2 BULLETIN *724, U. S. DEPARTMENT OF AGRICULTUEE. 



conveniently by naming both materials, as sand-cla5% sand-gumbo, 

 gravelly-clay, etc. 



Clay. — Clay is a soil of very fine texture which results from the 

 complete disintegration of rocks or minerals. Pure cla}^ is very re- 

 tentive of water and becomes plastic and unstable when wet. It 

 is very difficult to drain, and, on that account, when it occurs in the 

 subgrade of a road, the system of drainage should be designed with 

 a view to preventing water from reaching the subgrade rather than 

 to draining water from the cla3\ Most clays as they occur in nature 

 contain more or less sand or gravel, which has a stabilizing effect by 

 making the clay more easily drained and by furnishing a combina- 

 tion of mechanically interlocking grains. A mixture containing 

 about 50 per cent each of clay and gravel or coarse sand often makes 

 nn excellent soil of which to construct roadbeds. Such a soil, in addi- 

 tion to being easily drained, has less " capillarity " than clay, and 

 consequently does not need to be drained to so great a depth in order 

 to prevent water from rising to the surface. Furthermore, it is 

 moderately stable, even when wet. 



Sand. — Sand is made up of granular particles of mineral or stone 

 which occur in nature and which will pass a 5-inch mesh screen. 

 Particles which are too large to pass such a screen are called gravel. 

 Nearly all natural sand consists of particles which are composed prin- 

 cipally of quartz, individually very hard and durable. There is no 

 coherence between the different grains, however, and soils composed 

 principally of sand are unstable, unless properly confined and pro- 

 tected from undermining, in which case a good quality of sand may 

 make an excellent subgTacle for some types of road crusts and for 

 practically any kind of foundation. Sandy soils usually require very 

 little drainage except when the sand is mixed with a considerable per- 

 centage of fine silt. Sand so fine and evenly graded that it becomes 

 saturated readily with water and remains so for more or less lengthy 

 periods is called quicksand and has little dependable stability. 



Loam. — A mixture of soil composed of clay or sand and a consid- 

 erable percentage of decaj^ed vegetable matter is called loam. All 

 such soils usually contain both clay and sand in greater or less 

 amount, and the quality of the loam for road-building purposes is 

 largely dependent on the proportion in which the sand and clay are 

 present. Most loams are drained comparatively easily, but are subject 

 to the disadvantage that the vegetable matter they contain may be 

 incompletely decomposed and subsequent decomposition eventually 

 may cause more or less settlement and shrinkage. 



Gravel. — Gravel is made up of small angidar or rounded particles 

 of stone which occur in nature and which are sufficiently large to be 

 retained on a ^f-inch mesh screen. Gravelly soils are distributed 

 widely in the United States and vary greatly in quality. In general. 



