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Bible Encyclopedias
Tunnel
1911 Encyclopedia Britannica
or (Fr. tonnel, later tonneau, a diminutive from Low Lat. tonna, tunna, a tun, cask), a more or less horizontal underground passage made without removing the top soil. 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 begun 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 ft. wide and 23 ft. 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 beginning of the 19th century solids equivalent to the excavation of a 6 ft. by 3 ft. heading 9 m. long; and yet the water is perfectly clear and the daily flow is only the I both part of that pumped out of the great railway tunnel under the Severn. Tunnelling 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, or on the seaboard of the western Highlands of Scotland, 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 Rock, 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 ft. below sea-level, from the bottom of which 4 m. of tunnels or galleries were driven so as to completely honeycomb the rock. The roof rock ranged from 10 ft. to 24 ft. in thickness, and was supported by 467 pillars 15 ft. square; 13,286 holes, averaging 9 ft. in length and 3 ins. in diameter, were drilled in the pillars and roof. About 80,000 cub. yds. of rock were excavated in the galleries and 275,000 remained 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 loo ft. 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 obtained 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 compressed 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 Mineptah at Thebes was driven at a slope for a distance of 350 ft. into the hill, when a shaft was sunk and the tunnel projected a farther length of about 300 ft., 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. Flinders Petrie has traced the method of underground quarrying followed by the Egyptians opposite the Pyramids. Parallel galleries about 20 ft. square were driven into the rock and cross galleries cut, so that a hall 300 to 400 ft. wide was formed, with a roof supported by rows of pillars 20 ft. square and 20 ft. 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, reference may be made to the vaulted drain under the south-east palace of Nimrod and to the brick arched tunnel, 12 ft. high and 15 ft. wide, under the Euphrates. In Algeria, Switzerland, and wherever the Romans went, remains 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 3 a m. in length, and was driven under Monte Salviano, which necessitated shafts no less than 4 00 ft. 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 ft. high by 6 ft. wide, but its actual crosssection 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).
(From Agricola's De re metallica, Basel, 1621.)1621.) FIG. I. - Method of mining, 1621.
Crude methods of ventilation by 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 ft. wide, 12 ft. high and 2880 yds. long was begun on the Grand Trunk Canal, England, and completed eleven years later; and this was followed by many others. On the introduction of railways tunnelling became one of the ordinary incidents of a contractor's work; probably upwards of 4000 railway tunnels have been executed.
Tunnelling under Rivers and Harbours
In 1825 Marc Isambard Brunel began, and in 1843 completed, the Thames tunnel between Rotherhithe and Wapping now used by the East London railway. He employed a peculiar " shield," made of timber, in several independent sections. Part of the ground penetrated was almost liquid mud, and the cost of the tunnel was about £1300 per lineal yard. In 1818 he took out a patent for a tunnelling process, which included a shield, and which mentioned cast iron as a surrounding wall. His shield foreshadowed the modern shield, which is substituted for the ordinary timber work of the tunnel, holds up the earth of excavation, affords space within its shelter for building the permanent walls, overlaps these walls in telescope fashion, and is moved forward by pushing against their front ends. The advantages of cast-iron walls are that they have great strength in small space as soon as the segments are bolted together, and they can be caulked water-tight.
In 1830 Lord Cochrane (afterwards 10th earl of Dundonald) patented the use of compressed air for shaft-sinking and tunnelling in water-bearing strata. Water under any pressure can be kept out of a subaqueous chamber or tunnel by sufficient air of a greater pressure, and men can breathe and work therein - for a time - up to a pressure exceeding four atmospheres. The shield and cast-iron lining invented by Brunel, and the compressed air of Cochrane, have with the aid of later inventors largely removed the difficulties of subaqueous tunnelling. Cochrane's process was used for the foundation of bridge piers, &c., comparatively early, but neither of these devices was employed for tunnelling until half a century after their invention. Two important subaqueous tunnels in the construction of which neither of these valuable aids was adopted are the Severn and the Mersey tunnels.
The Severn tunnel (fig. 16), 43 m. in length for a double line of railway, begun in 1873 and finished in 1886, Hawkshaw, Son, Hayter & Richardson being the engineers and T. A. Walker the contractor, is made almost wholly in the Trias and Coal Measure formations, but for a short distance at its eastern end passes through gravel. At the lowest part the depth is 60 ft. at low water and 100 ft. at high water, and the thickness of sandstone over the brickwork is 45 ft. Under a depression in the bed of the river on the English side there is a cover of only 30 ft. of marl. Much water was met with throughout. In 1879 the works were flooded for months by a land spring on the Welsh side of the river, and on another occasion from a hole in the river bed at the Salmon Pool. This hole was subsequently filled with clay and the works completed beneath. Two preliminary headings were driven across the river to test the ground. " Break-ups " 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 cub. yds. The total amount of water raised at all the pumping stations is about 27,000,000 gallons in twenty-four hours.
The length of the Mersey tunnel (fig. 15) between Liverpool and Birkenhead between the pumping shafts on each side of the river is one mile. From each a drainage heading was driven through the 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 ft. 4 in. and at a rate attaining occasionally 65 lineal yds. per week. All of the tunnel excavation, amounting to 320,000 cub. yds., was got out by hand labour, since heavy blasting would have shaken the rock. The minimum cover between the top of the arch and the bed of the river is 30 ft. Pumping machinery is provided for 27,000,000 gallons per day, which is more than double the usual quantity of water. Messrs Brunlees & Fox were the engineers, and Messrs Waddell the contractors for the works, which were opened in 1886, about six years after the beginning of operations.
In 1869 P. W. Barlow and J. H. Greathead built the Tower foot-way under the Thames, using for the first time a cast-iron lining and a shield which embodied the main features of Brunel's design. Barlow had patented a shield in 1864, and A. E. Beach one in 1868. The latter was used in a short masonry tunnel under Broadway, New York City, at that time. In 1874 Greathead designed and built a shield, to be used in connexion with compressed air, for a proposed Woolwich tunnel under the Thames, but it was never used. Compressed air was first used in tunnel work by Hersent, at Antwerp, in 1879, in a small drift with a cast-iron lining.
In the same year compressed air was used for the first time in any important tunnel by D. C. Haskin in the famous first Hudson River tunnel, New York City. This was to be of two tubes, each having internal dimensions of about 16 ft. wide by 18 ft. high. The excavation as fast as made was lined with thin steel plates, and inside of these with brick. In June 1880 the northerly tube had reached 360 ft. from the Hoboken shaft, but a portion near the latter, not of full size, was being enlarged. Just after a change of shifts the compressed air blew a hole through the soft silt in the roof at this spot, and the water entering drowned the twenty men who were working therein. From time to time money was raised and the work advanced. Between 1888 and 1891 the northerly tunnel was extended 2000 ft. to about three-fourths of the way across, with British capital and largely under the direction of British engineers - Sir Benjamin Baker and E. W. Moir. Compressed air and a shield were used, and the tunnel walls were made of bolted segments of cast iron. The money being exhausted, the tunnel was allowed to fill with water, and it so remained. for ten years. Both tubes were completed in 1908.
The use of compressed air in the Hudson tunnel, and of annular shields and cast-iron lined tunnel in constructing the City & South London railway (1886 to 1890) by Greathead, became widely known and greatly influenced subaqueous and soft-ground tunnelling thereafter. The pair of tunnels for this railway from near the Monument to Stockwell, from Jo ft. 2 in. to 10 ft. 6 in. interior diameter, were constructed mostly in clay and without the use of compressed air, except for a comparatively short distance through water-bearing gravel. In this gravel a timber heading was made, through which the shield was pushed. The reported total cost was £840,000. Among the tunnels constructed after the City & South London work was well advanced, lined with cast-iron segments, and constructed by means of annular shields and the use of compressed air, were the St Clair (Joseph Hobson, engineer) from Sarnia to Port Huron, 1889-1890, through clay, and for a short distance through water-bearing gravel, 6000 ft., 18 ft. internal diameter; and the notable Blackwall tunnel under the Thames (Sir Alexander Binnie, engineer, and S. Pearson & Sons, contractors), through clay and 400 ft. of water saturated gravel, 1892-1897, about 3116 ft. long, 24 ft. 3 in. in internal diameter. The shield, 19 ft. 6 in. long, contained a. bulkhead with movable shutters, as foreshadowed in Baker's proposed shield (fig. 2). -- Numerous tunnels of small diameter have ';= a ,; ?'4 ?- - = 4 ?" -been similarly con structed under the - ?" l: -`'=' n, Thames and Clyde for electric and cable ways, several for sewers in Melbourne, and two under the Seine at Paris for sewer siphons.
The Rotherhithe tunnel, under the Thames, for a roadway, with a length of 4863 ft. between portals, of which about 1400 ft. are directly under the river, has the largest crosssection of any subaqueous tube of this type in the world (see fig. 3). It was begun in 1904 and finished in 1908, Maurice Fitzmaurice being the engineer of design and construction, and Price & Reeves the contractors. It penetrates sandy and shelly clay overlying a seam of limestone, beneath which are pebbles and loamy sand. A preliminary tunnel for exploration, 12 ft. in diameter, was driven across the river, the top being within 2 ft. of the following main tunnel. The top of the main tunnel excavation in the middle of the river was only 7 ft. from the bed of the Thames, and a temporary blanket of filled earth, usually allowed in similar cases, was prohibited owing to the close proximity of the docks. The maximum progress in one day was 12.5 ft., and the average in six days Io4 ft. The air compressors were together capable of supplying i,000,000 cub. ft. of air per hour.
Some tunnels of marked importance of this type - to be operated solely with electric cars - have been built under the East and Hudson rivers at New York. Two tubes of 15 ft. interior diameter and 4150 ft. long penetrate gneiss and gravel directly under the East River between the Battery and Brooklyn. They were begun in 1902, with Wm. B. Parsons and George S. Rice as engineers, and were finished in December 1907, under the direction of D. L. Hough of the FIG. 2. - B. Baker's pneumatic shield.
Scale of Feet Jo 5 o to 20 30 3. - Cross Sections of Tunnels under Rivers and Harbours.
