Ch. 7] RESIDUAL SOILS 129 



if he knows whether he is dealing with residual, fluvial, lacustrine, 

 glacial, or some other type of soil. 



There is no general agreement relative to the classification and 

 identification of soils; hence no common usage of terms and their 

 meanings exists. Soils may be residual or transported. If the former, 

 they may be mature or immature; if the latter, they may be fluvial, 

 lacustrine, eolian, volcanic, or glacial. In each of the various types 

 there are infinite variations and rarely any approach to homogeneity. 

 Jenny (1941) says that the soil is a physical system consisting of 

 various properties that are functionally related. This relationship 

 may be expressed as soil-function (climate, organisms, topography, 

 parent material, time) . Any one of the properties of a soil, porosity, 

 density, etc., is determined by the independent variables listed in the 

 parentheses. When one establishes and evaluates the independent 

 variables, the soil type is fixed. However, by the very nature of their 

 origin, there can be no single set of physical constants for any sedi- 

 mentary deposit. A sample taken and tested from a layer located 

 between bedding planes gives no hint of the weakness in a sample 

 which includes the bedding plane. In an alluvial fan, glacial deposit, 

 beach, delta, and many other types of deposits, the intertonguing, 

 crossbedding, variable laminae, grain-size variability, mixing, and the 

 infinite variations and successions of materials of different densities, 

 porosities, and permeabilities make a typical sample a figment of the 

 imagination. Continuous sampling and extremely close spacing of 

 borings such as to show the true physical characteristics would not be 

 an economic feasibility. Even then the weakest layer would be the 

 determining factor governing the settlement of a fill or any super- 

 posed structure. 



In spite of these apparent unsolvable obstacles, the soils-testing 

 program is extremely important, and it certainly can and does indi- 

 cate the direction that construction in and on soils must take. It must 

 not be forgotten, however, that geologic factors associated with origi- 

 nal deposition and, perchance, features located far beneath the tested 

 zones may govern the permanent stability of a fill, foundation, or 

 cut slope. 



Residual Soils 



These vary from a few inches to many feet in thickness. They also 

 vary in accordance with the nature of the parent rock, the slope, and 

 the weathering processes acting on the bedrock. The types of proc- 

 esses involved in the formation of residual soils do not evoke the con- 

 cern of the engineer, but end products do. Irrespective of whether 



