sensitivities were greater than 8. Samples were not dried prior to testing on limit 

 tests at BUDOCKS and the Hydrographic Office, following recommendations by 

 Casagrande (1948, p. 922) and a recommended British test method (Norman, 1959). 

 One other difference in test procedure from that prescribed by the ASTM is that the 

 limits were determined from the entire size fraction rather than from only that portion 

 f i ner than 1 . 25 J> (0 . 42 mm) . 



Dawson (1960) has shown that different operators may obtain slightly different 

 results in the limit tests, despite test standardization. Furthermore, the type of 

 material the base of the liquid limit machine is made of affects the measurement. 

 Differences between British and American machines recently were summarized by 

 Norman (1958). These and other problems in determining Atterberg limits are dis- 

 cussed by Bauer (1960). 



Dr. Martin (1961, written communication) kindly informed me that the Atterberg 

 limits of certain sediments are very sensitive to changes in interstitial water salt con- 

 centration, and that because the salt content is unknown in the samples investigated, 

 the resulting data may be unreliable. 



Results — Measured values of liquid limit range from 25 (core F 6) to 109 percent 

 dry weight (core D Ip). A more normal range is between 50 and 80 percent. Measured 

 values of plastic limit range from 15 (core F 6) to 46 percent dry weight (core F 15). 

 Most values lie between 20 and 30 percent. Emery (1960, p. 260) has reported that in 

 cores tested at the University of Southern California the limits reflect changes in water 

 content; cores with large variation in water content gave smaller variation for liquid 

 limit and smallest variation for plastic limit. In general, this relationship was found 

 in the cores tested . 



The plasticity index, PI (LL-PL), affords a quantitative measurement of the plastic 

 characteristics of a completely remolded sediment sample by defining the range of water 

 content in which the sediment is plastic. Previously, it was mentioned that the A-line 

 of the plasticity chart is an empirical boundary separating organic and inorganic clays, 

 the former located below the line and the latter above. Considering only inorganic 

 clays, those having liquid limits greater than 50 percent are highly plastic; the majority 

 of samples tested are in this category (Fig. 4). Inorganic clays with liquid limits'be- 

 rween 20 and 50 percent have low or medium plasticity: core F 6 and certain samples 

 from Area C and E cores. Terzaghi and Peck (1948, p. 35) place the division between 

 inorganic clays of low and medium plasticity at a liquid limit of 30 percent. Using 

 this classification, only the 4 samples from core F 6 having water contents less than 

 40 percent have low plasticity. Burmister (I960, p. 98) considers the overall plasticity 

 of clay sediments with plasticity indices greater than 40 to be very high. 



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