After one layer has been thoroughly consolidated add a second, and so on until the desired depth is reached. The first layer may be the poorer gravel, the best being reserved for the top. All the layers should be added in time to get well packed before the rains and frosts of winter soften the road-bed.
When finished the gravel should be deepest at the center and taper off to the sides. It is immaterial whether the first layer is the widest or the narrowest; there is a little advantage either way. The depth necessary will depend upon the nature of the soil, the quality of the gravel, the amount of traffic, the maximum weight per wheel, and the care given to maintenance; Wit under ordinary conditions, a depth of 8 or 10 inches of compacted gravel at the center is sufficient. The width should vary with the amount of traffic; but for a country road a depth of 6 inches at 4 or 5 feet from the center is sufficient. For data on the width of the actually traveled way on gravel and crushed-stone roads, see § 306.
The bottom of the trench should be rolled to consolidate it and to discover any soft places in the foundation. After rolling, any depressions should be filled and then re-rolled. The steam roller
is better for this purpose than the horse roller, since it is heavier and since the horses' feet do not dig up the subgrade. For a dis cussion of rollers, see § 336-40. For precautions to be taken in rolling the subgrade, see § 326.
A layer of 3 or 4, or at most 6, inches of gravel is placed in the trench, and the gravel is consolidated either by throwing the road open to traffic or by rolling. The latter is preferable, since teams in passing each other are liable to break down the edges of the trench and mix the earth with the gravel, and since the wheels are liable to break through the thin layer of gravel—particularly if a wet time intervenes. If the only gravel available contains an excess of large pebbles, they may be used in the lower layer, in which case the layer can not be compacted either by the wheels or by rolling. If the gravel is only slightly deficient in binding ma terial, it will be impossible to use a heavy roller, since the gravel will push along in front of it.
Additional layers are added as rapidly as the preceding one is compacted, until the desired depth is reached. Before rolling the last layer, the earth at the sides of the trench, i. e., the " shoulders " or " wings," should be thoroughly rolled; and then the rolling of the gravel should proceed from the sides toward the center, to prevent the gravel from slipping outward. The gravel will compact much better when damp; but if it is sprinkled, care should be taken that (1) the gravel is not made so wet that the earthy binding material becomes semi-fluid and collects on the surface, and (2) that the subgrade is not unduly softened.
No practical amount of rolling will cause a gravel road to " come down" in the sense that a crushed-stone road does; that is, a gravel road can not be rolled until the surface is as hard as it will probably be after it has been opened to traffic for a time, since even the heav iest rollers do not give as much pressure as the wheels of heavily loaded wagons. This difference between gravel and broken-stone roads is due to the fact that gravel has the binding material origi nclly uniformly distributed throughout the mass, while with broken stone the binder is spread upon the top and worked in by rolling and spriniding.