64: HIGHWAY CONSTRUCTION 



diameter, and weighing from 3 to 6 tons. Some forms are provided 

 with boxes in which stone or iron may be placed to increase the 

 weight, and some have closed ends and may be filled with water or 

 sand. 



Sprinkling=carts. Fig. 46 shows a convenient form of sprink- 

 ling cart for suburban streets and country roads. Capacity about 

 150 gallons. 



ROAD COVERINGS. 



Road coverings consist of some foreign material as gravel, 

 broken stone, clay, etc., placed on the surface of the earth road. 

 The object of this covering, whatever its nature, is (1) to protect the 

 natural soil from the effect of weather and travel, and (2) to furnish 

 a smooth surface on which the resistance to traction will be reduced 

 to the least possible amount, and over which vehicles may pass with 

 safety and expedition at all seasons of the year. Where an artificial 

 covering is employed, the wheel loads coming upon its surface are 

 distributed over a greater area of the roadbed than if the loads 

 come directly upon the earth itself. The loads are not sustained by 

 the covering as a rigid structure, but are transferred through it to 

 the roadbed, which must support both the weight of the covering 

 and the load coming upon it. 



Gravel Roads. Gravel is an accumulation of small more or 

 less rounded stones which usually vary from the size of a small pea 

 to a walnut. It is often intermixed with other substances, such as 

 sand, clay, loam, etc., from each of which it derives a distinctive 

 name. In selecting gravel for road purposes the chief quality to be 

 sought for is the property of binding. 



Gravel in general is unserviceable for roadmaking. This is 

 due mainly to the fact that the surface of the pebbles is smooth, so 

 that they will not bind together in the manner of broken stone. 

 There is also an absence of dust or other material to serve as a 

 binder, and even if such binding material is furnished it is difficult 

 to effectively hold the rounded and polished surface of the pebbles 

 together. 



In certain deposits of gravel, particularly where the pebbly 

 matter is to a greater or less extent composed of limestone, a con- 

 siderable amount of iron oxide has been gathered in the mass. 



