TUNNEL. 



TUNNEL. 



423 



or railway over elevated ground, and the enormous expense of such an 

 open excavation as would be necessary in order to preserve the re- 

 quisite level. Those of less extent are frequently constructed to avoid 

 the opposition of landowners, or to afford uninterrupted passage under 

 a road, canal, or river. Many tunnels of the latter character differ in 

 no material point from bridges ; but in the case of oblique crossings a 

 tunnel is distinguished from an oblique or skew bridge by its faces 

 being at right angles with the direction of the lower passage, instead of 

 being parallel with the direction of the upper passage. Of this 

 character are the tunnels under the Hampstead Road, between Euston 

 Square and Camden Town, and the Kensall Green tunnel, under the 

 Harrow Road, both on the line of the London and North- Western rail- 

 way ; and on the West London railway, passing under the Paddington 

 canal at Wormholt Scrubbs. The Thames tunnel is the most re- 

 markable example of tunnelling under a river, and, although far less 

 extensive than many other tunnels, it is, from the almost insuperable 

 difficulties of its situation, perhaps the most astonishing work of the 

 kind ever executed. Another class of tunnels are those made under 

 towns, in order to form a canal or railway communication with points 

 which are inaccessible by an open passage, except at great expense. 

 The Regent's canal, for example, passes under Islington, London, by a 

 tunnel three-quarters of a mile long ; and the Liverpool and Man- 

 chester railway is conducted from the station at Edge Hill, on the 

 outskirts of Liverpool, to the docks at Wapping. for goods traffic, and 

 to Lime Street, in the centre of the town, for passenger traffic, by two 

 tunnels, each of which is about a mile and a quarter long. 



The construction of tunnels is by no means of recent origin, although 

 it is only of late that they have become common. The outlet for the 

 drainage of the lake Copate in Bceotia [BCEOTIA, in GEOO. Div ] is one 

 of the oldest monuments of the labour of man. The great tunnel in 

 Samoa, which was seven stadia, or 4200 Greek feet, in length, was 

 driven through a mountain 900 feet high, for the purpose of serving 

 as the bed of a channel to convey water from a natural source to the 

 city of Samoa. (Herod., iii. 60.) The Posilipo near Naples, which is 

 at least as old aa the beginning of the reign of Tiberius, is a tunuel 

 three-quarters of a mile long. The tunnel which was made at an 

 early period in the Roman republic for the partial drainage of the 

 Alban Lake is above a mile in length. Of ancient works of this 

 character there ia a remarkable example in the subterranean canal from 

 lake Fucinus, or the lake of Celano. to the river Sirio, originally formed 

 by the emperor Claudius, and cleared out some years since by order of the 

 Neapolitan government. Thia extensive tunnel, which is about three 

 miles long, thirty feet high and twenty -eight wide at the entrance, and 

 nowhere less than twenty feet high, passes in part through solid rock, 

 and is lined in other places with masonry ; and it appears to have been 

 constructed in a manner resembling that now usually followed, the 

 excavation having been carried on by several parties or gangs simulta- 

 neously, by means of vertical shafts, and inclined passages or galleries 

 from the sidea of the mountain. A minute account of the tunnel, 

 as it appeared during the clearing-out, ia given in the thirty-eighth 

 volume of ' Blackwood'a Edinburgh Magazine' (p. 657), in a paper 

 entitled ' Kight Days in the Abruzzi.' The object of thia tunnel is to 

 carry off the superfluous waters of the lake ; but in more recent times 

 similar works have been executed for navigable canals. These are 

 generally in England of small transverse dimensions, being calculated 

 for the passage of single boats, and very often without towing-paths, 

 in which case the boats are either hauled through by a rope or chain, 

 worked by a steam-engine, or propelled by men lying on their backs 

 on the deck, or on projecting boards provided for tlie purpose, and 

 thrusting against the sides or roof of the tunnel with their feet. This 

 dangerous practice has occasioned much logs of life, and is also objec- 

 tionable on account of its tediousness, aa boats are often detained for a 

 long time at one end of the tunnel while a boat is coming from the 

 opposite end. In the evidence before the House of Lords on the 

 Great Western Railway bill, in 1835, it was stated that great delays 

 were experienced at the Islington tunnel when any accidental derange- 

 ment prevented the steam-engine and chain from working ; because, 

 although boats were occasionally "legged" through in as little as 

 seventeen minutes, the ordinary time required for working a light 

 barge through the tunnel, by two men, was half an hour, and for a 

 loaded barge three-quarters of an hour, or frequently an hour. In 

 such cases boats arriving in the opposite direction had to wait at the 

 mouth of the tunnel, until, frequently, as many as half a dozen were 

 collected, which, when their turn arrived, passed through in a train. 

 At some of the longer tunnels this inconvenience was even greater. 

 At the Harecastle tunnel, on the Trent and Mersey, or Grand Trunk 

 canal, two hours were formerly required to effect a passage of little 

 more than a mile and a half. " This place is so frequented," observes 

 the Banm Dupin, in his ' Commercial Power of Great Britain,' " that 

 at the moment when the passage begins, a file of boats a mile long is 

 often seen." To prevent confusion, those going towards Liverpool 

 were allowed to pass in the morning only, and those in the contrary 

 direction in the evening. This tunnel, which was formed by Brindley, 

 and was one of the earliest works of the kind executed in this country, 

 was commenced about the year 1763. It is 2830 yards long, 12 feet 

 wide, and 9 feet high, and is in some parts as much as 70 yards 

 beneath the surface. It is lined with a semicircular brick arch, and 

 was completed for the unall sum of SI. 10s. Sd. per yard. Increased 



traffic upon the canal having rendered it necessary either to construct 

 a new tunnel or to enlarge the old one, the former alternative was 

 adopted ; and in 1822 Telford was engaged to superintend the work. 

 The new tunnel, which runs parallel with that of Brindley, is 2926 

 yards long, 14 feet wide, and 16 feet high ; and, notwithstanding its 

 greater dimensions, it was executed in less than three years : the 

 original tunnel occupied eleven years. The new tunnel has an iron 

 towing-path, so supported as to allow the water to play freely beneath 

 it, which gives the advantage, so far as the play of the waves is con- 

 cerned, of a waterway of the full width of the tunnel. It is perfectly 

 straight, and the light can be seen from end to end ; and so agreeable is 

 the travelling through it, that one of the bargemen said, after passing 

 it, that he wished it extended all the way to Manchester. 



Although in some cases the adoption of a tunnel on a line of railway 

 or canal may be decided by the necessity of non-interference with pro- 

 perty on the surface, it is more generally a question of expediency, 

 which involves the consideration of many important points, among 

 which the nature of the ground is one of the first. The ground should 

 be examined by numerous borings, because sudden breaks or faults in 

 the strata, which may occasion great difficulty and expense in tunnel- 

 ing, may otherwise escape notice. The Kilsby tunnel, on the London 

 and North- Western Railway, presents a case in point ; the trial shafts 

 having been accidentally sunk just beyond the limits of a bed of 

 sand and gravel, so full of water as to resemble a quicksand, occa- 

 sioned so much difficulty that the contractors had to relinquish the 

 work, which had been let for 99,000 , but ultimately cost upwards of 

 320,OOOZ., or about 133/. per yard. If the first small borings appear 

 satisfactory, shafts of at least four feet diameter should be sunk along 

 the line of the tunnel, down to its extreme depth ; and the quantity of 

 water which appears in these shafts in a given time be noted, in order 

 to ascertain, as nearly as possible, what draining power is requisite. If 

 the trial shafts be judicioxisly placed, they may subsequently be used 

 as working shafts, which will render the expense of forming them of 

 large diameter immaterial. When the quantity of water and the 

 nature of the strata have been thus tested, tbe engineer possesses data 

 for calculating the comparative cost of a tunnel and an open cutting, 

 in doing which it is necesaary to consider the adjoining works of the 

 line. If embankments be required within a reasonable distance of the 

 proposed tunnel, it may become a question whether it will be more 

 economical to make a cutting, and to carry the excavated earth to the 

 embankment, or to adopt a tunnel, and to obtain the required quantity 

 of earth for the embankment from side-cutting, or in any other way. 

 Sometimes it may be necessary to deposit the material taken out of 

 the tunnel in spoil-banks, in which case the power of obtaining sites 

 for them, and of making convenient temporary roads or tramways for 

 the removal of the earth, must be considered in selecting the positions 

 for the working shafts. These are only a few of the points to be con- 

 sidered in estimating the expense of a .tunnel, among the less prominent 

 of which is the probability of being called upon for damages owing to 

 the intersection of springs, which may occasion mischief at a great dis- 

 tance. Cases have occurred in which the water has been drawn from 

 wells a mile from the tunnel. This evil may often be remedied by 

 sinking the wells to a greater depth, but in some cases it is better to 

 offer compensation at once. The cost of the actual making of the 

 tunnel varies very greatly, according to the nature of the ground and 

 the amount of brickwork required. Lecount, in the article ' Tunnel" 

 in the ' Encyclopaedia Britaunica,' states that many of the old canal 

 tunnels were made for less than 4/. per lineal yard, and that railway 

 tunnels of the ordinary dimensions vary from about 201. per yard, in 

 sandstone rock, which is at once easy to excavate, and able to stand 

 without t any lining of brickwork or masonry, up to from 1001. to 140^. 

 per yard in very loose bad ground, such as a quicksand, which may 

 require a lining of brickwork twenty-seven inches thick. The cost 

 of the Thames tunnel was about 12001. per yard; but in this case, in 

 addition to the unparalleled difficulties attending the excavation, the 

 amount of brickwork is much greater than in ordinary tunnels, and 

 there are two arches, each of which may be considered a distinct 

 tunnel. 



Rocky strata, if the stone be of a nature to work freely, are usually 

 the cheapest for tunnelling, owing to the absence of lining, and the 

 power of saving labour by the use of gunpowder. In the extraordinary 

 tunnels and rock excavations at Bishopton, on the Glasgow, Paisley, 

 and Greenock railway, 314 tons of gunpowder were used in a length of 

 2300 yards in hard whinstone, some veins of which were so hard that 

 the rate of progress at each face of the excavation varied from three 

 feet six inches to six inches only per diem. The Box tunnel, on the 

 Great Western railway, passes for a considerable distance through 

 strata of Bath freestone, geologically termed the great oolite formation, 

 and presents some features worthy of notice. Major-General Pasley 

 was deputed by the Board of Trade to examine this tunnel, and 

 from his report, dated August 12, 1842, the following particulars are 

 derived : -" The tunnel occurs on a gradient or inclination of 1 in 100, 

 ascending from west to east, while the natural dip of the oolitic 

 strata is about the same in the opposite direction : they are nearly 

 horizontal in a direction from north to south, or transverse to that 

 of the tunnel. The strata vary in thickness from 24 to 4 feet and 

 upwards, and are intersected by vertical fissures, of trifling width, but 

 of considerable height ; the direction of which is generally at right 



