304 FARM DEVELOPMENT 



the subsoil. And when the water does enter the sur- 

 face the substructure is better if constituted of sand 

 gravel or stones, so that it will allow the water to per 

 colate freely downward or to seep off sideways through 

 the open layers of the material of the roadbed Or it 

 should be underdrained in such a manner that the 

 ground water will sink to at least a few feet below the 

 surface. The upper part of the substructure can some- 

 times be made of coarse material, through which the 



water can seep sideways, 

 even though the lower part 

 of the grade is made of im- 

 pervious clay. 

 Puddling is a character 



Figure 178. A, wheel on hard surface; peculiar tO Soils made Up 

 B, wheel on soft, yielding surface. J 



largely of clay. They be- 

 come soft and mushy when wet, but if thoroughly mixed 

 and worked up while wet, they do not allow water to 

 pass through them. If puddled soils are dried rapidly 

 they become hard and brittle. Thus, the roadway is 

 often cut up into ruts by the wheels of vehicles and the 

 tramping of horses ; depressions in these puddled places 

 retain water as does a dish, and when these become 

 dry the road is made very rough by the hard clods 

 of earth. In some cases, these soils become softened 

 again when soaked with rain, though with some soils 

 the clods remain hard for an extended time. Materials 

 which become soft when wet, even though they are 

 hard when dry, are very poorly suited for road surfaces. 

 These clays are sometimes useful to mix with gravels 

 or even with sand to help combine the coarser 

 materials into a surface which will not be soft 

 in wet weather nor too easily crumbled when 

 dry. Puddling is caused by a readjustment of the 

 particles of the clay when wet. The cementing mate- 

 rials harden, holding together the particles of soil, 



