108 kaye. PRINCIPLES OF SOIL MECHANICS [Ch. 5 



has caused many serious slides in deep cuts that had been perfectly 

 stable for years. Testing of samples of these clays for shearing 

 strength gives absolutely no indication of the specific weakness from 

 which the soil eventually failed. 



Many, if not most, landslides and soil failures are due to the de- 

 velopment of high pore-water pressures. The evaluation of the hy- 

 draulic properties of soil is based primarily on geological conditions 

 which often are so insignificant as to escape detection by all but the 

 trained geological observer. Small pervious or impervious layers 

 may influence the path of pore-water movement and the localization of 

 pore-water pressures. The eventual failure may occur in a way en- 

 tirely neglected in the stability analysis. In such cases, the strength 

 of the predominant soil has no bearing on the strength of the slope, 

 and it is the geological interpretation of structural relations that gives 

 the clue to specific weaknesses. 



(2) The geologist can reconstruct the history of a clay deposit and 

 in this way roughly compute the type, duration, and amount of the 

 preconsolidation load. Knowledge of this kind is of value in inter- 

 preting consolidation, settlement, and shearing-strength data in many 

 clays. 



Surficial clays may possess very much higher strength than deeper 

 clays, owing to changes of the ground-water table. With each lower- 

 ing of the ground-water table, surface clays experience a renewal of 

 consolidation due in part to desiccation and in part to the increase in 

 load resulting from the reduction of buoyancy. This process may not 

 necessarily affect the deeper clays. In situations of this sort, the 

 engineer, when adequately forewarned, can resort to deeper sampling, 

 where he may find important differences in soil strength. 



The geologist is inclined to wonder whether some examples of recent 

 earth movement are not perhaps due more to the consolidation of 

 clays than to tectonic disturbances. The recently reported findings of 

 a Roman city many fathoms beneath the surface of the Mediterranean, 

 just off the mouth of the Rhone River in France, may be an example 

 of the extreme consolidation of thick, soft deltaic deposits rather than 

 a case of crustal subsidence of the area. 



(3) The geologist can study the granular characteristics of fine- 

 grained soils, particularly clays, and correlate his observations with 

 data on mechanical behavior. The mineralogy of clays, the inter- 

 granular structure of clays, and the physicochemical characteristics of 

 clays are frontiers of research which will in time yield information of 

 the greatest usefulness to soil mechanics. 



It is well known that the shape of sand and silt grains affects to 



