G. SHEAR STRENGTH 9 



Introduction — Compressive strength and laboratory vane shear test analytical 

 procedure and depth related to strength previously was described and discussed (part 

 one). In brief, shear strength, s, is defined by the relation 



s = c + a" tan £ (18) 



where c is the apparent cohesion, a is the effective stress normal to the shear plane 

 (total stress minus pore pressure), and / is the angle of shearing resistance. Fine- 

 grained, saturated sediments stressed without change in water content behave with 

 respect to applied stress as if they were purely cohesive materials having an angle 

 of shearing resistance equal to zero (^ = 0) . In this special instance s = c . Com- 

 pressive strength, p c , is related to the cohesion or shear strength by 



P 

 c 



(19) 



The compressive strength test also is known as the U (for unconfined) test. Labora- 

 tory vane shear tests give results directly in shear strength. 



Cohesion -- Values of cohesion in the samples tested range from 4.2 to 234 

 g/cm2. Most cores have minimum cohesions near the top and maximum values usually 

 at some depth other than the bottom . Table 4 summarizes surface or near surface 

 cohesions. An average of the fifteen to 5 cm cohesion measurements of predomi- 

 nantly terrigenous sediments is 19.8 g/cm^, The stronger calcareous Area D sediments 

 have an average of 43.2 g/cm^ (2 measurements) in the to 5 cm interval . 



Cohesion generally shows a direct relationship to wet unit weight and an inverse 

 relationship to water content or porosity. Relation to median diameter, percentage 

 of the sand- or clay- size fraction, and plasticity index is variable, sometimes direct 

 and sometimes inverse. 



An extremely important book by the American Society of Civil Engineers (1961) on its 

 research conference on shear strength of cohesive sediments appeared after this report 

 was written. Unfortunately, it has not been possible to incorporate recent results 

 reported in this paper, except for a few instances, because other commitments upon my 

 time precluded an opportunity to assimilate its contents. 



35 



