PART II.

CONSTRUCTION.

101. THE Field-work of engineering during Construction may be divided into two parts, the first (A) dealing with the setting out of the work, and the second (B) with the estimating of the labor and material employed in its execution; and in this order it will be well to consider the subject.

A. THE SETTING OUT OF WORK.

102. An engineer, when given a subdivision of a road to look after during its construction, often finds merely the centreline staked out at every 100 feet,—with hubs indicated by Guard-stakes at the transit stations,-and bench-marks every half-mile or so apart. He is provided with a copy of the location profile and of the transit-notes and bench-marks, and with the notes and plans connected with any special features in the construction on his subdivision for which he will be held responsible-such as plans of bridge-sites, culverts, etc.

If in a timber country, the first thing he has to do is to see to the Clearing of the Right of Way, which he does by marking out the limits-if the clearing is to be carried to the full width-by blazing the trees at distances of a hundred feet or so apart on either side of the centre-line, and inscribing the letter C.

While the clearing is being done, he usually has time to examine the country along the line with an eye to the location of culverts and the size of openings necessary, and to make a closer examination of the probable classification of the cuts than the location party probably had the opportunity of doing. 103. In order to obtain a correct idea as to what size of openings may be necessary, he is guided by the flood-marks

along the water-courses; and if there is any doubt about these in the neighborhood of the line, he must follow them up until he finds some definite indication of the amount of flow, or else forms a more or less accurate estimate of it for himself, by an examination of its source.

In selecting the points for culverts and the sizes required, the engineer must bear in mind the effect of drainage upon the natural well-defined water-courses: for instance, water that before the construction of ditches ran more or less broadcast over the country, -as is frequently the case in low marshy land, thereby perhaps in a dry season showing no indications of its existence at another time of the year, or which in a wet season may be simply indicated by a saturation of the soil, may, when conducted by ditches to the mouth of a culvert, present a very decided reality.

Often too, by cutting a small ditch, two streams can be brought together at a less cost than would be involved by the construction of two separate culverts. For a masonry culvert is an expensive article in the first place, and the usual substitute —a timber one-a still more expensive article in the long run. When the dump is low, open wooden culverts are the best to use as temporary expedients, for any defects in them are readily visible, and masonry culverts can be built to replace them with very little trouble. For small openings piping does admirably, but should be well bedded; as a temporary substitute for pipes, small plank culverts may be inserted, which may afterwards serve as a means of inserting the pipes themselves.

104. A thorough system of drainage along each side of the road-bed should be one of the first points to which the attention of the engineer should be given, for it is often possible to greatly decrease the cost of construction by constructing ditches some little time before the commencement of the work.

As regards the form and size of such ditches, it is usually sufficient to make them with slopes of 1 to 1, but with plenty of width in the base: as a rule, for each foot of water likely to be in the ditch there should not be less than three feet of base; and the rate of fall should be made as uniform as is compatible with the cost of construction. For small ditches, the rate of fall should not be less than 0.2 p. c. if possible; but a large ditch which is likely to have a depth of water of not less than

one foot will draw tolerably well with a fall of only 0.1 p. c. Neither should the fall be so great as to permit scouring to any large extent.

Small extra ditches are usually staked out with centre-stakes only, and the amount of excavation calculated from the centreheights. But for larger ones slope-stakes should be set, and if the surface is irregular it must be properly cross-sectioned.

105. It is often the case that the cross-sectioning of the work has been done by a party detached from the main location party: if so, the engineer usually has time to check the benchmarks and insert new ones for himself at points which he may consider suitable. These B.M.'s should not be less than 10 stations apart; their positions should be such as to do away as mnch as possible with turning-points. They should be marked B.M., and the elevation of each inscribed on it. At each bridge-site there should be a bench-mark close at hand. It is a good plan also, if there is time, to check the alignment from the transit-notes. Any error discovered, either in the levels or the alignment, should be at once reported. For discrepancies arising in the checking of the alignment by using short chords, see Part I.

106. When, however, the subdivision engineer has the cross-sectioning to do himself, if the construction is being started at various points on his work almost simultaneously with his taking charge, he then has his time from the very first fully occupied in taking cross sections.

The amount of work which this involves depends a good deal on the manner in which the grading is to be measured. If measured in excavation only, then it is merely the cuts that have usually to be cross-sectioned; but if measured in cut and fill, both must receive equal attention. In the former case, where borrowing has to be done, it is often necessary, however, to have the fills also cross-sectioned, for, owing to the impossibility of measuring the borrow-pits correctly, the work may have to be measured in the fills, and this must be borne in mind at the time of cross-sectioning. Also, to obtain a correct estimate of the over-haul it is necessary to have the fill connected with it cross-sectioned. At all points, too, where the question of the distribution of material is likely to arise, crosssections of the fills are useful, but these need not be taken with

the same accuracy as those required for the measurement of the work.

To cross-section properly, five men are wanted besides the engineer, namely, a rodman, a man to carry stakes, another to drive them and another to mark them, and a tapeman,-for though the setting of slope-stakes is sometimes done separately from the cross-sectioning, it usually saves both time and expense to do both at once.

Before starting to cross-section, the engineer will do well to construct a small table for each different width of road-bed and set of slopes which he is likely to use, giving the distances out" to the slope-stakes for various amounts of side-heights. For though he rapidly acquire these after a little practice,and should be checked in his calculations of them by the rodman,-still, by having a table before him, he saves considerable mental work and insures greater accuracy. He should also be provided with a small scratch-block.

The best way to explain the method of cross-sectioning is by means of an example.

b B

A

D

H

FIG. 55.

Let bBAC, in Fig. 55, represent a surface which we wish to cross-section. We first take the elevation at the centre A, which should correspond within a tenth or so with that given on the location profile. By subtracting the grade at the station from this elevation we thus have H, the centre cut at A. The rodman then goes to the left and holds the rod at some point b near where he judges the slope-stake will come. If on obtaining the side-height for bit is found that the proper distance out from A for this height does not agree with the distance out as actually measured, other points must be tried until a point is obtained, such as B, where these two correspond. An error of only a few tenths in distance can be estimated for by eye without taking a separate reading to correct for it, so that two or three trials are usually all that are required to fix the

position for the slope-stake; and on comparatively level ground the point can be usually hit off by a good rodman at the first trial.

Similarly on the right the point C must be fixed.

If there are any decided irregularities in the surface, such as is represented at D, the elevations of such points must also be taken.

The following rules give all that is required as regards the actual levelling:

1. When H.I. is above grade.-If the rod-reading exceed the difference in elevation of the H.I. and Grade, the excess = the fill; but if it is less, the deficiency = the cut. Consequently, when the rod-reading = the difference of H.1. and Grade that point is a Grade-point.

2. When H.I. is below Grade, the rod-reading + the difference of H.I. and Grade

=

the fill.

Cut is always indicated by a positive, and Fill by a negative sign.

The following is a good form for keeping the notes:

There is no need to work out the elevations in the field, but so doing in the office afterwards forms a useful check on the work, since H.I. - F.S. (which of course is the elevation) should agree within a tenth or so with the sum of grade ± centre-height, F.S. representing the rod-reading at the centre. We see from the above that it is the Difference of H.I. and Grade which is the foundation of the calculation at each station, and this, when worked out for the next station after a turningpoint, can be modified for the succeeding stations by merely adding or subtracting the difference in grade. Thus the calculation is simpler than it at first appears from the above rules. The slope-stakes should be marked S.S. on the outer sides