CONSTRUCTION OF SUBGRADE.
It is necessary to say that the subgrade is the ultimate support of any pavement, and that both the cost and the efficiency of a pavement depends upon the supporting power of the soil upon which it rests. There are only two ways of increasing or supplementing the supporting power of the subgrade: (1) by underdrainage, or (2) by constructing a pavement that will distribute the concentrated load of the wheel over a considerable area of the subgrade. Usually the former is both the cheaper and the more effective. Tile drainage is cheap to construct, is certain in action, and costs nothing for maintenance. With all soils, except clean dry sand, the cost of both the construction and the main tenance of the pavement can usually be materially decreased by proper underdrainage. Unless the subsoil is very open and porous, it is economical to lay a tile under each edge of the pavement, 2 or 3 feet below the surface of the subgrade.
Not only should the subgrade be properly drained, but it should be thoroughly rolled to compact the surface and also to reveal any soft spots. Usually just before a pavement is to be constructed, the street is dug up to lay sewers and water and gas pipes and to connect these with the private property ; and almost universally the trenches are re filled in such a manner that great care and skill are required to con struct a pavement which will not ultimately settle over the trenches; much to the damage of the pavement and the disfigurement of the street. It is always specified that the subgrade shall be thoroughly rolled; but it needs only a casual inspection of the pavements of any city to show that this is seldom in a manner to prevent settlement. The cheapest and surest method of preventing such settlement is to properly refill the trenches; but usually this is indifferently done, owing to the ignorance or the carelessness of the proper municipal officer, and as a consequence the remedy of this defect is left to the paving contractor.
The method to be employed in refilling trenches so that they will not settle, depends upon the kind of soil and also upon the relative cost of labor and water. The problem of the proper filling of trenches is too intricate to permit a thorough discussion here; but briefly it may be said that except in the case of comparatively clean sand and gravel, back filling can be thoroughly done only by tamping; and to make this method successful it is necessary (1) that the material shall be moist enough to be plastic, but neither too wet nor too dry, (2) that it shall be deposited in layers not more than 3 or 4 inches thick and (3) that each layer shall be thoroughly tamped. To secure thorough tamp
ing the relative number of tampers and shovelers is sometimes specified; but this alone is ineffectual since there is a natural tendency for the tampers to work less energetically than the shovelers, and besides more labor is required to tamp the soil around a pipe than higher up. No kind of municipal work should be more rigorously inspected than the filling of a trench over which a pavement is to be laid. The nearly universal result of a neglect in this respect is that a pavement built at great expense is disfigured or damaged by settlement, the repair of which will cost many times as much as it would have cost to properly fill the trench originally.
The subgrade should be rolled both longitudinally and transversely with a steam roller weighing not less than five tons. If the sheet is rolled in only one direction, only one set of trenches will be compacted. The subgrade can not be rolled transversely with a horse roller; and be sides the horses' feet tear up the subgrade nearly as much as the roller compacts it, particularly when the rolling is almost completed. The roller should pass over the surface several times to. settle the filling into the trenches and also to compact the surface by the kneading action of many passes of the roller. Unfortunately the specification requiring the use of a steam roller adds somewhat to the cost of a pavement, since that implement is expensive in first cost and also in maintenance; and since ordinarily it can be used only a comparatively short time each year; but its use is believed to be worth its cost, particularly if the trenches were not properly back-filled.