TUNNEL. 



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HI 



should be bedded with wooden mllet, and the joint*, if in mortar. 

 well flushed up. The thickness of the lining U regulated by the nature 

 of the ground ; but Lecount says that 27 inches at the top and aide*, 

 and 18 in the invert, if laid in cement, will be sufficient, even in a 

 quicluand : there are, however, instance* of a thickness of 10 ring*, or 

 45 inche*. It U sometimes considered advisable to lay the first or 

 inner ring of the roof without mortar, and then to grout it, by which 

 means an equable pressure U insured. 



A brick drain, built in Roman cement, with the joints left open for 

 about half an inch to admit water from the ballasting, should be laid 

 along the centre of the tunnel ; and if the shafts let in water, it should 

 be collected and conducted down the inside of the arch by pipes. 

 Water should be excluded as much as possible during the building of 

 the tunnel and shafts, by puddling with clay, or such other means as 

 the circumstances may dictate; but whatever precautions may be 

 used, water will frequently percolate through the brickwork to a serious 

 extent At the Chevet tunnel, near Wakefield, this inconvenience 

 has been remedied by lining the roof with sheet zinc. In the Thames 

 tunnel there is an interior lining of cement, behind which channel* 

 are provided in the brickwork for the passage of water. A remarkable 

 instance of difficulty arising from this cause occurs in the Beechwood 

 tunnel, 302 yards long, upon the London and North- Western railway. 

 It passes through alternate strata of rock and marl abounding in 

 springs ; and, in the first winter after its erection, a chemical action 

 took place, which partially destroyed many of the bricks. It was pro- 

 posed to line the arch with cement ; but an apprehension was enter- 

 tained that it would not adhere, owing to the constant dropping, and 

 it was determined to apply an interior lining of brickwork, 9 inches 

 thick, and to cut chases in the old work, which, when closed in by the 

 new arch, should become so many drains, 4 4 inches square, to conduct 

 the water to the central drain or culvert The tunnel was divided 

 longitudinally by a temporary partition, and the work was executed in 

 one-half of the tunnel at a time, without stopping the passage in the 

 other half. After executing as much as possible of the brickwork in 

 this way. a series of bearers was laid overhead supporting a close 

 flooring, on which the men stood to complete flic arch. The details 

 of this curious operation, which was completed in forty days, in the 

 latter end of the year 1840, were fully detailed by Mr. T. M. Smith, in 

 a paper laid before the Institution of Civil Engineers. 



In laying the roadway in a railway tunnel care should be taken to 

 avoid the use of any ballasting of a character Kkcly to retain water. 

 The ballast is sometimes thrown down the shafts on to an inclined 

 plane at the bottom, which conducts it in the right direction. In 

 spreading it, it should be well beaten down with wooden rammers, and 

 the blocks or sleepers should be bedded with great care. As it is 

 especially desirable to avoid all risk of accident in travelling through 

 tunnels, while their darkness might prevent the immediate detection of 

 any derangement of the rails, some engineers, for greater security, place 

 the sleepers or points of support closer together in tunnels than on other 

 part* of the line. The temperature being more uniform than in the 

 open air, renders it easy, with proper care, to provide more accurately 

 than usual for alterations in the length of the roils by expansion and 

 contraction. 



Although, in the majority of cases occurring in railway and canal 

 practice, tunnels are constructed in the manner just described, there 

 are exceptions which require notice. In tunnelling near the side of a 

 hill expense is occasionally saved by driving horizontal or nearly hori- 

 zontal passage*, which are called yalleria, from the face of the hill to 

 the line of the tunnel, and removing the excavated earth through them. 

 The double tunnel through the Shakspere Cliff, near Dover, on the 

 line of the South- Eastern railway, was constructed in this way. A 

 benching or road was formed along the face of the cliff, to afford the 

 means of access for the workmen ; and the tunnel was excavated by 

 means of seven galleries opening in the face of the cliff, and inclining 

 towards the sea at the rate of 1 in 1 76. Their average length was about 

 400 feet ; their width feet, and then- height 7 feet ; and the excavated 

 chalk was convoyed along them in small tram-waggons, and tipped into 

 the sea. There are also seven vertical shafts of 6 feet diameter, and of 

 an average depth of 1 80 feet The tunnel consists of two arches or 

 nsssQi, 12 feet wide, separated by a wall of chalk 10 feet thick : th. y 

 are about 1 9 feet high to the springing of the arch, which is of a Pointed 

 or Gothic form, and about 80 feet high in the centre ; and each has 

 a single track or line of railway laid through it. The chalk of which 

 the cliff is composed is very hard, but it consists in many place* of 

 small detached msura, so that brick lining is required for about two- 

 thirds of the whole length of the tunnel. The arching consists gene- 

 rally of three half-brick rings, and is strengthened at intervals of 12 

 feet by counter-forts, which are carried up and stepped back, so as to 

 sustain the weight of any flat beds of chalk that appear of doubtful 

 stability. 



Another variation from the ordinary process occurs in those tunnels 

 which are formed by means of an open cutting, and subsequently 

 covered in Such are called opfn tunnrtt, and are sometimes preferred 

 where the object of the tunnel is to avoid the permanent severance of 

 lands rather than to penetrate ground too elevated for an open cutting. 

 The short tunnel on the London and North-Western railway, at 

 Kensall Green, and parti of the Underground railway, in London. 

 were formed in this way. In such case* the aide* of the cutting are 



made nearly vertical, and supported by timbers until the brickwork i- 

 executed. 



While the project* for some of the earliest English railways were 

 before parliament, much discussion took place relative to the ventila- 

 tion and lighting of tunnels, and to the effect which they might have 

 upon the health of persona riding through them. Most of the objec- 

 tions raised against tunnels during the period referred to are now 

 exploded, and some of them appear not a little ridiculous. It was 

 urged by their opponents that the damp cold air common to all sub- 

 terraneous excavations would prove highly detrimental to health ; that 

 the noxious gases emitted from the locomotive engines would accumu- 

 late and render the air irrespirable ; and that the sudden transitions 

 from light to darkness, and rife rerid, would be very injurious t 

 sight The discomfort arising from these evils, so far a- 

 exist, and from the deafening noise of tunnel-travelling, are amply 

 sufficient to give a preference to an open cutting, when such a In., is 

 obtainable at moderate expense, but they by no means bear out the 

 predictions of the alarmists. Some interesting experiment* made in 

 the tunnel on the Leeds and Selby railway are recorded in a paper by 

 Mr. Walker 'On Ventilating and Lighting Tunnels,' in the ' Transac- 

 tions of the Institution of Civil Engineers,' vol. i. p. 95. While the 

 tunnel alluded to was in progress, it was determined to leave the 

 working shafts open, to promote ventilation and to admit light. The 

 former object was sufficiently attained to prevent any serious incon- 

 venience to passengers, but, so far as the trains are concerned, little 

 benefit is derived from the light admitted by them, although attempts 

 were made to diffuse it by means of tin reflectors. The experiment 

 succeeded BO far as to enable a person to read the larger print in a 

 newspaper advertisement in any part of the tunnel ; Imt it is 

 that, owing to the rough and dirty state of th'j walls ami tin- l>lii|iiity 

 of the rays thrown upon them, the rays reflected from them were too , 

 feeble to be useful in a case of such sudden transition from the light 

 of day as that experienced by persons passing through with a train. 

 Reflectors would also be rendered useless during the passage f a tr.iin 

 by the quantity of steam emitted by the engine. In order to settle the 

 question as to the supposed unhenlthiness of tunnels, in February, 

 1837, Dr. Paris, Dr. Watson, Mr. W. Lawrence, lecturer on anatomy 

 and surgery, Mr. R. Phillips, lecturer on chemistry, and Mr. Lucas, 

 surgeon, were requested to visit and report upon the Primrose Hill 

 tunnel. Although the ventilation was then imperfect, owing to the 

 western extremity of the tunnel being unfinished, and the steam was 

 allowed to escape from the engine for a space of twenty minutes, 

 during which it remained stationary in the tunnel, those gentlemen 

 reported that for so many feet above their heads the atmof 

 remained clear, and apparently unaffected by steam or effluvia of .un- 

 kind, and that neither damp nor cold was perceptible. They further 

 express their opinion "that the dangers incurred in passing through 

 well-constructed tunnels are no greater than those incurred in ordinary 

 travelling upon an open railway or upon a turnpike-road ; and that 

 the apprehensions which have been expressed that such tunnels are 

 likely to prove detrimental to the health or inconvenient to the feel- 

 ings of those who go through them are perfectly groundless." These 

 opinions are fully corroborated by the observations of Dm. Davy, 

 Williamson, and Reid, upon the Leeds and Selby railway tunnel, 

 which, as well as the report on the Primrose Hill tunnel, \\ei. 

 in evidence before the Select Committee of the House of Commons on 

 the London and Brighton railways, in 1837. The objee; 

 from darkness is obviated on the London and North-Wester 

 many other railways by the use of lamps in the roofs of the carriages, 

 which afford an agreeable though small degree of light in the interior ; 

 hut in a few cases the tunnels themselves are lighted by gas-lamps 

 attached to the side walls. 



The above description of the practical operations of tunnelling is 

 founded upon the methods used by railway engineers ; but there is 

 no difference between the execution of a railway or of a canal tunnel. 

 A list of some of the most important tunnels is added, and their 

 cost per yard lineal is affixed wherever it is possible so to do. It 

 may, however, be added that all the works enumerated seem likely 

 to be surpassed in magnitude by the tunnel in course of execution 

 under the Alps of the Mont C<5uis range : it is proposed to be about 

 13,787 yards in length, under a mountain nearly 9000 feet above the 

 level of the rails. 



raixririi. C.is.n TTNXEU. Length. Cost per yiitd. 



Yank. .. d. 



Thamra and Medway . . . 3730 28 



Hiiroca.tlc (by Bilndley) . . . J880 3 10 8 



Lapal (Dudley Canal) . . . 1776 



Gouty Hill (Dudley Canal) ... 623 



Tipton Green 29S6 



Klpley (Cromford Canal) . . . 2966 700 



Dli.worth (Grand Junction) . . 3080 13 13 



Anperton (Hereford and Olouceitrr) . 1320 



Oicnhall . S19J 



Murrdrn (Huddeirneld) . . . 9600 



Foulbri.lze (L*ed and Liverpool) . 1640 



Fumy Compton (Oxford) . . . 1188 



I-lington (Regent's Canal) . . 900 



Maids Hill ... 370 



Bapperton (Thamei and Severn) . 4180 S9 



