water it was frequently necessary to completely roof the timbering wdth felt or corrugated iron before the bricklayers could commence the arching. The total amount of water raised at all the pumping stations is about 27,000,000 gallons in twenty-four hours; but the total pumping power provided is equal to 66,000,000 gallons in twenty-four hours. The ventilation is effected by a fan of the Guibal pattern, 40 feet in diameter and 12 feet wide, making forty-three revolutions and drawing 447,000 cubic feet of air per minute from the tunnel through an 18-feet shaft at Sudbrooke (Monmouth).
Another example of subaqueous tunnelling, second only in importance to the foregoing, is the Mersey tunnel, the length of which between the pumping shafts on each side of the river is 1 mile. From each shaft a drainage heading was driven through the red sandstone with a rising gradient towards the centre of the river. This heading was partly bored out by a Beaumont machine to a diameter of 7 feet 4 inches, and at a rate attaining occasionally 65 lineal yards per week. All of the tunnel excavation, amounting to 320,000 cubic yards, was got out by hand labour, since heavy blast-ing would have shaken the rock. The minimum cover between the top of the arch and the bed of the river is 30 feet. Pumping machinery is provided for 27,000,000 gallons per day, which is more than double the usual quantity of water ; and ample ventila-tion is secured by two 30 - feet diameter and two 40 - feet diameter Guibal fans. Messrs Brunlees and Fox were the engineers, and Messrs Waddell the contractors for the works, which were opened in 1886, about 6 years after the commencement of operations.
Proposals for the construction of a tunnel about 30 miles in length to connect England and France have been brought forward periodically from the commencement of the 19th century, but nothing was done until 1881, when preliminary works of some im-portance were commenced by Sir Edward Watkin and the South-Eastern Railway Company. At the proposed point of crossing the deepest part of the channel is 210 feet, and, as the beds on the English side, and those on the French side, so far as relates to the grey chalk and chalk marl, are each 225 feet thick, it is assumed that those strata are continuous and that the tunnel would be driven through a water-tight material. Shafts have been sunk near Folkestone, and experimental headings have been driven 2000 yards under the sea, on the line of the tunnel. The heading, 7 feet in diameter, was cut by a Beaumont boring machine, having two arms with steel teeth, and driven by compressed air ; the usual rate of progress was 15 lineal yards per day.
A partially constructed subaqueous tunnel now lies drowned under the Hudson river at New York. An attempt was made to drive a double tunnel through the mud and silt forming the river bed. In 1880, when about a hundred yards had been completed, the water burst in, and twenty men were drowned. Work was sub-sequently resumed on the following plan (see fig. 2). A pilot tunnel, consisting of an iron tube of 6 feet 6 inches in diameter, was advanced from 30 to 40 feet ahead of the main tunnel, to form a firm support for the iron plates of the latter by means of radial screws. Compressed air, pumped into the tunnel at a pressure of about 20 lb per square inch, prevented the weight of silt and water from crushing the plating and flowing into the tunnel. The excavated silt was mixed with water and ejected by compressed air. Between the shafts the length of the proposed tunnel is 1 mile, and about

FIG. 2.—Hudson river tunnel—method of work.

one-eighth of the distance had been accomplished when the works were stopped for financial reasons.
Small subaqueous tunnels have been driven through clay without difficulty under Lakes Michigan and Erie, and elsewhere in America. In England a heading was driven nearly across the Thames in 1807, and eighty years later two 10 feet 6 inch iron-lined tunnels were constructed under the river close to the foundation of London Bridge by Mr Greathead, with the aid of a simple annular shield advanced by six hydraulic presses. Where open gravel or water has to be tunnelled through a diaphragm must be fitted to the shield. Mallet proposed in 1858 to carry in this way a tubular tunnel across the English Channel. Various plans have been suggested for the removal of the soil in advance of the shield. Mr Greathead would effect it by the circulation of a closed current of water, carrying the stuff through the shield from front to back; and an American plan provides for forcing it bodily out of the way by a plough-shaped shield, aided by jets of water at a very high pressure.
Tunnelling through Mountains.—Where a great thickness of rock overlies a tunnel, it is necessary to do the work wholly from the two ends, without intermediate shafts. The problem resolves itself into devising the most expeditious way of excavating and removing the rock, and there are none of the uncertainties and difficulties which make subaqueous tunnelling of so high an interest. Ex-erience has led to great advances in speed and economy, as will e seen from the following particulars of the three tunnels through the Alps, the longest yet constructed.

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In 1857 the first blast was fired in connexion with the Mont Cenis works; in 1861 machine drilling was introduced; and in 1871 the tunnel was opened for traffic. With the exception of about 300 yards the tunnel is lined throughout with brick or stone. Little interest now attaches to the method of tunnelling adopted at Mont Cenis, as it is in several respects obsolete. During the first four years of hand labour the average progress was not more than 9 inches per day on each side of the Alps ; but with compressed-air rock-drills the rate towards the end was five times greater.
In 1872 the St Gotthard tunnel was commenced and in 1881 the first locomotive ran through it. Mechanical drills were used from the commencement. Tunnelling was carried on by driving in advance a top heading about 8 feet square, then enlarging this sideways, and finally sinking the excavation to invert level (see figs. 3 and 4). Air for working the rock-drills was compressed to seven atmospheres by turbines of about 2000 horse-power. Six to to eight Ferroux drills, making about 180 blows a minute, were mounted on a carriage and pushed up to the point of attack. From thirteen to eighteen holes were drilled by the machine and its sixteen attendants to depths of from 2' 7" to 4' 3" in three to five hours, and the work of charging wdth dynamite, firing, and clearing away was then done by twenty-two men in three to four hours. The charge per hole averaged If lb, and after firing a strong current twenty-four hours, which rendered a progress of 13 feet per day in such rock as gneiss attainable in each heading.
The driving of the Arlberg tunnel was commenced in 1880 and the work was completed in little more than three years. The main heading was driven along the bottom of the tunnel and shafts were opened up 25 to 70 yards apart, from which smaller headings were driven right and left. The tunnel was enlarged to its full section at different points simultaneously in lengths of 8 yards, the excava-tion of each occupying about twenty days, and the masonry 14 days. Ferroux percussion air drills and Brandt rotary hydraulic drills were used, and the performance of the latter was especially satisfactory. After each blast a fine spray of water was injected, which assisted the ventilation materially. In the St Gotthard tunnel the discharge of the air drills was relied on for ventilation. In the Arlberg tunnel over 8000 cubic feet of air per minute were thrown in by ventilators. In a long tunnel the quick transport of materials is of equal importance with rapid drilling and blasting. In the Arlberg, to keep pace wdth the miners, 900 tons of excavated material had to be removed, and 350 tons of masonry to be in-troduced, daily at each end of the tunnel, which necessitated the transit of 450 wagons. This traffic was carried on over a length of 3J miles on a single track of 27-inch gauge with two sidings. When the locomotives ran into the tunnel the fires were damped down, and, as the pressure in the boiler was fifteen atmospheres, the stored-up heat in the water furnished the necessary power. The cost per lineal yard varied according to the thickness of masonry lining and the distance from the mouth of the tunnel. For the first 1000 yards from the entrance the prices per lineal yard were £11, 8s. for the lower heading; £7, 12s. for the upper one ; £30, 10s. for the unlined tunnel; £45 for the tunnel with a thin lining of masonry; and £124, 5s. with a lining 3 feet thick at the arch, 4 feet at the sides, and 2 feet 8 inches at the invert.
Long Tunnels.—The new Croton aqueduct tunnel from Croton dam to the reservoir in New York is worthy of note both for its great length and the rapid progress made with it. The distance is 33J miles and practically the whole is tunnelled through rock. Shafts were sunk about 1J miles apart and headings driven each way. Ingersoll drills were chiefly used, and the rate of advance with the headings was in 1886 1J miles per month. The old Croton aqueduct was 7 feet 8 inches wide by 8 feet 5 inches high ; the new one is 13 feet 7 inches in width and height.
Tunnelling in Towns.—Where tunnels have to be carried through soft soil and in proximity to valuable buildings special precautions


FIGS. 5 and 6.—Great Northern Railway tunnel. Method of tunnelling under the Metropolitan cattle market, London.

have to be taken to avoid settlement. The important Metropolitan tunnels constructed by Sir John Fowler have already been de-scribed under RAILWAY (vol. xx. p. 239). Another successful example of such work is the tunnel driven in 1886 by Mr Johnson, the Great Northern Company's engineer, under the Metropolitan cattle market. Where clear of buildings the tunnel was executed in 12-feet lengths measured from the finished brickwork, the excavation extending another 5 feet. The face of the excavation was carried out in four sections, the first between the head trees and the first sill was formed with a rake of 1 in 4J, the second and third with a rake of 1 in 6, and the fourth was vertical, the whole face being close boarded (see figs. 5 and 6). The arch and side walls were eight rings and the invert six rings thick. A 12-feet length was completed in 12 to 14 days, and the subsidence in the ground was about inches. Under buildings and roads the tunnel was executed in 6-feet lengths. The crown bars, 15 inches in diameter, alternating six and seven in number, were built in wdth solid brickwork in cement and hard wood wedging. The skeleton centres for the arching were supported by props notched into the ribs and provided with wedges for tightening up. A 6-feet length was built in six days, and the surface subsidence, consequent upon the impossibility of exactly fitting the poling boards to the clay, was only from 1 inch to If inches. Several heavy buildings were tunnelled under without any structural damage arising.
Where open ballast and running sand heavily charged with water are met wdth a tun-nel cannot be driven on the ordinary system without seri-ously endangering adjoining buildings. To meet such cases, and also to provide a safe means of tun-nelling under dock basins, canals, and rivers, the pneu-matic shield (see fig. 7) was designed by Mr Benjamin Baker. The shield is supported against external pressure by vertical girders about 6 feet apart. Horizontal shelves of steel plates wdth cutting edges are spaced about 4 feet apart, and the face of the shield is closed by vertical plates and slides ; the arrangement is such that any slide can be opened to admit of the ballast or sand being excavated, whilst the compressed air filling the tunnel pre-vents the influx of water during the process. Where hard water-tight clay is encountered, sections of the shield plates are unbolted to admit miners. When sufficient material has been excavated the shield is advanced by hydraulic pressure and the brick arching built.
See AQUEDUCT and RAILWAY ; also Drinker's Tunnelling, New York, 1878 (a most important work); and Proc. Inst. Civ. Eng., art. "Tunnels." (B. B.)