70 HIGHWAY CONSTRUCTION 



tion of such soil is effected by drainage and by the application of a 

 layer of suitable material to entirely separate the surface from the 

 road material. This material may be sand, furnace ashes, or other 

 material of a similar nature, spread in a layer from 3 to 6 inches 

 thick over the surface of the natural soil. 



When the road is formed in rock cuttings it is advisable to 

 spread a layer of sand or other material of light nature, so as to 

 fill up the irregularities of the surface as well as to form a cushion 

 for the road material to rest on. 



Spreading the Stone. The stone should be hauled upon the 

 roadbed in broad-tire two-wheeled carts and dumped in heaps and 

 be spread evenly with a rake in a layer which should be of a depth 

 of 4J inches. 



Watering. Wetting the stone expedites the consolidation, 

 decreases crushing under the roller, and assists the filling of the 

 voids with the binder. It should be applied by a sprinkler and 

 should not be thrown on in quantity or from the plain nozzle of a 

 hose. 



Excessive watering, especially in the earlier stages, tends to 

 soften the foundation, and care should be exercised in its appli- 

 cation. 



Binding. As the voids in loosely spread broken stone range 

 from 35 to 50 per cent of the volume, and as no amount of rolling 

 will reduce the voids more than one-half, it is necessary, in order to 

 form an impervious and compact mass, to add some fine material 

 which is called the binder. It may consist of the fragments and 

 detritus obtained in crushing the stone. When this is insufficient, 

 as will be the case with the harder rocks, the deficiency may be 

 made up of clean sand or gravel. The proportion of binder 

 should slightly exceed the voids in the aggregate; it must not be 

 mixed with the stones, but should be spread uniformly in small 

 quantities over the surface and rolled into the interstices with the 

 aid of water and brooms. 



As the quality of -the binding used is of vital importance, the 

 employment of inferior material, such as road scrapings or material 

 of a clayey nature, should be avoided, even if the initial ciDSt of the 

 work should be greater when a good binding material is used. 



