Other data obtained from consolidation testing relates to time 

 rate of consolidation. The two important parameters are the coeffi- 

 cient of consolidation, c , and the coefficient of secondary compres- 

 sion, C . Techniques for using both of these in settlement calcula- 

 tions are given in Navy Design Manual DM-7 (U. S. Navy, 1971). The 

 coefficient of secondary compression, C a , was obtained directly from 

 plots of displacement versus the log of time. It is equal to the 

 sample axial strain per log cycle of time for times clearly beyond the 

 termination of primary consolidation (Lambe and Whitman, 1969, p. 421). 

 The coefficient of consolidation, c , was calculated using the log time 

 fitting method (Lambe and Whitman, 1969, p. 412). A plot of the cal- 

 culated values of these parameters as a function of applied vertical 

 effective stress is given in Figure 5. The values of c-y given are 

 relatively low and the values of C a are relatively high as compared 

 with cohesive soils in general. In practical terms this means that 

 these pelagic clays respond slowly to applied stresses and that signif- 

 icant movement may continue for a great deal of time into the future. 



The distribution of vertical effective stresses and densities with 

 depth below the seafloor may be estimated from consolidation data by 

 assuming that laboratory compression behavior reproduces sedimentation 

 behavior. Stresses are applied which simulate vertical overburden 

 pressures at some depth. The soil density which the sample acquires is 

 taken as the density at that assumed depth. The stress distribution 

 may be obtained by converting the void ratios to densities and then 

 integrating these densities incrementally with respect to sub-bottom 

 depth. A plot of estimated vertical effective stress was obtained for 

 Box A and is given in Figure 6. This information will be used in the 

 next section to determine how the soil shearing strength increases with 

 sub-bottom depth. 



Shear Strength 



Vane Shear . As soon as the box cores were received at NCEL, the 

 box sides were removed and a number of vane shear tests were performed 

 on the intact material. The tests were conducted with the vane axis 

 perpendicular to the core axis. Small cylindrical cores were taken 

 from the box samples and sealed for longer-term storage. Triaxial and 

 consolidation tests were performed on these, and as the last phase of 

 the test program, a series of vane shear and water content tests was 

 performed on one of the small cores . A plot of all of the vane shear 

 strength data as a function of depth in the core is given in Figure 7. 

 There is considerable scatter to the data; however, the following con- 

 clusions appear justified. 



1. There is little difference between the two cores. 



2. The one-year storage had little influence on the strength. 



3. There is no consistent trend of vertical variability, although 

 there is some random scatter. 



4. The mean vane shear strength for both cores is approximately 

 1 psi. 



11 



