The first three conclusions are similar to those developed from 

 the water content measurements. The last conclusion is significant. 

 Strength values for surficial pelagic clays have been given in the 

 literature (Keller, 1967) as being less than 0.5 psi. These earlier 

 data were obtained from tests on small diameter gravity or piston cores. 

 It appears likely that these corers disturb the sediment significantly, 

 and that samples taken with them are much weaker than the material 

 would be in-situ. The earlier results, therefore, appear to be 

 incorrect. The in-situ strength of the box core material may be 

 slightly higher than the laboratory strength, and it would be desirable 

 to estimate the disturbance introduced during box coring using tech- 

 niques involving residual pore pressure measurements. (Lee, 1973b). 

 This is difficult, however, because certain disturbance parameters 

 cannot be estimated on the basis of two-foot cores. When longer piston 

 cores and in-situ data are available, an estimate of disturbance will 

 be made. For future reference, the residual pore pressure retained by 

 the box cores was measured and found to be equal to about -0.3 psi. 



An estimate of soil sensitivity was made by rotating the vane 

 rapidly through one revolution following a determination of the origi- 

 nal strength. A second "remolded" strength is measured and a sensi- 

 tivity is calculated as the ratio of original to remolded strength. 

 The measured sensitivities varied considerably from a low of 3.1 to a 

 high of 11.5. The mean sensitivity for both cores was about 6; this 

 would classify the soil as a sensitive clay, requiring special atten- 

 tion during bottom operations. 



Triaxial Shear . Five consolidated- undrained triaxial shear tests 

 with pore pressure measurements were performed on carefully trimmed 

 specimens obtained from Box A. Since the water content and vane shear 

 tests indicated little difference between the two cores, it appears 

 likely that the test results obtained are applicable to both cores and 

 possibly to pelagic clays in general. 



Triaxial testing has certain advantages over such simple tests as 

 vane shear and unconfined compression. One advantage is that during 

 the consolidation phase of the triaxial test it is possible to apply 

 stresses which simulate conditions well below the level of sampling. 

 It is, therefore, possible to estimate the variation of strength with 

 depth. A second advantage of triaxial testing with pore pressure 

 measurement is that it allows the determination of soil response in 

 terms of effective stresses as defined by 



a = a - u (1) 



where a = effective normal stress 



a = total normal stress 

 u = pore water pressure 



The vast majority of soil mechanics research indicates that it is 

 the effective stresses which determine soil engineering behavior rather 



15 



