Vane Shear Strength 



As discussed earlier, the shear strength profile of each core 

 was established by utilizing a vane shear testing apparatus. Although 

 the vane shear device has received almost universal acceptance as a 

 tool for defining the undrained shear strength of cohesive soils, 

 several theoretical limitations exist. In several instances, these 

 limitations assist the reader in understanding the applicability of 

 the results, while, in other cases, the limitations restrict the 

 usefulness of the strength data. 



The simple relationship between vane shear strength and torque 

 is based upon several assumptions, which can be stated as follows 

 (Skempton, 1948; Kenney and Sandva, 1965; Brand, 1967): 



(1) The soil is purely cohesive ( <j> = 0) ; no drainage takes 

 place during shearing. 



(2) The soil is homogeneous and isotropic. 



(3) Insertion of the blade causes no disturbances. 



(4) Shearing takes place by shearing over the surface of the 

 cylinder generated by the rotating vane. 



(5) No progressive failure takes place in the soil, and the 

 shear strength is fully mobilized on the surface of the cylinder at 

 failure. 



The validity of these assumptions determines the significance of the 

 test's results. 



Theory assumes that the soil is purely cohesive. The consequence 

 of a purely cohesive soil is essentially no drainage during the 

 shearing process. The validity of the assumption depends upon the 

 type of soil tested and the rate of vane rotation. 



When the soil is not purely cohesive ( <\> = 0) , test results from 

 the vane shear test become very difficult to interpret. Although 

 certain individuals (Evans and Sherratt, 1948) have advanced expressions 

 which account for a finite value of the angle of shear resistance (<(>) , 

 none of these expressions have proved to be applicable to more than a 

 few soil types with very low values of <)>. When the angle of shear 

 resistance does not equal zero, such factors as dilatancy during 

 shearing and the mode of failure (failure ceases to take place along 

 the bounds of a cylinder) must be considered (Brand, 1967). 



For a particular cohesive material, Sridharan and Madhav (1964) 

 found that the shape of the stress-strain curve depends upon the 

 rate of strain. The strength increased with increasing strain rate 

 while the strain at failure decreased. Sridharan and Madhav attribute 



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