429 



TUNNEL. 



TUNNEL. 



430 



of the shaft ; but the substance of the brickwork must vary with 

 circumstances, and engineers are by no means uniform as to the dimen- 

 sions of their shafts. Those of the Box tunnel are mostly 25 feet in 

 diameter, while some tunnels have been excavated with shafts of 

 only 3 or 4 feet. Down to a depth of 50 or 60 feet the earth may be 

 removed by means of a simple roller or winch worked by two men ; 

 and when the depth ia greater a horse-gin may be erected to draw up 

 the loaded skips or buckets. The depth at which the horse-gin be- 

 comes preferable is however dependent in some degree upon the nature 

 of the strata to be excavated ; because when they are hard, the men 

 above may have to wait occasionally for the coming up of the skip ; 

 and it is better to have only two men at a stand-still instead of a 

 horse, a horse-boy, and a banksman or skip lander. The roll or winch 

 is, at the commencement of the shaft, supported by four bars or cills 

 laid across each other on the ground ; and the excavation is continued 

 until the earth exhibits signs of weakness. A wooden curb or riug 

 about 3 inches thick, and as wide as the brick lining is to be, is then 

 laid in, and upon this the brickwork is laid. When the lining of this 

 portion of the shaft, which is frequently but half a brick, or 4 4 inches 

 in thickness, is completed, the excavation is recommenced, and is 

 carried down in a vertical line even with the inner surface of the brick- 

 work, which is then supported by the earth left under the curb, which 

 may be further supported if necessary by diagonal timber props. 

 When the excavation has been carried so much deeper that the ground 

 again appear* weak, a second curb is inserted in a groove cut in the 

 earth, and the ground between the two curbs is divided into four, six, 

 or eight vertical masses, of which one or two are removal to a depth 

 equal to the thickness of the brickwork. The wall is then built up in 

 its place, and a further portion of earth ia removed, and so on until 

 the lining is complete. When the shaft is carried down to the full 

 depth, the miners begin to excavate laterally by forming a heading or 

 driftway along the level of the upper part of the tunnel. Sometimes 

 uch a drift is formed throughout the whole length of the tunnel 

 before any part is opened out to the full size ; but in other cases it is 

 made in short portions, little exceeding the lengths in which the exca- 

 vation of the tunnel itself is carried on, which may vary, according to 

 the ground, from 3 to 15 feet. In the former case the driftway, which 

 is about 4 feet wide and 5 feet high, affords a satisfactory tent of the 

 strata to be passed through, and of the probable quantity of water to 

 be met with, for which it may in some cases serve as an adit or drain ; 

 and for these reasons such a heading is occasionally formed before 

 letting the contracts for the tunnelling. 



In addition to the working shafts, the contractor is usually allowed 

 to sink any number of small air-shafts of 3 or 4 feet diameter, as may 

 be necessary to prevent the accumulation of foul air in the workings of 

 the tunnel ; provided that no such shaft shall open into a public road 

 or be within 50 feet of a working shaft. These are formed in a similar 

 manner to the working shafts, and both are finished at their lower 

 ends by resting upon a cast-iron curb or ring imbedded in the masonry 

 of the roof of the tunnel, and at the upper ends, after the works are 

 completed, by building them about 10 feet above the surface, and 

 coping'them with stone. In very long tunnels one or more large per- 

 manent shaft* are desirable for the purpose of ventilation, and also to 

 admit light, so as in some degree to lessen their gloom. In the Kilsby 

 tunnel, which is between a mile and a quarter and a mile and a half 

 long, there are two such ventilating shafts, 60 feet in diameter and about 



10 and 132 feet deep respectively. These were built from the top 

 iown wards, in the manner above described, in portions 10 feet deep, 

 and from fi to 12 feet wide. Lecount gives the following directions 



the bnckwork of such a shaft, if made in unfavourable ground : 

 The tunnel itself at the point of intersection should be of stone The 

 lower part of the shaft, to the height of 48 feet, should be 3 feet 



lick ; the next 17 feet, 2 feet 8 inches thick ; the next portion, of the 

 s extent, 2 feet 3 inches; and the uppermost 16 feet, 1 foot 10 

 inches. The bricks should be laid in alternate courses of headers and 

 stretchers, and each brick should be well Bushed up. The top may be 

 finished with a stone coping and an iron railing, and protected by an 

 iron fence wall, to prevent the risk of anything being thrown down the 

 shaft e.ther by accident or design. In some tunnels a Large oblong ex- 

 cavation, called an eye, is introduced in lieu of a circular ventilating shaft. 

 In the Bishopton tunnel there is such an eye 300 feet long ; and there 

 are two similar openings in the Glasgow tunnel of the Edinburgh and 

 Glasgow railway. 



After the completion of the driftway, either through the tunnel or 



> the length of a single stage only, the miners excavate the tunnel to 



ill dimensions, beginning by cutting downwards, and propping up 



lie earth with timbers as they proceed ; those which support the roof 



it such an elevation as to allow the centering to be set up and 



3 brick arch to be built beneath them. The bars which immediately 



support the earth, extending from the top or outside of the completed 



brickwork to a framework placed against the face of the excavation 



side bar,, and are in favourable earth required at the upper 



t of the tunnel only ; but in bad ground they are required some- 

 times as low as the springing of the inverted arch which forms the 

 bottom of the tunnel When a complete briuk lining is required thu 

 . the part first built, and it is completed by a course of stone 



. along each side, at the point where the side walls spring from it. 

 hese courses of stone, which are marked a, a, a, a, in the subjoined 



cuts, figs. 1 and 2, consist of blocks about three feet long, well bedded 

 in mortar, upon a few courses of brickwork laid as a footing. The 



Fig. l. 



Fig. 2. 



side walls are next raised, with such a batter or curvature as may 

 enable them best to sustain the pressure of the external earth ; and 

 when they are raised to the level of the springing of the arch, beams 

 of wood, called ciUi, are laid across the tunnel and built in with the 

 bnckwork. Upon these cills the trusses of the centering are set up 

 and adjusted with wedges to the proper height. Laggins, or pieces of 

 wood stretching longitudinally from one centering to another, are then 

 added, and upon these the bricks are laid. In some cases the cills are 

 supported upon trestles, instead of being built into the walls; and 

 where this is not the case, the holes left in the masonry must, after 

 the removal of the cills, be carefully filled up. The excavation should 

 always be made as nearly as possible of the size and shape of the 

 intended masonry ; and as the building proceeds, every cavity left out- 

 side the brickwork should be carefully filled up. Well-pounded clay 

 may for this purpose be rammed under the invert, and almost any 

 other material may be used for the sides and roof. The rammin" of 

 the sides may be performed after the laying of every second course of 

 brickwork, and that above the arch aa frequently as convenient. The 

 timbers used to support the superincumbent earth are in most cases 

 removed as soon as the arch is completed ; but in very bad ground it 

 is sometimes necessary to leave them imbedded in the earth When 

 the work has proceeded so far that the excavations from the two 

 adjoining shafta are within about fifty yards of each other if no drift- 

 way have been previously made through the tunnel, it is advisable to 

 drive a heading through the intervening earth, to insure a perfect 

 junction of the two shifts. 



Many of the earlier tunnels were constructed with vertical sides and 

 a semicircular arch ; but it is now more usual to have the sides curved 

 or battered, the degree of curvature, as well as the shape of the arch 

 which forms the rOof, being varied according to the nature of the 

 ground, as soft semi-fluid ground will press much more equally in 

 every direction than strata of a harder and drier character, and will 

 therefore require a nearer approach to the circular form. The annexed 

 cuts represent two varieties of form, the details of which are taken 

 froni the first series of Brees's ' liailway Practice.' Fig. 1 is the form 

 of the Primrose Hill tunnel, in movable London clay. The invert 

 which consists of three concentric half-brick rings, is a curve of 25 feet 

 radius ; the arch, of four half-brick rings,* is struck with a radius of 

 11 feet 9 inches ; and the sides are arcs of 27 feet inches radius 

 The width of the tunnel is 21 feet 5 inches at the springing of the 

 invert, and 24 feet 8 inches at the widest part. The clear height of 

 the tunnel is 21 feet 8 inches, the remaining depth of 3 feet 4 inches 

 being occupied by the ballasting, drain, &e. The side walls are 



8 inches thick, like the arch which constitutes the roof. /'/ 2 

 which represents the transverse section of the Linsdale and Kilsby' 

 tunnels, both on the North- Western railway, has an elliptical arch con- 

 sisting of several circular arcs, of which the lower pair, extending from 

 the invert to the point marked 4, are struck from radii of 42 feet 

 8 inches, the centres being upon an horizontal line 8 feet 4 inches 

 above the springing of the invert; the portion from 6 to c has radii of 



1 feet, the centres being on the same horizontal line ; that from c to 



(/has radii of 14 feet 44 inches ; and the crown of the arch is a curve 



9 feet radius. In this case also the invert, instead of being struck 



from a centre in the crown of the arch, has its centra some feet below 



1 he internal height of this tunnel is altogether rather more than 



27 feet, and its greatest width is 24 feet. These dimensions are rarely 



much exceeded on railways of the ordinary gauge, where two tracks 



are provided for; and for a single track perhaps 12 feet wide and 



T i , g , \ may . be taken aa an avera S 8 - Uoth the invert and the 

 arch should be built in half-brick rings, care being taken to put in the 

 proper number of bricks to each ring, that the bearing may be uniform. 

 In tunnels of the ordinary dimensions, each ring should contain five 

 more bricks than that immediately within it. The side walls may be 

 built in what is called English bond, consisting of alternate courses of 

 headers and stretchers. The bricks should always be of the best 

 quality, and, when the form of the tunnel requires it, moulded of a 

 taper shape. In the laying also care is requisite, and every brick 



As before stated, it was found necessary, in most parts of this tunnel to 

 increase the tuickncsa of the brickwork from 13 to 27 inches. 



