Buffington, Hamilton, and Moore 



The vane- shear machine, mounted rigidly on the brow of 

 DEEPSTAR in a frame with the two thin-walled tube samplers, 

 operates at ambient pressures with a signal and power lead through 

 to the pressure hull. The machine features a logarithmic accumula- 

 tor which supplies torque to the vane and allows good sensitivity at a 

 wide range of strengths. The data read-out is in electrical resist- 

 ance calibrated to torque. For this test, the machine was set up to 

 measure strength in the interval 20 to 22. 5 cm beneath the sediment 

 surface with a 2. 5 x 2. 5 cm vane. The tube samplers were 30-cm 

 long with an inside diameter of 6, 67 cm and were fixed at a distance 

 of 26 cm on both sides of the vane. Thus, as the vane was inserted 

 into the sediment for the in situ strength test the sample tubes also 

 penetrated the sea floor and were filled. The samples were carefully 

 handled and stored under sea water in their collecting tubes until the 

 laboratory measurements were made. 



The measurement in the sea floor gave a shear strength of 

 22. 13 gm/ cm^ and the adjacent samples tested at exactly the same 

 depth intervals within the sediment showed strengths of 21. 81 and 

 27. 91 gm/ cm2 when later measured with a calibrated laboratory 

 vane- shear strength machine. It is apparent that, in this test, the 

 variation in sediment strength between the two cores taken 56 cm 

 apart is significantly greater than the variation between _in situ and 

 laboratory measurement of shear strength. With this limited data, 

 the tentative conclusion is that laboratory measurements of vane- 

 shear strength are valid on gravity type cores at least 20-cm long, 

 assuming a well-designed core tube and proper handling between 

 collection and testing. 



For this type of comparative test, it is imperative that labora- 

 tory measurements of shear strength be made on cores collected as 

 closely adjacent to the in situ measurement as is practical, and at 

 exactly the depth interval in the core which corresponds to the in situ 

 depth measurement. Without this precaution the comparisons may be 

 meaningless as lateral variations and, particularly range in strength 

 with depth may be great. The laboratory vane strengths of these 

 sediments, for example, varied from 5. 79 gm/ cm^^ at 3 to 5. 5 cm 

 beneath the sediment surface, to the previously stated values at 20 

 to 22. 5 cm which are about four times as great. 



90 



