HIGHWAY CONSTRUCTION 61) 



stone are to endure friction and to shed water; its thickness must 

 be regulated by the quality of the stone, the amount of traffic, and 

 nature of the natural soil bed. Under heavy traffic it is advisable 

 to make the thickness greater than for light traffic, in order to pro- 

 vide for wear and lessen the cost of renewals. 



When the roadbed is firm, well drained, and not likely to be 

 affected by ground water, it will always afford a firm foundation 

 for the broken stone, the thickness of which may be made the mini- 

 mum for good construction. This thickness is four inches. When 

 this thickness is employed the stone must be of exceptionally fine 

 quality and the road must be maintained by the " continuous " 

 method. With heavy traffic the thickness should be increased over 

 the minimum a certain amount, say 2 inches, to provide for wear. 

 Where the foundation is unstable and there is a tendency on the 

 part of the loads to break through the covering, the thickness of 

 the stone must be made the maximum, which is 12 inches. In such 

 a case it may be advisable to employ a Telford foundation. Where 

 the covering exceeds six inches in thickness, the excess may be 

 composed of gravel, sand or ledge stone, the choice depending 

 entirely on the cost, for all are equally effective. 



Foundation. The preparation of the natural soil over which 

 the road is to be constructed, to enable it to sustain the superstruc- 

 ture and the weights brought upon it, requires the observance of 

 certain precautions the neglect of which will sooner or later result 

 in the deterioration or possible destruction of the road covering. 

 These precautions vary with the character of the soil. 



Soils of a siliceous and calcareous nature do not present any 

 great difficulty, as their porous nature generally affords good natural 

 drainage which secures a dry foundation. Their surface, however, 

 requires to be compacted; this is effected by rolling. 



The rolling should be carried out in dry weather, and any de- 

 pressions caused by the passage of the roller should be filled with 

 the same class of material as the surrounding soil. The rolling 

 must be repeated until a uniform and solid bed is obtained. 



The argillaceous and allied soils, owing to their retentive 

 nature, are very, unstable under the action of water and frost, and 

 in their natural condition afford a poor foundation. The prepara- 



