SHAFTS.
Shafts are vertical walls or passages sunk along the line of the tunnel at one or more points between the entrances, to permit the tunnel excavation to be attacked at several different points at once, thus greatly reducing the time required for excavation. Shafts may be located directly over the center of the tunnel or to one side of it, and, while usually vertical, are sometimes inclined. During the construction of the tunnel the shafts serve the same purpose as the entrances; hence they must afford a passageway for the excavated materials, which have to be hoisted out, and also for the construction tools and materials which have to be lowered down them. They must also afford a passageway for workmen, draft animals, and for pipes for ventilation, water, compressed air, etc. The character of this traffic indicates the dimensions required, but these depend also upon the method of hoisting employed. Thus, when a windlass or horse gin is used, and the materials are hoisted in buckets of small dimensions, the dimensions of the shaft may also be small; but when steam elevators are employed, and the material is carried on cars run on to the platform of the elevator, large dimensions must be given to the shaft. Generally the parts of the shaft used for different purposes are separated by partitions. The elevator for workmen and the various pipes are placed in one compartment, while the elevator for hoisting the excavated material and lowering construction material is placed in another.
Shafts may be either temporary or permanent. They are temporary when they are filled in after the tunnel is completed, and permanent when they are left open to supply ventilation to the tunnel. Permanent shafts are usually made circular, and lined with brick, unless excavated in very hard and durable rock. When sunk for temporary use only, shafts are usually made rectangular with the greater dimension transverse to the tunnel. They are strutted with timber. A pump is generally located at the bottom of the shaft to collect the water which seeps in from the sides of the shaft and from the tunnel excavation. The dimensions of this pump will of course vary with the amount of water encountered, as will also the capacity of the pump for forcing it up and out of the shaft, which has always to be kept dry.
The majority of engineers prefer to sink shafts directly over the center line of the tunnel. Side shafts are employed chiefly by French engineers. The chief advantage of the former method is the great facility which it affords for hoisting out the materials, while in favor of the latter method is the non-interference of the shaft with the operations inside the tunnel. Were it not that the side shaft requires the introduction of a transverse gallery connecting it with the tunnel, it would be on the whole superior to the center shaft; but the side gallery necessitates turning the cars at right angles, and consequently the use of a very sharp curve or a turntable to reach the shaft bottom, which is a disadvantage that may outweigh its advantages in some other respects. It is impossible to state absolutely which of these methods of locating shafts is the best; both present advantages and disadvantages, and the use of one or the other is usually determined more by the local conditions than by any general superiority of either.
When side shafts are employed they are sometimes made inclined instead of vertical. This form is used when the depth of the shaft is small. By it the hauling is greatly simplified, since the cars loaded at the front with excavated material can be hauled directly out of the shaft and to the dumping-place, surmounting the inclined shaft by means of continuous cables. The short galleries connecting the side shafts with the tunnel proper usually have a smaller section than the tunnel, but are excavated in exactly the same manner. Another form of side shaft sometimes used is one reaching to the surface when the tunnel runs close to the side of cliff, as is the case with some of the Alpine railway tunnels.