SECTION V.

TUNNELLING.

(1.) PREVIOUS to setting out the earthwork of a tunnel, the levelling operation must be repeated with great care, which should also be checked by the method given in Art. (22.) Section I., especially if the tunnel pass under a very high summit: for, if the section be incorrect, the gradient or gradients, on which the tunnel is formed, will not meet at the points shown thereon, and thus embarrass the mining operation.

(2.) If the tunnel is formed on one gradient, as B D, Plate XIII., the gradient must incline to one of the extremities of the tunnel, as at D, in order to discharge the water generated therein. Strong poles or masts must be firmly fixed on the surface, in the intended direction of the tunnel, of which one must be on the summit of the hill; at which place a temporary observatory is frequently erected, especially if the summit be a very high one, and the tunnel a very long one; but such an observatory can be of no use, except as a shed to shelter the engineers occasionally when they are superintending the mining operations in stormy weather. Shafts must be sunk at the distance of four or five chains from one another, in the direction of the poles, (and observatory, if there be one), in order to ventilate the tunnel, as well as to check the accuracy of the work, as it proceeds. If the tunnel be a long one, it would be preferable, if convenient, to form it on two gradients, inclining to its opposite extremities to liberate the water, and thus to aid the mining operation, which is commonly commenced at both ends of the 'tunnel at the same time.

(3.) When it is necessary to have a curve in the direction of a part or of the whole of the tunnel, that direction must be carefully laid down on the surface, by the methods given in Section III., making allowance for acclivities and declivities, poles or masts being fixed therein, as pointed out in Art. (2.), that the shafts may be sunk so as to meet the mining operations of the tunnel, as well as to check their accuracy in point of direction, and this will be the more especially necessary in the curved part of the tunnel.

commence when

The width and

(4.) The mining operation of the tunnel should the depth of the cuttings at each end is 60 feet. depth of the excavation of a tunnel, on the narrow gauge, should be about 30 feet each, and must be dug 5 or 6 feet below the intended line of the rails, to give space for the inverted arch and the ballasting, excepting where the excavation is made through rock sufficiently

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hard to form the side-walls of the tunnel, in which case 22 or 24 feet in width, and about 26 feet in height, will be sufficient, the excavation in this case being terminated below by the balance line, or formation level. The depth and width of the excavation for a tunnel on the broad gauge must, in both cases, be proportionately larger.

The annexed figure is a cross-section of the masonry of a tunnel, with the ballasting below the rails, which, of course, is such as is required where the tunnel is made through loose earth, only the arch above being required when made through hard rock.

NOTE. The remarks at Articles (25.) and (27.), Sect. II., refer only to the projection of tunnels on parliamentary maps.

SECTION VI.

VIADUCTS, AQUEDUCTS, SKEW ARCHES, ETC.

WHAT are here given on these subjects are necessarily only outlines, referring to existing structures, without entering into the details of specifications, working drawings, &c., which are foreign to the nature of this work, and properly belong to the department of the architect. These outlines are such as may be given by any engineer, if required, for such particular cases as may occur in his practice, when applying to parliament for power to construct a railway or canal, without his being a professional architect.

Railway Viaducts. It would be impossible to enumerate the whole of the viaducts constructed since the introduction of railways. Structures in timber, brick, stone, and iron, of various designs, have been erected; and in some cases there is a novelty of principle accompanied with great boldness of execution.

The brick viaduct at Maidenhead, constructed by Mr. Brunel for the Great Western Railway, is one of the best examples in that material it is composed of two elliptical arches spanning the Thames, each 128 feet, with a versed sine of 24 feet; the pier between the two arches is 30 feet in width. The arch, in the middle, is 5 feet in thickness, which gradually increases towards the abutBesides these two grand brick arches, on each bank of the river are four others; those on the abutments span 21 feet each, and the six others 28 feet each.

ments.

The viaduct over the Ouse, near York, constructed for the Great

North of England Railway, consists of three arches, each 66 feet span; the piers are 10 feet in thickness, and the width of the arches 28 feet on the soffite, and their thickness at the keystone 3 feet, the voussoirs gradually increasing towards the springing.

This is a specimen of a stone viaduct combining great strength and elegance in its construction.

Aqueduct of Pont-y-Cysyllte is 4 miles from Chirk, over the river Dee; its length is 1007 feet, and the river is 127 feet below the water level of the canal carried over it.

To construct an aqueduct upon the usual principles, with piers and arches above 100 feet in height, and of a sufficient breadth and strength to afford room for a puddled water-way, would have been not only expensive but extremely hazardous. Telford, who had already carried the Shrewsbury canal by a cast-iron trough 16 feet above the level of the ground, formed the idea of doing the same in the present instance, which was approved and finally adopted. The foundation on which the piers are erected is a hard sandstone; their height above low water in the river is 121 feet; at the bottom they are 20 feet by 12, at the top 13 feet by 7. For a height of 70 feet above the foundations, they are built solid, and the remaining 50 feet hollow, the walls being only 2 feet in thickness, with one cross inner wall by this means the centre of gravity of the piers is thrown lower, and the masonry economised. The width of the water-way is 12 feet; of which the towing path covers 4 feet, leaving 7 feet for the boat : as the towing path is supported by iron pillars, the water fluctuates, and recedes freely as the boat passes.

There are 18 of these stone piers, besides the abutments; the span of the arches is 45 feet, and their rise above the springing 7 feet, and the total expense of its construction was £47,018.

This aqueduct almost rivals the works of a similar kind left us by the ancient Romans: the introduction of iron, however, for a watercourse, is a novelty with which they were unacquainted: in this instance it has proved admirably well fitted for the purpose to which it is applied: had the channel been constructed with stone or brick at this great elevation, it would have been less secure from the passage of boats, as there would have been a constant leakage from the motion and friction of the water within it.

One of the skew bridges, on the London and Birmingham line, perhaps the finest erection of this kind, is 23 feet in height from the surface of the road below to that of the rails, making an angle of 32° with the road: the direct span of the arch is 21 feet, and oblique span 40. The arch, which is 2 feet thick, is the segment of a cylinder, the internal radius of which is 12 feet, and the versed sine 5 feet. The angle at which the coursing joints of the soffite cross the axis of the cylinder is 53° 25', and the joints of the face of the arch all converge to a point 321 feet below the axis of the cylinder, and 45 feet below the crown of the arch.

another variety of

The Midland Counties Railway presents skewed brick bridge, the span of which is 42 feet, and versed sine 11 feet; the arch consists of six ribs, 2 bricks in depth, and 4 feet in thickness, the total breadth of the bridge, measured at right angles to the face, being 24 feet.

Manchester and Birmingham Railway. - In Fairfield Street is a skew bridge, the oblique span of which is 1283 feet, with a versed sine of 12 feet; the width from face to face is 31 feet.

Six ribs of iron abut on as many independent walls, and project before each other 13 feet; an ornamental stone parapet and cornice crown this viaduct, presenting a novel and agreeable appearance.