Ch. 34] WATER SUPPLY 645 



6. River flood plains; complex of well-drained and poorly drained soils. Tra- 

 ficability hampered by local areas of muddy soils. Flood hazard, especially 

 in winter and spring. 



Predictions of this degree of detail have been proved accurate by 

 field use, but a more quantitative expression of traffic ability character- 

 istics of soil is necessary; accordingly instruments are being devised 

 whose measurements of bearing strength of various soil horizons can be 

 correlated with trafficability properties. 



In contrast to the detailed data given by bearing-strength tests and 

 similar measurements is the highly important military requirement 

 for prediction of trafncability on a regional basis. Soil maps, the 

 primary basic data for such predictions, are based on a genetic clas- 

 sification and are designed primarily for use in connection with agri- 

 cultural problems. This vast amount of detailed information can 

 be applied to trafncability prediction, as well as to the use of soils 

 in construction, by redefining agricultural soil map units (or groups) 

 in terms of various laboratory tests. This is now being done in a 

 study of Fort Benning, Georgia, which is to be published by the Office 

 of the Chief of Engineers, U. S. Army. Samples were taken of each 

 of the major agricultural soil types in the area. These were sub- 

 jected to the following tests at the laboratory of the Waterways Experi- 

 ment Station, U. S. Engineer Department, Vicksburg, Mississippi: 

 mechanical analysis (percent sand, silt, clay) ; Atterberg limits; ratio, 

 expressed as percent, of weight of contained water to weight of solid 

 particles ; and optimum dry weight (pounds per cubic foot) . On the 

 basis of these tests, each of the 64 soil series, types, and phases was 

 assigned an airfield classification group symbol (U. S. War Depart- 

 ment, 1944b). Each such group includes from two to ten agricultural 

 soil units and is defined in terms of drainage, plasticity, dry strength, 

 void ratio, compaction characteristics, optimum compaction (pounds 

 per cubic foot) , and California bearing ratio for compacted and soaked 

 samples. Besides these general soil characteristics each group is de- 

 fined in terms of its value for embankment, foundation, and base 

 course, the effect on it of frost action, and its shrinkage, expansion, 

 and elasticity. 



Water Supply 



Water for troop use is secured at the surface wherever possible, be- 

 cause of the greater speed and convenience involved. Ground water 

 must be sought, of course, in arid regions; and even in humid climates 

 it is often necessary to seek subsurface water because of contamina- 



