34 trask. PRINCIPLES OF SEDIMENTATION [Ch. 1 



longer the load has been applied, the lower is the water content and 

 the less the response of the sediment to added load or added stress. 

 The more permeable a bed is, the more rapidly the water is forced 

 out of it and the more quickly the sediment comes into equilibrium 

 with the stress applied. The finer the sediments and the more poorly 

 they are sorted, the higher is their uniformity coefficient and the less 

 permeable they become. 



When considering the release of pore-water pressure under added 

 stress, the engineer and geologist would do well to look on this con- 

 dition as a dynamic state and not as a static state. The water moves 

 between the constituent grains, thus forming a greater proportion of 

 the unit volume of a sediment in some places and a lesser proportion 

 in others. If the stress continues long enough or is sufficiently strong, 

 shear cracks develop through which water can move. Thus any 

 measurement of shearing stress made in the laboratory must be inter- 

 preted in light of conditions as they exist in the ground. A laboratory 

 test may indicate a high angle of internal friction, but, when a load 

 is placed upon the sediment in the field and the water begins to move 

 through the constituent grains, the relations of water to individual 

 grains change. The engineer should realize that, if the water forces its 

 way between the grains so that the grains do not come to rest upon 

 other grains in as many contacts per grain as they did previously, the 

 shearing stress at such places becomes less and, if several such places 

 should suddenly develop in a plane, a very different condition of shear- 

 ing stress results than during a laboratory test in which the movement 

 of the water within the sediment sample may be materially different 

 from that in the field. It is perhaps for this reason that the apparent 

 angle of internal friction is low along the planes on which sediments 

 sometimes fail. 



Clay minerals differ in their affinity for water. Individual mineral 

 grains or flakes of clay react differently to water under different con- 

 centrations of cations in pore water (Grim, 1942; Kelley, 1939). This 

 subject has not yet been thoroughly explored, but it is one that should 

 be considered seriously by the engineer when working in areas where 

 previous construction experience is scanty. (See Chapter 25.) Very 

 likely the different relationships between liquid limit and plasticity 

 index described by Casagrande (in Terzaghi and Peck, 1948, p. 36) 

 depend upon the type of clay mineral. Otherwise why should all the 

 sediments in a given area show such a constant relationship between 

 liquid limit and plasticity index, whereas all the sediments in another 

 area exhibit a different but equally consistent relationship? 



Alternating freezing and thawing of the ground in cold climates 



