Atterberg (1911j p. 20-22) originally defined seven limits of plasticity. Two of 

 his limits, the liquid limit ("Fliessgrenze") and the 'Vol I -out'! limit ("Ausrollgrenze")— 

 now called the plastic limit, were adapted by Terzaghi (1925a, p. 20-29; 1925b, 

 p. 799) for use in soil mechanics. Liquid limit is defined (ASCE) as the water content 

 (in percent of Ovendry weight) corresponding to the arbitrary limit between the liquid 

 and plastic states of consistency of a sediment; plastic limit is the water content corre- 

 sponding to an arbitrary limit between the plastic and semisolid states. The numerical 

 difference between the liquid and plastic limits was called the plasticity number by 

 Atterberg (1911, p. 29-30; 1913, p. 293); it 5s now known as the plasticity index. 

 Determination of Atterberg limits is discussed later. 



A system of classification developed by Casagrande (1948) for application to 

 design and construction of airfields uses a plasticity chart relating plasticity index to 

 liquid limit (Fig. 4). in this chart the A-line represents an important empirical bound- 

 ary (Casagrande, 1948, p. 919) between typical inorganic clays generally above the 

 line, and plastic sediments containing organic colloids and typical inorganic silts and 

 silty clays below it. The plasticity chart shows two especially interesting relation- 

 ships that have been emphasized by Terzaghi (1955, p. 564): (1) Atterberg limits of 

 grain-size fractions of ground minerals plot in a straight line roughly parallel to the 

 A-line and may be located above or below the line depending on mineralogical com- 

 position, and (2) points representing different samples from a geologically well-defined 

 sedimentary deposit also are located on such a line because of the likelihood of similar 

 mineralogical composition of the clay-size fraction. A corollary of the last statement 

 is that if points representing two members in the plasticity chart are located on differ- 

 ent lines then it is almost certain that the sediments have different sources (Terzaghi, 

 1955, p. 565). Trask and Rolston (1950; 1951, p. 1092) confirmed these relationships 

 for San Francisco Bay sediments. These relationships also are valid for the sediments 

 investigated. All sediments shown in Figure 4, with two exceptions, have a similar 

 depositional environment, despite the wide variation in water depth and geographic 

 location. These sediments are composed of terrigenous material and plot on a line 

 above and parallel to the A-line. One of the two exceptions was the Area D cores, 

 which are of calcareous material, the other was core B 83, which was collected 

 farthest from land and probably consists of mixed terrigenous material and deep-sea 

 (Foraminifera) ooze; samples from core B 83 plot as points on and below the A-l?ne 

 between liquid limits of 50 and 60 percent. The marked difference 5n liquid limit 

 and plasticity index of Area D gravity cores 1 and 2 (very low plasticity index) and 

 piston core 1 (high plasticity index) probably results from the greater proportion of 

 sand-size material in the gravity cores. It is noteworthy that samples from the cal- 

 careous D Ip core, although plotting above the A-line, fall on a dissimilar slope 

 compared to those samples having a terrigenous origin. The sediments in Area F with 

 Sow liquid limit are exclusively from core 6 and have different physical properties 

 compared to other cores from the same area (see Fig. 3). 



11 



