ROLLING THE SUBGRADE. The finished subgrade should be thoroughly rolled to consolidate the surface and also to discover any soft places—particularly over trenches that have not been solidly filled. If the roller reveals a low place, it should be filled with earth and be rolled again. The roller, whatever its weight, should be passed over the subgrade more than once, since the successive passages have something of a kneading action and add to the solidity of the soil. Several passes with a light roller give better results than a few passes with a heavy one. It is well to specify both the weight of the roller and the number of times it is to pass over the road-bed. For some hints applicable in rolling the subgrade, see § 326.
It is customary to use a horse roller (§ 337) for this purpose but a steam roller (§ 338) is much better because it is heavier, and, still more important, because with it the street can be rolled trans versely. The street is full of trenches made often just before the pavement is laid, in connecting the houses with the sewer, the water, and the gas; and as these trenches run both longitudinally and transversely, it is necessary to run the roller in both direc tions if the trenches are certain to be solidly filled. With a horse roller, it is practically impossible to roll the street trans versely, but with a steam roller this is comparatively easy.
Unless the back-filling of a trench has been unusually well tamped, a roller run transversely over a trench will leave a de pression. In most soils, the back-filling will not of itself settle into its former solidity, however long it is left to the action of traffic and to the forces of nature; and whatever the foundation of the pavement, the heavier traffic is nearly certain to cause a settlement over these same trenches, unless the subgrade is well rolled. Traffic consolidates only a thin layer near the surface,
which is usually removed when the pavement is constructed. Ordinarily, if the subgrade is rolled both longitudinally and tram..
versely with a steam roller weighing 10 or 12 tons, there will be no settlement of the pavement.
In rolling, if a depression is produced over a trench, it should be filled and then again rolled. If the depression is of considerable depth, it shows that the trench was badly filled or was very deep, or both; and therefore it is wise to re-consolidate the trench. One way of doing this is to make numerous openings through the crust and keep the depression filled with water until the earth in the bottom of the trench has become thoroughly soaked; and then after the ground has dried out below, the roller should again be passed over the surface. The surest way to prevent settlement over trenches is to pack the soil solidly when the trench is first filled. For a discussion of various methods of back-filling, see § 540.
Insufficient tamping in filling trenches or inefficient rolling of trenches is a very common defect in pavement construction, nearly every block presenting one or more such depressions. One Of the purposes of a guarantee of the pavement (§ 450) is to secure a thorough consolidation of the soil in the trenches.