DRAINAGE OF THE ROAD. 217 



The shrinkage of gravelly earth and sand may be taken at 8, of 

 clay 10, loam 12, surface soil 15, and of " puddled " clay 25 per 

 cent. The increase of bulk of rock is 40 to 60 per cent. 



To make use of all these data in calculating the probable cost of 

 a piece of road, there is of course still wanting the equally es- 

 sential factor which gives the number of cubic yards to be dug 

 and moved and the distance of transport. These are got from 

 the plan, profile and cross-sections of the proposed work, an 

 engineer's knowledge being requisite to make the necessary 

 drawings and calculations. 



Drains and Culverts. 



The drainage of roads is of two kinds, surface and sub-drain- 

 age. The first provides for a speedy removal of the rain-fall on 

 the surface of the road and the cutting and embankments on 

 which it is carried ; the second, for the removal of that part of 

 the rain-fall which nevertheless does penetrate into the body of 

 the road-covering. With a perfect sub-drainage the winter's 

 frost, having no water to act upon within the body of the road, 

 is robbed of its great power to destroy the same, and it also pre- 

 vents the road-surface from becoming soaked and thence destroyed 

 in the summer. The need of surface drainage is self-evident. 

 This last named is to be provided for at this stage of the build- 

 ing of the road, the sub-drainage being more properly a part of 

 the building of the road-covering or top. For this purpose 

 ditches, one on each side generally, are absolutely necessary, both 

 when the road is on a level with the surrounding country and 

 when it is in a cutting. They may become necessary also in the 

 case of embankments ; for example, when an embankment is 

 built across low ground. Where these side ditches cross under 

 the embankment we have a culvert ; also whenever any small 

 valley, having a constant or intermittent stream of water, is 

 crossed by such an embankment. It is very bad policy to make 

 such culverts of wood, unless indeed they are so situated as to 

 be constantly under water ; the cost of replacing them after the 

 embankment and road has been built over them is dispropor- 

 tionately great. They should be made of stone, or brick ; lately, 

 cement drain-pipe, oval or egg-shaped, has been used to advan- 

 tage in their construction. 



All ditches, drains and culverts should have a fall throughout 



28* 



