Improvements to the road-bed of an existing coun try road may have for their object the reduction of gradient upon steep inclinations by cutting the material from the road-bed and lowering the surface of the road on the upper part of the grade, and filling in correspondingly on the lower part, or they may be intended to provide better drainage by raising the road across low ground.
In the construction of new roads, the formation of the road-bed consists in bringing the surface of the ground to the grade adopted for the road. This grade should be carefully established upon an accurate pro file of the line, in such manner as to give as little earthwork as possible, both to render the cost of con struction low, and to avoid unnecessarily marring the appearance of the country in vicinity of the road. The most desirable position of the grade line is usually that which makes the amounts of cut and fill about equal to each other, especially where room for borrow pits, or spoil-banks, would be expensive, and it is desirable to make .the embankment for the most part of the material taken from the road excavations. On side-hill work, one side of the road is commonly in cut and the other in fill, and where the side slopes are steep, it is usually better to make the road mostly in cut on account of the difficulty of forming stable embankments on steep ground. In balancing cuts and fills, it is necessary to estimate the quantities for the full width, including side ditches, as the grade should be placed High enough to permit using the material cut from the ditches in the embankment.
Shrinkage. Earth, in embankment, will compact closer than it is found in its natural state, and allow ance for shrinkage must be made in estimating the amount of excavation necessary to form a given embankment. On an average, ordinary soil may be expected to shrink Io to 12 per cent of its bulk; gravel or sand will shrink a little less than this, 8 or 9 per cent; light surface soils a little more, 14 or 15 per cent. The shrinkage may also be somewhat affected by the method of construction used in forming the embank ment, being slightly less for work placed by wagons than for that by scrapers, and still less for wheelbarrow work.
Settlement. In forming an embankment, allowance must sometimes be made for subsequent settlement, by raising the top of the embankment above the required grade. Where scrapers are used, the earth will usually be well compacted in placing, and no allowance is necessary; with dump carts or wagons the compacting is not so thorough, and a small allowance should be made; while when wheelbarrows are used or the earth is thrown into place with shovels, an allow ance of Io or 12 per cent must be added to the height of the embankment, in order to allow for the final shrinkage. Rock occupies more space in embank ment than in excavation, and does not need allowance for shrinkage.
Embankments. When embankments are to be con structed, brush and weeds should be removed from the site and at points where the filling is thin, it is desirable to remove all vegetable matter and soft material, to prevent unequal settling and the formation of soft and spongy places in the surface of the road-bed.
In constructing embankments across wet and unstable ground, it is frequently necessary to form an artificial foundation upon which to place the earth embankment. This may be accomplished in some cases by excavating a little of the soft material and substituting sand or gravel, or in other cases it may be advisable to employ layers of brushwood or fascines as a support for the enbankment. Sometimes it may be possible to drain the soft material by deep ditches, so as to render it capable of sustaining the road, and in all cases drainage should be provided in so far as possible to make the embankment more secure.
When embankments are to be found on sloping ground, the surface of the ground should be stepped off, in order to hold the earth-filling from sliding upon the natural surface at the line of contact between the two, until it becomes sufficiently settled for the develop ment of cohesion to cause it to become one solid mass.