direct shear box test

[znhcnhoj]

in the direct shear box test, there are two steel plates (top and bottom) that sandwich the sample. one side of the steel plate is smooth but the other side has a undulated profile that is in contact with the soil sample. I watch a video which say that when placing the steel plate, the direction of the undulated profile is at right angle to the direction of shearing.

Can any one know what is the reason?

 

[Panars]

Because you want the soil sample to shear within the sample, not in at the interface between the soil and steel plate.

 

[znhcnhoj]

Thanks for the response.
My understanding is that in the direct shear box the failure surface is already pre-determined at the interface between the top and bottom half, the undulated profile in the steel plate is to facilitate drainage during shearing, so it should not matter how it is placed.

 

[oldestguy]

No, it is to keep the stuff in the box from moving along that top and bottom plate, to create a total zone of constant properties of soil throughout with no pre-determined weaker areas. Otherwise a shorter failure plane may slope upward (or downward)to that weaker shear area at the plates.

 

[znhcnhoj]

Thanks Panars and oldestguy..