The net results of all these departures are summarized in Table 3. 

 The table also indicates the magnitude of the effect. 



Table 3. Summary of Factors Affecting Vane 

 Shear Strength 



Factor 



Measured Vane Strength 

 R Actual Undrained Strength 



Magnitude of 

 Problem in 

 Seafloor Strength 

 Measurements 



Finite value for 

 angle of shear 

 resistance ( $ = 0) 



S R > 1.0 



Important 



Rate of rotation 

 greater than 6°/min 



S > 1.0 (assuming strength 



at 6%nin equals actual) 



Important 



Anisotropic soils 

 (S H > V 



S R > 1.0 (for S v ) 

 S R < 1.0 (for S R ) 



Small 



Vane insertion 



S R > 1.0 



Moderate 



Enlarged failure 

 surface 



S R > 1.0 



Moderate 



Incomplete stress 

 mobilization 



S R < 1.0 



Moderate 



Since the number of studies conducted on the vane shear testing device 

 is limited, no quantitative magnitude can be placed on the factors 

 influencing strength. Empirical evidence suggests, however, that the 

 results of the vane shear test provide a good indication of the 

 undrained strength of cohesive soils. The results tend to be 

 greater than results obtained by unconfined compression (Gray, 1957) 

 but compare well with in-situ failure evaluations (Carlson, 1948) and 

 consolidated undrained tests (Lea and Benedict, 1952). 



13 



