The general method of directing the excavation in enlarging the profile by referring all points of the profile to the vertical axis is the one usually employed in tunneling, and gives good results. It is considered better in actual practice to have the excavation exceed the profile somewhat than to have it fall short of it, since the voids can be more easily filled in with riprap than the encroaching rock can be excavated during the building of the masonry. In tunnels where strutting is necessary the excavation must be made enough larger than the finished section to provide the space for it. In soft-ground tunnels it is also usual to enlarge the excavation to allow for the probable slight sinking of the masonry. The proper allowance for strutting is usually left to the judgment of the foreman of excavation, but the allowance for settlement must be fixed by the engineer. 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.