ROADBED CONSTRUCTION. The first task in the building of a railway is the construction of the roadbed or permanent way. In its broadest meaning the permanent way of a railway com prises all structure upon which track is laid, but the term is often given a more limited appli cation which excludes culverts, trestles, viaducts, bridges, etc.; the broader application of the term will be chosen here. The actual work of con struction of the roadbed of a railway begins with the clearing of the right of way. This right of way is a strip of land usually 100 feet wide, or 50 feet each side of the centre line of the road, And the clearing from it of all obstructions is obviously necessary before the work of actual construction can be begun. The amount of clear ing required varies according to the natural con ditions; on an open prairie section it is merely nominal, but in thickly wooded country, where trees have to be felled and their stumps pulled up, it forms quite an item in the expense of con struction. As soon as the right of way is cleared, the work of excavation and embankment con struction is begun. In locating the road the en gineer has settled upon certain stretches and grades which are right lines and which define the top surface of the roadbed. The lines of these levels and grades ill some places cut the natural surface of the ground and in other places they he above the ground surface so that the roadbed has to be filled in. So far as he can do so with out sacrificing more important things, the engi neer endeavors to make the adjacent cuts and fills balance each other; that is, he tries to ar range the grades so that the material excavated from the cuts will be sufficient in quantity to construct the adjacent fills or embankments. When this desirable end cannot be accomplished the extra earth necessary for the embankments is secured by excavating pits called burrow pits at points convenient to the embankment to be built. Sometimes also it is preferable to take the material from burrow pits even where the amount of cuttings is more than enough to form the fills, since it is less expensive to do this than to haul the material excavated from the cuts to the points where it is required for em bankment construction. A cut is simply a trench whose bottom is at the plane of the grade line and somewhat wider than the required road bed, and whose sides slope upward and away from the track to the ground surface at greater or less angles, determined by the slope at which the material will stand without sliding. The natural slope of different materials runs from a nearly vertical plane in firm rock to planes as flat as one foot rise in a horizontal distance of four feet, or technically defined, a slope of one on four. Slopes of 1 on or 1 on 2 are perhaps the most common. Evidently the width of the cut at its top will depend upon its depth and the slope of the sides; it may easily reach 100 feet. When it exceeds this width it becomes necessary for the engineer to figure upon the de sirability of sustaining the sides of the cut by retaining walls (see RETAINING WALL), or per haps to consider the substitution of a tunnel for an open cut. Cuts are always made with a bot tom width enough wider than the roadbed to allow a ditch to be built at each side to carry away the water from rain or melted snow which runs down or seeps through the side slopes.
The excavation of cuts is accomplished by any of the ordinary means of earth and rock excava tion. For earth excavation the steam shovel (q.v.) is the tool most commonly employed. A fill may be described as the reverse of a cut; in fact, were it possible to take out a cut in a single solid piece and to deposit this piece on the ground bottom side up, it would serve as a fill. The manner of constructing a fill is to de posit the material from cuts and burrow pits along the line until an embankment is formed whose top is at grade and somewhat wider than the required roadbed and whose sides slope down ward and outward at angles depending upon the natural shape of the material. Care is taken to make the embankments solid, since they must carry heavy trains, and to construct them so that the water falling on them will drain away as soon as possible. t Tsnally there is not much at tempt to use selected material, except for the upper section on top of which the track ballast will come. Fills, and, more particularly, deep fills, are often constructed by building a rough timber trestle onto which the material cars are run and their contents dumped until the trestle is entirely buried in an embankment of earth. Often also a trestle is at first built to carry the trains with the intention of filling it in after wards. This hastens the construction and cheap ens the first cost of the road, thus allowing the owners to begin operations and to earn money while the final embankment awaits some conven ient time for its construction. The method of constructing embankments by filling in trestles is often resorted to in order to carry the roadbed across morasses of swampy ground.
It often happens that the problem of carrying an embankment across a morass is one of the most difficult which falls to the lot of the rail way engineer. Where streams have to be crossed it is necessary to provide openings in the embankment for their passage. For small streams these openings are provided by means of culverts (see CULVERT) and in the case of large streams bridges are built. (See BRIDGE.) Bridges or viaducts are also employed to carry the road across gorges and deep valleys. Where the con trary condition exists and the engineer is called upon to carry his line over or mountains where an open cut is riot possible. because of its size and cost, and a direct climb is not practi cable, because of the steep grade-s. he either re sorts to the construction of a tunnel or of a switchback. The conditions which call for a tunnel and the methods of constructing such works are discussed in the article on TUNNELS.
A switchback is a line which zigzags back and forth along the side of a mountain and thus gradually climbs to the summit level at which a direct crossing is possible. These structures are expensive to operate, because of their length and steep grades, and railway managers usually sub stitute tunnels for them as soon as the finances of the road and the amount of its traffic will war rant so costly an undertaking. Another method of overcoming steep mountain grades is to use a rack railway or a cable incline railway, and these special forms of road are described in suc ceeding paragraphs.