How does torque tell load capacity of helical piles

[Sparky4598]

I think I have a decent understanding of the concept behind how helical piles work, but what I am struggling to understand is how the installation torque translates to a load bearing capacity. I know it's different for each size pile probably, but how can torque tell you bearing capacity? It seems to me the torque needed screw it in at 5ft depth would be about the same as the torque 20ft down, aside from the slightly additional friction from the longer shaft. What am I missing?

 

[IRstuff]

Probably a similar reason the air pressure at 30 kft is different than the air pressure at 0 ft.

 

[Sparky4598]

Probably a similar reason the air pressure at 30 kft is different than the air pressure at 0 ft.

Possibly so I suppose. Indeed the pressure would likely be higher the deeper you go. But I wouldn't think it would be enough to matter. Soil is more or less a solid where air is a fluid. I mean, it's no harder to drill a hole at the bottom of a concrete wall that is 20ft tall vs the top. I may be wrong though.

 

[dik]

Tests have shown there is a strong correlation between torque and load capacity for cohesive soils. I used to have literature, but left it at my old place of employment. I had to design a 3' thick transformer pad for six 400K transformers (they work for large loads, too).

It is likely due to the frictional resistance of the auger helix being greater for better soils.