Earth Roads. 457 



may be too great to permit it to act as a binding material 

 and as the water-holding power of the clays is greater than 

 the loams, they more quickly come into the condition of 

 over saturation during long rains and so the loam which is 

 intermediate between the two extremes makes the best earth 

 road, sand tending most of the time to retain too little 

 water and the clay retaining too much for tight binding. 



With this principle to direct practice it is clear that if the 

 right amount of finer soil particles can be obtained to in- 

 corporate with the sand of sandy roads their firmness will 

 be increased. It is unfortunately too often true that in 

 districts where sandy roads prevail there is no clayey or 

 loamy material available, either to incorporate with the 

 sand or to place above it. 



568. The Use of Straw, Sawdust and Tan Bark on Sandy 

 Roads. It is well known that these materials when applied 

 to sandy roads have temporarily a beneficial effect. The 

 fundamental principle underlying this improvement is that 

 stated in the last paragraph; that is, in the power they 

 have of maintaining a higher per cent, of water in the sand, 

 which is necessary in order to bind the grains together. 

 The sawdust, tan bark and straw act in two ways to main- 

 tain the needed amount of water in the sand. At first 

 they act as a mulch, lessening the rate of evaporation from 

 the surface. Later, when they begin to disintegrate, they 

 form a humus-like material, in its physical effects, which 

 increases the capillary power and diminishes the rate of 

 percolation downward after rains. 



The reason why these materials are only temporary in 

 their effect is because they rapidly decay, being converted 

 into soluble salts and gaseous products which finally leave 

 the sand as if nothing had been added. 



569. Road Gravel. It occasionally happens that natural 

 gravel beds are found which possess the right characteris- 

 tics for making roads, and when the gravel is just right ex- 

 cellent roads may be made from it. 



