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 arc taken from the first series of Brees's Railway Practice.' Fig. 1 is the form of the Primrose Bill 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 6 inches radius. The width of the tunnel is 21 feet 5 inches at the springing of the invert, and 24 feet 8 inches at tho 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, &c. The side walls are 18 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 b, 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 b to c has radii of 21. feet, the centres being on the same horizontal line; that from c to d has radii of 14 feet 44 inches ; and the crown of the arch is a curve of 9 feet radius. In this case also the invert, instead of being struck from a centre in the crown of the arch, has its centre some feet below it. The 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 16 feet high may he taken as an average. Both 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 should be bedded with a wooden mallet, and the joints, if in mortar, well flushed up. The thickness of the lining is regulated by the nature of the ground; but Lecount rays that 27 inches at the top and aides, and IS in the invert, if laid in cement, will be sufficient, even in a quicksand : there are, however, instances of a thickness of 10 rings, or 45 inches. It is 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 is 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 channels 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, 4i 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 *he 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.
lu laying the roadway in a railway tunnel care should be taken to avoid the use of any ballasting of a character likely 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 parts 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 rails 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 passages, which are called galleries, 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 176. Their average length was about 400 feet ; their width 6 feet, and their height 7 feet ; and the excavated chalk was conveyed 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 180 feet. The tunnel consists of two arches or passages, 12 feet wide, separated bye wall of chalk 10 feet thick ; they are about 19 feet high to the springing of the arch, which is of a Pointed or Gothic form, and about 30 feet high iu the centre ; and each has a single track or line of railway laid through iL The chalk of which the cliff is composed is very hard, but it consists in many places of email detached masses, 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.