Jh. 5] 



CONSOLIDATION 



105 



the soil mass can come about only by the extrusion of some of the 

 pore water. This process of the reduction in the volume of soils due 

 to the extrusion of pore water is called consolidation in soil mechanics, 

 a usage not to be confused with that of geology. 



Because the rate at which water flows through clays is generally 

 very low, there is a considerable time lag between the moment of 

 application of a stress to the clay and the completion of the strain. 

 This time lag is a function of the permeability of the soil. 



In the laboratory, saturated sam- 

 ples of clays are tested for consolida- 

 tion in a device called the consol- 

 idometer. In this device a small 

 disk of soil, confined on the sides 

 and bounded top and bottom by 

 porous stones allowing free drainage 

 of the sample, is subjected to load- 

 ing. The consolidation, or change in 

 volume, is measured by the displace- 

 ment of the top surface of the sam- 

 ple. The loading is done in incre- 

 ments, and each succeeding load is 

 applied only after consolidation has 

 ceased for the previous load. The 

 data from this test are generally 

 plotted on semi-logarithmic paper, 

 with the total consolidation for each 

 load expressed by void ratio e (the 



ratio of total volume of voids to total volume of solids in the sample) 

 as the ordinate and the load per unit of area or pressure p as abscissa. 



For the geologist, the interesting point is that it has been established 

 that the shape of this e-log p curve (Fig. 3) reflects to a certain extent 

 the history of loading to which a clay had been subjected during the 

 geologic past. Clays, which at one time had been covered by the 

 weight of a thick overburden or of an ice sheet, or in which there have 

 been fluctuations in the water table, undoubtedly retain residual effects 

 of this former early consolidation. Figure 3 represents a typical e-log p 

 curve of a clay with a previous history of consolidation. The break in 

 the slope of the e-log p curve is considered to be the expression of this 

 earlier preconsolidation, and the pressure value corresponding to this 

 break in the curve is roughly that of the former load. To establish 

 the preconsolidation pressure more precisely, Casagrande (1936) has 

 suggested a construction in which point b (Fig. 3) indicates the pre- 



Pressure p (log scale) 



Fig. 3. Method of determining 



preconsolidation pressure. (After 



Casagrande, 1936.) 



