Ch. 1] IMPACT OF SEDIMENTS ON PRACTICAL ENDEAVOR 33 



cia. Mineral or rock composition also is used for some of the coarser 

 deposits. Coarse deposits consisting primarily of material of granitic 

 origin, in which the rock and mineral fragments are distributed in 

 essentially the same proportion as in the original rock, are called 

 arkoses. The term graywacke is used by some for a sandstone com- 

 posed mainly of feldspar and quartz embedded in a clay matrix, but 

 others use the term to indicate material derived from basic rocks in 

 contrast with arkose, which is derived from acid rocks. 



The proportion of calcium carbonate in a sediment causes con- 

 fusion in terminology. A sediment should contain at least 30 per- 

 cent calcium carbonate to be called limestone. If it contains between 

 10 and 30 percent the sediment is commonly called a calcareous 

 sandstone or shale, depending upon the average grain size of the clastic 

 constituents. Some workers object to the use of the term calcareous 

 for this purpose, as they would like to distinguish a sandstone or shale 

 cemented together by calcium carbonate from a sediment in which 

 detrital particles of calcium carbonate are intimately mixed with par- 

 ticles of terrigenous origin. 



Confusion also exists with respect to names for the sediments after 

 they have been consolidated into rock. A consolidated sand is almost 

 invariably called a sandstone, but a silt is classed as a siltstone or 

 shale, depending on the degree of lamination of the rock. If massive, 

 the rock is called a siltstone; if laminated, it is called a shale. De- 

 posits of clay particles similarly are designated as mudstone and shale, 

 respectively. If a worker is concerned about misinterpretation of the 

 terms, he should describe the basis for the classification he uses. 



IMPACT OF SEDIMENTS ON PRACTICAL ENDEAVOR 



The practical applications of sediments depend on the properties of 

 the sediments and the processes that affect them. The strength of 

 sediments and the changes in strength under added or reduced stress 

 are of fundamental importance to the construction engineer, whether 

 he be interested in bearing strength of foundations, stability of slopes, 

 or the construction of tunnels (Terzaghi and Peck, 1948; Taylor, 

 1948). The strength of loosely consolidated sediments and earth ma- 

 terials depends on the water content, the grain size, the mineral com- 

 position (particularly the type of clay minerals) , the stresses to which 

 the sediments have been subjected, and the length of time the stresses 

 have been applied. 



The water content depends on, among other factors, the load, the 

 grain size, and the clay mineralogy. The greater the load and the 



