The building of retaining walls entirely of dry stone is very questionable economy, and entails a constant expenditure in maintenance and renewal. The working out of one stone loosens the surrounding portion of the wall, and if not at once repaired, a length of the wall will fall down, bringing with it a large quantity of the earthwork.
If readily obtained, large heavy stones should be selected for the coping of retaining walls, so as to minimize as much as possible the chance of their disturbance or displacement. Where lighter stones have to be used, or bricks laid on edge, they should be bedded and pointed in cement.
In many places it is necessary to form wide and massive foundations of concrete on which to build the retaining wall; and in some cases of soft, treacherous ground, timber piling may be necessary.
Tunnels.—It would be difficult to assign a date to the first examples of subterranean works constructed for utilitarian purposes. Nature had furnished so many grand specimens of caves, grottoes, and underground passages formed in the solid rock, that man soon grasped the principle, and essayed to carry out similar works on his own account. The early attempts would probably be limited to forming places of shelter, storage or security. Advantage would be taken of those rocks which from their locality, accessibility, and compactness of material, promised favourable results. The appliances being few and
primitive, the work of construction would be laborious and slow. So long, however, as the workers restricted their operations to the solid rock, they had merely to contend against the hardness of the material, as the opening or passage-way, once made, required no further support or attention; but as the wave of progress swept onward, man was compelled to deviate from the lines originally followed by nature, and had to form his subterranean pathway through softer material, where the workings required substantial support. The search for minerals of various kinds led to the driving of long headings or galleries underground, and as these had frequently to penetrate through strata of a soft and yielding character, strong timber framework had to be introduced to afford stability to the works, and safety to the workers. For ordinary mining operations, strong rough timber supports may meet all requirements, and may last until the heading is worked out and abandoned; but for subterranean passages or tunnels which are intended to form permanent means of communication, the strongest and most durable materials must be used to protect the interior as far as possible from deterioration or decay. Heavy timbering might be sufficient for mere temporary purposes, but substantial masonry or brickwork side walls and arching became necessary for permanent work in those portions where the tunnel required artificial support.
The first tunnels of any importance were most probably those constructed for canal purposes. Many of them were of considerable magnitude, and in some instances were from two to three miles in length. They were substantially lined with masonry or brickwork at all places where the tunnel passed through soft material or loose rock, and from the solid nature of the work, and the many years they have been in existence, they thoroughly testify to the ability of the constructors.
The introduction of railways involved the making of a large number of tunnels, perhaps more so in the beginning, when it was thought that the use of the locomotive would be confined to very moderate gradients, and when engineers hesitated to adopt the steeper inclines and sharper curves which form the practice of modern times. Another element of consideration also consisted in the fact that the first railways were designed to connect the most populous and busiest districts, where the prospects of heavy traffic would appear to warrant a large outlay for works of construction. As the system spread and
railways extended further away from the important centres, the probabilities of traffic would become less promising, and efforts would be made to keep down cost of construction, and avoid tunnel work as much as possible.
It is not easy to define where cutting should end and tunnelling begin. There is no practical difficulty in making a cutting 50, 60, or 70 feet deep, with slopes to suit the material excavated, and the estimated cost per yard forward may even compare favourably with the cost of average tunnel-work. But there are other questions which must be kept in view—the time required to form the cutting, the space to be obtained on which to deposit the enormous quantity of excavated material, and the probable difficulty in obtaining the large area of land necessary for the cutting.
Before deciding the actual position of a tunnel, both as to line and level, it is necessary to obtain the most reliable information possible regarding the strata through which it has to pass. In addition to the geological indications on the surface and in the locality, borings should be made, and trial holes or shafts sunk along the proposed centre line of the work, and from these an approximately accurate longitudinal section can be laid down on paper, showing the respective layers of material to be cut through, and the angle at which they lie. With these particulars before him, the engineer may, in some cases, consider it more prudent to change the position of the tunnel in preference to incurring specially difficult or tedious work in dealing with some recognized unfavourable material. Occasionally the route may be slightly varied and better material obtained, but very frequently there is little to be gained except by a wide deviation from the original line.