DRAINAGE METHODS FOR' COUNTY ROADS. 59 



but very quick bonding and satisfactory gravel. The soil was poor, 

 being a micaceous clay, and the base was laid in two courses, with 

 the first thoroughly harrowed to shake the finest to the bottom and 

 close the foundation against t*he clay. No effort was made to close 

 the top foundation course as the gravel topping served adequately 

 to do this. 



Figure 15 shows a typical cross section for a crushed-stone foun- 

 dation and indicates the limiting dimensions used in good practice. 



The proper manner of spreading and compacting the stone prob- 

 ably can best be described by stating certain precautions it is neces- 

 sary to observe in order to secure satisfactory results. In doing 

 this it will be necessary to repeat several of the precautions already 

 mentioned as being applicable to gravel foundations, because the 

 two types of construction are very similar in many respects. In fact, 

 where the gravel used is such as to compact readily without the pres- 

 ence of clay the construction methods are practically the same as 

 when crushed stone is used. 



1. The first precaution in constructing a crushed-stone or any otlier type of 

 foundation, of course, is to see that the subgrade is well prepared of good 

 material and that it is firm at the time the stone is spread. 



2. The stone should be delivered in vehicles adapted to spreading it out in a 

 •jniform layer to the required loose depth. If such vehicles are not available, 

 the stone should be dumped on platforms and spread to the required depth with 

 hand shovels. This requirement is of especial importance because it is prac- 

 tically impossible to secure a uniformly compacted foundation unless some such 

 method is followed. Where whole loads are dumped on successive spots of the 

 subgrade and spread by merely raking off the tops of the piles thus formed, a 

 dense core will be formed where each load is dumped and the area between 

 cores will be much less dense. When the layer of stone is rolled, the roller 

 will tend to ride on the cores with the evident result that the foundation will 

 jiot be uniformly compacted and while its appearance may be satisfactory when 

 first /Completed, lack of uniform density is almost certain to result later in 

 Irregularities, and these will continue to occur long after completion of the top 

 or wearing course. 



3. If in hauling and spreading the stone, the fine and coarse particles be- 

 come more or less separated, as frequently is the case, each layer of the founda- 

 tion should be harrowed with a tooth harrow until the different sized particles 

 are distributed uniformly throughout the mass. When moderate care is used in 

 handling the stone harrowing should not generally be necessary, though with 

 some varieties of stone there seems to be a strong tendency for the dust and 

 fine particles to become segregated while the material is being loaded into 

 vehicles and in such cases harrowing may be very desirable. 



4. After eacli layer of the stone is spread, and harrowed if necessary, it should 

 be rolled with a power roller weighing about 10 tons, until it is thoroughly and 

 uniformly compacted. The rolling should begin preferably at the edges of the 

 road and progress toward the center, and should continue until no appreciable 

 settlement is caused by additional rolling. 



5. In general, the voids in the top layer of the foundation should be filled 

 by spreading coarse sand or stone screenings over this layer while the rolling is 

 in progress. With some varieties of rather brittle stone the voids may be filled 



