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drawn most closely to agree with this line, and as a rule the smaller the minimum radius of curve the closer will the location agree with the "surface line." A longitudinal section of this proposed location may now be plotted from the contours and examined with a view to adoption or otherwise. If not satisfactory, a new line with cross sections on it may have to be surveyed and the operation repeated until a satisfactory location is obtained.
It is perhaps unnecessary to say that all alterations must be within the limits of deviation as marked on the Parliamentary plans.
As regards sharpening curves and steepening gradients, there are certain specified limits which must not be exceeded, for which see a copy of the regulations. These matters must of course be strictly attended to, as before the inspection or opening of the railway plans and sections of the final and Parliamentary lines must be handed to the Board of Trade for approval. In certain cases, if sufficient cause can be shown, exception may be made to the regulations under special circumstances.
Pegging out Line. The line having been definitely fixed in this way and laid down on the 25 in. Ordnance sheets, these should be pasted together and mounted, and cut up into continuous rolls convenient for use in the field, or a tracing on linen made for field use. The next operation is now to set out the line by driving in pegs 1 chain or 66 ft. apart along the centre line. The field details of this operation are entered into later (see page 233 et seq.).
Pegs. The pegs should be set out with the theodolite, and should be about 1 to 2 in. square, and 15 to 18 in. long. At every 10 chains an index peg should be driven in slantwise, slightly to the right hand of the centre line peg, with the chainage marked on it in miles and chains with a blue pencil.
Tangent Points.-Tangent points should be distinguished by pegs driven in slantwise, one on each side of the centre line peg, and to right and left of it.
Chainages and Survey of Existing Detail. The chainage of all roads, ditches, fences, &c., crossed by the railway must be carefully taken and entered in the field book. Any alterations to fences, boundaries, new buildings, &c., must be carefully sur
veyed, and the 25 in. map in fact corrected and brought up to date. The survey of existing features should extend for about a chain beyond the probable position of the railway fence as nearly as that can be judged. This may be estimated approximately from the probable height of bank or depth of cutting.
When important works, such as large bridges, &c., occur, it may be necessary to survey the road or river, &c., crossed for some distance on each side of the railway.
Longitudinal Section.-The line having been pegged out, a careful section is next taken along it, levels being taken at every chain peg and also at any intermediate points where the slope of the ground changes.
Cross Sections.-Cross sections are also to be taken wherever the ground slopes at right angles to the centre line, or where a section at right angles to the centre line is rough and broken.
Working Plan.-The working plan is prepared by tracing from the 25 in. Ordnance on which the line was laid down or by replotting from it and the field notes. The chainage should be marked at every chain and numbered at every 10 chains. The radii of the curves, the tangent points and their chainage, the intersection points, the lengths of the tangents, and the intersection angles of the curves, are also to be marked on the plan, together with span, width, height and description of bridges, number of the drawing which shows the details of the bridge, the level crossings, road and stream diversions, accommodation works, culverts, pipes, &c. &c.
The fence lines should also be shown as well as the lines representing the tops and bottoms of the slopes, and the space between the latter, i.e., the excavation or embankment, should be coloured red.
The centre line, fence lines, tops and bottoms of slopes, and in fact all new works, are to be in red ink, all other details existing on the ground being in black ink.
Working Section. The section is usually plotted to the same horizontal scale as the plan, viz., ; the vertical scale is then usually 30 ft. to 1 in. The levels are invariably referred to Ordnance datum, and the datum line may be plotted at any convenient height above or below Ordnance datum, it being so stated on the section.
The line representing the formation of the railway, ie, the bottoms of the cuttings or the tops of the banks, or the "road bed," is drawn on the section in red ink, the surface line being in black ink, as also the datum line. Blue vertical lines are drawn upwards from the datum line, extending as far as the surface or formation line, as the case may be, at every chain, and every 10 chains is numbered along the datum line. All chainages should be in miles and chains.
Fixing Gradients on Working Section.-The gradient line will be limited by the ruling gradient, and the gradients on the Parliamentary plans together with the limits of vertical deviation. It is drawn on the section so as to equalise the cuttings and embankments as much as possible, consistently with at the same time getting economical and uniform working gradients. A good method of fixing the gradient line is to use a piece of fine thread stretched tight, moving it up or down until the cuttings and banks are as nearly as may be judged by the eye equalised. Transparent set squares are also useful for this. In laying down the gradients it should be remembered that the slopes of the embankments will generally be much flatter than the slopes of the cuttings, and that the banks are wider at the top than the formation widths of the cuttings. Thus a rock cutting may have slopes of to 1 or nearly vertical, while the adjacent embankment may have slopes of 1 to 1, and this is to be allowed for in equalising cuttings and banks on the section.
It should also be remembered that there is shrinkage to be allowed for in the case of earth embankments, while rock expands in bulk when put into bank.
Levels on Working Section.-Having thus decided upon the gradient line, which will be considerably influenced by the necessity of crossing roads on the level and by the headroom required for bridges, the levels at each change of gradient are next to be figured on near the datum line, and the gradients calculated out and printed on in red ink, as 1 in 100, 1 in 50, and so on. The changes of gradient should as far as possible be selected at even chains, and should be indicated by red in place of blue vertical lines ruled up from the datum line.
The level of the surface of the ground at each chain should be figured on alongside the vertical blue line at the chain to which
it refers, next to the datum line and in black ink; the level of the formation of the railway calculated from the gradient should be figured on at each chain next above the ground level, and in red ink; finally, the difference between the two, being the depth of cutting or height of bank, as the case may be, should be figured above the formation level, in red ink when cutting, and in blue ink when bank.
Cuttings and Banks on Working Section. - The spaces included between the ground line and the formation line of the railway, being the cuttings or banks as the case may be, should be coloured red.
A blue line representing the surface of the rails is also often drawn on parallel to the formation line.
New Works and Existing Details on Working Section. All new works, such as bridges, culverts, pipes, &c., should be described in writing above the section at the points. where they occur in red ink, all existing details, such as roads, streams, &c., being similarly described in black ink.
The setting out and the preparation of the general plan and section as above described may be said to complete the surveying work proper, the preparation of the detail working drawings of bridges, culverts, accommodation works, &c. &c., the specification and bill of quantities, coming under the head of the engineer's work.
Examples of Working Plan and Working Section of Railway taken from actual practice.-Figs. 170 and 171, Plates IX. and X., are examples of the general working plan and working section of a railway actually constructed, executed as described in this chapter.
Ruling Gradients.-The ruling gradient is the steepest gradient which occurs oftenest on the line. The considerations which fix the ruling gradient are
1. Train load.
3. Power and weight of locomotive and train resistance.
1. Train Load. The train load must be fixed from local considerations of traffic, extent of rolling stock, &c. Ordinary 10 ton coal waggons, built for use over main lines to railway