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-Westeru 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 eo much difficulty that the contractors had to relinquish the work, which had been let for 99,000l, but ultimately cost upwards of 320,0001., 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 judiciously 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, the engineer possesses data for calculating the comparative cost of a tunnel and an open cutting, in doing which it is necessary 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 tramwaye 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 iu 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 Britannica; etatea 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 20/. per yard, in sandstone rock, which is at once easy to excavate, and able to stand without any lining of brickwork or masonry, up to from 100/. 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 1200/. 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, am 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 polite 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 angles with that of the tunnel, but in some cases 'obliquely across it.' The eastern end of the tunnel is, for a short distance, lined with masonry; beyond which. for about half a mile, the excara tion is left without any support from masonry or brickwork, its extreme width being, in thin part, 30 feet, and its height varying from 30 to 40 feet, in order to avoid finishing above in any stratum of doubtful character, which, though sufficiently strong in the skies, might not be so trustworthy as a roof. The sides of the excavation are cut in the form of an oblong elliptical or nearly a Gothic arch ; but the uppermost bed of oolite, which forms the roof of the tunnel, has not been completed in that form, but has, for greater strength, been left flat for a width of two or three feet ; and in order to avoid the risk of exfoliation which would have attended the cutting of the edges of the strata to a thin wedge shape, had the curved line of the tunnel been continued unbroken where it intersects them in the sides of the roof, they are cut in the form of steps, each stratum presenting a thick blunt edge ; so that, in the words of Pa.sley's report, " the upper strata of the oolite being supported by those below them, beyond which they each successively project in a moderate degree, like corbels in architecture, the whole enter into that sort of combination which has the strength of an arch without its form ; provided that the vertical fissures do not cut through them longitudinally," of which no symp tom was apparent. The remaining portion of the Box tunnel, passing in the centre through beds of fullers'-earth and clay below the great oolite formation. and towards the western extremity through the infe rior oolite, is arched over with brickwork, varying in thickness from four to seven concentric half-brick rings, and at the western entrance contsining nine such rings. An inverted arch is introduced excepting where the oolite forms the foundation or bottom of the tunnel ; and In some parts a portion of the side walls bass been formed of the inferior oolite, unprotected by brickwork, although it has not in any instance been trusted for the roof. The judicious adoption of each changes in the amount of masonry forms an important part of the duty of the engineer, and is essential to the construction of tunnels at a moderate expense, although it may occasionally lead to failure, as in the case of the Summit tunnel at Littleborough, on the Manchester and Leeds railway, where, in passing through a stratum of blue shale, the invert had been discontinued for a short distance ; but the shale, although apparently hard and firm, was so effected by the atmosphere as to soften and yield under the pressure of the sides, rendering it necessary eubee qnently to put in a strong invert of masonry.