Other relationships derived from the Atterberg limits are the liquidity Index' 

 (Terzaghi, 1936), B, which relates the water content to the limits and is defined as 



w -PL 



PI 



x 100 (17) 



and colloidal activity (Skempton, 1953a, 1953b), a c , which is defined by equation 

 21 . The liquidity index denotes the ratio of excess natural water content above the 

 plastic limit to the plasticity index (Capper and Cassie, 1960, p. 57). At a liquidity 

 Index of unity, or 100 percent when expressed as a percentage as recommended by the 

 ASCE, the water content equals the liquid limit. Terzaghi (1955, p. 563) notes that 

 although the liquidity index is close to 100 percent in the surficial layer of a normally 

 consolidated" clay deposit, it is always greater than 200 percent in a sediment formed 

 on the bottom of a vessel filled with a clay suspension — an under-consolidated deposit. 

 On page 566 of his paper, however, Terzaghi mentions that marine clays in their orig- 

 inal state have liquidity indices of about one hundred percent. The surface samples of 

 the cores examined possess liquidity indices of approximately 200 percent. Appreciably 

 higher liquidity indices are found in several cores. A surface layer liquidity index 

 greater than 1,200 percent, as a result of very low plasticity index, was found in the 

 two calcareous gravity cores from Area D . The consolidation history of the cores will 

 be discussed in a paper being prepared, as was previously mentioned. 



The liquidity index was computed for each sample having a measured water content. 

 In most instances, the plastic limit and the plasticity index were usually averaged 

 over several samples rather than determined on each sample because of small variabil- 

 ity in the limits. Liquidity indices will be somewhat less than shown in Appendix B 

 tables in the few instances where a marked decrease in water content occurs over the 

 interval measured for liquid limit. 



p 

 A sediment deposit that has never been subjected to an effective pressure greater 



than the existing overburden pressure and one that is also completely consolidated by 



the existing overburden is normally consolidated (ASCE). 



34 



