required, and at many points there will be detached threatening masses of rotten rock which must be cleared away to a much flatter slope for safety. In cuttings of this description it is frequently found necessary to clear out a portion of the loose pieces of the lower cavities and build in their place a facework of masonry to support the superincumbent rock. Springs of water rising in the rock, or running over any part of the rock slopes, must be properly provided for, and conducted to the nearest channel. They should be carefully watched during the winter season, when the frost, acting on the water penetrating the crevices, splits and separates large pieces which were previously firm and secure.
Instances will occur where a cutting has to be made through a thick bed of rock and several feet of soft loose strata underneath. The effect of forming a cutting through the soft strata is to induce the heavy bed of rock above to squeeze or force out the softer material below, and unless proper means were taken to avert such a disturbance, the entire cutting would have to be excavated to a very flat slope. The method adopted in such a case is to build strong face-walls of masonry, brickwork, or concrete, underneath the rock, as shown in [Fig. 51], with strong inverts placed at short distances. Suitable arrangements must be made to take away the drainage water which will collect at the back of the walls, and weeping-holes or outlets must be left in the lower part of the walls to convey the water into the water-tables on the line.
Where there is a depth of earth cutting on the top of the rock, the earth should be cut away so as to leave a bench or space of 3 or 4 feet between the edge of the rock cutting and the foot of the earth slopes, as shown on [Fig. 52].
In cases of shelving rock, with earth or clay on the top, as shown in [Fig. 53], it is frequently found necessary to remove the whole of the clay on the high side to prevent the possibility of its sliding off the rock on to the line below.
In large cuttings it is usual to push forward a gullet of sufficient width for one or two lines of waggons, as shown in [Fig. 54]. When this has advanced some distance, strong planks or half balks of timber are placed across the gullet, and the sides or wings of the cutting can be excavated, the material wheeled to the gullet, and tipped from the barrows into the waggons beneath. By this arrangement the work can be carried on very
expeditiously, as one set of men can be engaged advancing the gullet and laying the track, while others are following up and taking down the sides. A large number of waggons can thus be filled in a day, and a small locomotive kept fully employed.
Occasions will arise where the material from a large cutting, situate on a continuous gradient, as in [Fig. 55], has to be carried in both directions to embankment.
In wet weather, or if the cutting is at all wet, it would be almost, if not quite, impossible to carry on the excavation at the upper end to the proper formation level. The water would collect at the lower level, and not having any means of escape, except by pumping, would stop the work. In such a case the best way is to take out the cutting at the upper end to a slight rising gradient, as shown in the sketch, sufficient to carry away all water, and afterwards take out the lower portion in the working from the other end of the cutting.