G3 



10-20 



G8 



20-30 



G9 



20-30 



G 10 



20-30 



H 13 



16.5-21.5 



TABLE 4. SUMMARY OF SURFACE AND NEAR SURFACE COHESION (Cont'd) 



Cohesion 

 Sample Cohesion ' Area Average 2 

 Core (cm) (g/cm 2 ) (g/cm 2 ) 



14.4* 

 25.2* 

 14.3* 

 17.1* 



71.7* 



The logarithm of cohesion related to porosity, with an eye-fitted straight line 

 representing the suggested average, is. shown for each area (Fig. 11). Scatter of 

 values about the straight line varies from small in Area B to large in Areas E and G, 



It was stated previously that the slopes of lines representing averages In each 

 area, shown on a porosity-percentage of clay-size graph, denote relative rates of 

 deposition In the sequence (from slow to rapid): H — D — F and F 6 — G — A' — E' — 

 C — B (Fig. 10). A similar correlation between relative rates of deposition and the 

 slope of lines representing averages appears on a logarithm of cohesion-porosity graph 

 (Fig. 12). The sequential relationship shown in Figure 12 is identical to that of Figure 

 10, except that the position of Area B and core F 6 in the sequence is reversed in 

 Figure 12. 



Since there is a clear relationship between porosity and percentage of the clay- 

 size fraction, a similar relation between the latter and cohesion was expected but 

 not found (Fig. 13). The variable relationship of cohesion to the percentage of the 

 clay-size fraction in each area is shown graphically In this figure. 



For most engineering purposes, cohesion or shear strength in fine-grained, co- 

 hesive sediments from a given area can be estimated with reasonable accuracy from 

 a knowledge of the water content (compare Figs. 11 and 12) and slope of the line 

 representing the average relationship of porosity to cohesion . Measurement of water 

 content by neutron moderation '" and measurement of wet unit weight by gamma-ray 



The number of moderated or slow neutrons detected per unit time is a measure of the 

 concentration of hydrogen atoms that are contained in molecules of free water, assumi 

 absence of organic material . A slow neutron count thus becomes a measure of the sed 

 ment water content . 



37 



