Ch. 7] CONSTRUCTION ON SEDIMENTARY ROCKS 131 



ping Triassic strata. The extremes, however, were not so great be- 

 cause the series of strata consisted of clay shales, sandstones, and 

 conglomerates. In this area some of the sandstones were most 

 strongly influenced because of the solubility of the cementing ma- 

 terial between the sand grains. 



In an area of sparse surface outcrops, a contractor must gamble in 

 estimating the percentage of common and rock excavation, but a 

 careful geological study of the area and intelligent interpretation of 

 the subsurface findings can reduce the gamble to a negligible factor 

 and, in consequence, the contract bid price. 



The variations in the physical characteristics of most transported 

 soils may be extreme in both horizontal and vertical section. They 

 are most significant in glacial deposits, alluvial fans, deltas, terraces, 

 and beach deposits, but they may be less serious in eolian, flood-plain, 

 lake-bed or deeper marine deposits. The rapid variations in the dep- 

 ositional features and physical characteristics of these sediments in- 

 troduce many complications involving moisture content, density, por- 

 osity, permeability, and, ultimately, in bearing values. The many 

 combinations embrace too many factors for detailing in this brief 

 discussion. Nevertheless, an identification of the soil type and its 

 origin in advance of construction eliminates many possibilities and 

 permits intelligent analysis of those known to be associated with the 

 particular soil types concerned. 



CONSTRUCTION ON SEDIMENTARY ROCKS 



The principal sedimentary rocks may be divided into those me- 

 chanically deposited and those chemically or biochemically deposited 

 (U. S. Bur. Reclamation, 1942). 



Chemically or 

 Mechanically Deposited Biochemically Deposited 



Shale (consolidated clay) 04) Calcareous 



Siltstone (consolidated silt) Limestone (CaCOs) 



Sandstone (consolidated sand) Dolomite (CaC03 • MgCOs) 



Conglomerate (consolidated gravel or Marl (calcareous shale) 



cobbles, rounded) Caliche (calcareous soil) 



Breccia (angular fragments) Coquina (shell limestone) 



(B) Siliceous 



Chert 



Flint 



Agate 



Opal 



Chalcedony 



(C) Others 



Coal, phosphate, salines, etc. 



