« PreviousContinue »
RULES FOR TRACING THE LINE OF A NEW ROAD.
THIS business of tracing the line of a road should never be undertaken without the assistance of instruments; and all local suggestions should be received with extreme caution.
To guard against errors in this important point, it is essentially necessary not to trust to the eye alone, but in every case to have a survey made of the country lying between the extreme points of the intended new road. For this purpose an experienced surveyor should be employed to survey and take the levels of all the various lines that, on a previous perambulation of the country, appear favourable. It is only by
such means that the best line can be determined. These surveys should be neatly and accurately protracted and laid down on good paper, on a scale of sixty-six yards to an inch for the ground plan, and of thirty feet to an inch for the vertical section.
The map should be correctly shaded, so as to exhibit a true representation of the country, with
all its undulations of high grounds and valleys, streams and brooks, houses, orchards, churches, ponds of water adjacent to the line of road; and all other conspicuous objects should also be laid down in the map. A vertical section should be made, and the nature of the soil or different strata should be shown over which each apparently favourable line passes, to be ascertained by boring; for it is by this means alone that the slopes at which the cuttings and embankments will stand can be determined and calculated. If it be necessary to cross rivers, the height of the greatest floods should be marked on the sections; and the velocity of the water, and the sectional area of the river, should be stated.
If bogs or morasses are to be passed over, the depth of the peat should be ascertained by boring; and the general inclination of the country for drainage should be marked.
All the gravel-pits or stone quarries contiguous to the line should be described on the map, with the various roads communicating with them; and the existing bridges over the streams or rivers which are immediately below the proposed point of crossing them should be carefully measured, and the span, or waterway, stated on the section.
These preliminary precautions are absolutely necessary, to enable an engineer to fix upon the best line of road, with respect to general direction, and longitudinal inclination. Without the unerring guide of actual measurement and calculation, all will be guess and uncertainty.
It may be laid down as a general rule, that the best line of road between any two points will be that which is the shortest, the most level, and the cheapest of execution: but this general rule admits of much qualification; it must, in many cases, be governed by the comparative cost of annual repairs, and the present and future traffic that may be expected to pass over the road. Natural obstructions also, such as hills, valleys, and rivers, will intervene, and frequently render it necessary to deviate from the direct course.
In every instance of laying out a road in a hilly country, the spirit-level is essentially necessary to show the proper line of road to be selected. The general rule to be followed in surveys is to preserve the straight line, except when it becomes necessary to leave it to gain the rate of inclination that may be considered proper to be obtained, without expensive ex
ation is made for this purpose, it becomes necessary to proceed in a direct line from a new point.
Thus, for instance, if it be decided to have no greater rate of inclination than 1 in 35, on a new line of road, from A to B (Plate I. Fig. 1.), and the surveyor, when he arrives at the point a, finds a greater inclination than this, he must incline from the direct line to b. Having then gained the summit of the hill, he does not endeavour to get back into the original straight line AB, but pursues the direct line b в, unless he is again obliged, from a similar cause, to deviate from it. This part of the survey being accomplished, it will then become necessary to examine the practicability of making a direct line of road, between a and b, instead of going to the point a.
When hills are high and numerous, it sometimes appears, from a perambulation and inspection of the country, to be advisable to leave the straight line altogether from the beginning, in order to cross the ridges, at lower levels, by a circuitous course, in the way represented by the dotted lines A c d, in the above figure.
It constantly happens that although inclinations which do not exceed the prescribed rate can be had without quitting the straight line, the ridges may be crossed, at many feet of less perpendicular height, by winding the road over lower points of
them; but the propriety of doing so will depend upon the length that a road will be increased by going round to avoid passing the ridges in the direct line. The saving of perpendicular height to be passed over by a road, though a matter of so much importance and practical utility, has not hitherto received that attention from engineers which it deserves. For this reason it has been deemed advisable to bestow much consideration on it; but, as the investigation requires minute and extensive details, and cannot be conducted with full effect, without having recourse to algebraical formulæ, it has been transferred and given in note A.
When expeditious travelling is the object. the maximum rate of inclination that never should be exceeded in passing over hills, if it be practicable to avoid exceeding it, is that which will afford every advantage in descending hills, as well as in ascending them. For, as carriages are necessarily retarded in ascending hills, however moderate their inclinations may be, if horses cannot be driven at a fast pace in going down them, a great loss of time is the result. This circumstance is particularly deserving of attention, because the present average fast rate of driving over any length of road can be accomplished