Surveys are location surveys and construction or final surveys. The instrumental location of a railroad does not differ materially from that described for a new wagon road, but greater accuracy is desirable, and a much more careful adjustment of curves and grades is indispensable. As with common roads, the grade will mainly follow the natural surface, but the line must be so located as to keep these grades within the adopted limit, which will usu ally be 2 per cent, though in exceptional cases and for short grades 4 per cent is allowable.
Natural drainage lines present the most reg ular and easiest gradients, and in a broad sense it may be said that every railroad location fol lows lines of drainage. When the head or source of one drainage line is reached, the location crosses the divide to the next. With few exceptions, drainage lines have slopes not exceeding those permissible for railroad loca tion. The first requisite in considering a rail road location is to get the lines of drainage clearly in the mind. This done, start the ex ploration from the initial point on a straight line for the objective.
Before the survey is started, the maximum grade and degree of curvature should be de cided upon, and in sure eying the line the in strument men will know what their limitations are and will locate the line accordingly. This maximum grade should be the compensated grade. This is, when the grade occurs on a curve, an allowance of .04 per cent of grade should be added to the actual grade for each degree of curvature to give the compensated grade. When the level man finds the grade running steeper than the limiting grade, he will take a sufficient amount of side notes to determine the amount of excavation or filling that will be necessary to keep the grade within the prescribed limits ; and if he finds the amount of excavation excessive, he may have to call back the transit man and have the line relo cated. This should not happen frequently as the transit man should low the maximum al lowable angle of slope and should locate his line accordingly.
The virtual grade i. the actual grade cor rected to take advantage of the velocity head due to the speed of the train when the various grades are encountered. Its principal applica tion is for economical construction, but even at moderate speeds it might occasionally permit the use of grades steeper than the so-called maximum. A pusher grade is an isolated case that cannot be kept down to the ruling grade nor compensated for by the velocity head. Here it is best to pass at once to nearly double the ruling grade and provide for a regular engine for that special grade. They should not occur oftener than once or twice in a division.
Roadbed.— The roadbed or the support for the track generally consists of the foundation and the ballast. The latter should be a material the consistency of which is not affected by water, and especially which does not become slippery when wet. Sand will do if nothing else can be had; gravel is better, and broken stone is the best of all. Cinders, shells, burnt clay and other materials are also used. The surface of the foundation on which the ballast rests is called the subgrade. Unless the natural
ground is very unfavorable, it will not be neces sary to use a separate material for ballast and the subgrade really disappears. Even then, the earth between and immediately under the ties which is dug into in surfacing the track is called ballast. Such roads are usually called mud roads. The cross section of the roadbed must be decided upon before the level party starts on its work, in order that they may know the dimensions of the roadbed for which the stakes are to• be set. The dimensions of the roadbed of a two foot six inch track would be, approxi mately, 11 feet from shoulder to shoulder on a fill and 10 feet from shoulder to shoulder in a cut. The dimensions for a track of three-foot gauge will be about 12 feet from shoulder to shoulder on an embankment and about 11 feet in a cut. In excavations, plenty of depth should be allowed for ditches in order to ensure a dry roadbed.
Wherever the grades permit, the track is laid directly on the surface of the ground, and the necessary leveling is done after the track has been laid.
Bridges.— The engineer in the field during wartime on coming to an opening where a bridge is required plans the bridge so that the openings between the abutments, or between bents of piles or trestles, shall be from the lengths previously agreed upon and for which the chief engineer is prepared to send material without delay. This classification can only apply to single openings, because with several spans it will be impossible to include anything general about the necessary piers. Only in ex ceptional cases will openings over 50 feet be provided for. Larger spans will be subdivided into spans of less than 50 feet by temporary piers wherever possible. In any stream the rate of the current and the amount of drift are im portant factors in determining the span to be used. The length of time a bridge will be in use is also important, as the river may be at such a stage that one length of span can be used, when if it were at a higher or lower stage an entirely different one would be neces sary. On arriving at a bridge that has been destroyed, the first consideration is to get some sort of a line across to connect with the road on the other side. This can frequently be done by making a deviation which will carry the roadbed into the river bottom and up the other bank, necessitating only a very short, low bridge. This work is done by the advance party, and a second party follows and constructs the perma nent bridge at the grade of tfie permanent track. Sharp curves on this diversion line should be avoided, since steep grades will be inevitable, and the combination may cause frequent derail ments. Before the beams or trusses for the necessary spans can be decided upon, it is necessary to decide upon the location of the piers and abutments. The local conditions as to the banks and the bottom of the stream will, in general, determine the location and kind of abutments and piers that are to be used In general terms, they will be either of piles, cribs or trestles.