Torque in Helical Piles 2

[DocDirt]

Helical pile advocates use torque to determine bearing capacity of helical piles. What is the theoretical basis for calculating torque of the machine to bearing capacity of geomaterials?
DocDirt

 

[JAE]

I believe it is simply empiricle values derived over time based upon in-situ tests of piles vs. torque - for different types of soils.

See this link:

http://www.abchance.com/resources/technical/01-9410.pdf

for an example test summary report by A.B. Chance.

 

[PSlem]

Friction against the helix is the main component of the torque. Take a 14" diameter helix with a 3" pitch and an area of 1.05 sq. ft. on a minimal shaft in a clay with a cohesion of 5000 PSF. The helix area times the cohesion of the soil times the cos(avg. helix angle) times the alpha friction factor x midpoint of area will give you a torque. 1.05 SF x 9 x 5000 x cos(11*) x 0.3 x (.58' x .707)= 5700 lbs-ft. If pile capacity is 1.05 x 9 x 5000 = 47,250# per the Standard Bearing Capacity Equation then Kt = 47,250/5700 or 1/(cos(11*) x 0.3 x (.58' x .707))or 8.3.

 

[DocDirt]

Thanks JAE
Special Thanks to PSLEM. Could you please tell me as to how to calculate the area, in this case of 1.05 sq ft from the diameter of 14in and pitch of 3 in?
Will be much obliged.
DocDirt

 

[PSlem]

The area is just a 14" circle. 14/12 =1.17'. 1.17^2 x PI/4 = 1.07. I did net area less shaft of 1.05 from memory. It should be 1.07 here as I said minimal shaft. The pitch is small so can be ignored. The outer perimeter of a 14" disc is 14 PI or 44". Considering the pitch it is SQRT(44^2+3^2)= 44.1".

 

[PEinc]

Literature from the helical anchor manufacturers will also give the helix areas for different, available helices.

 

[DocDirt]

Thanks again PSlem for your valuable insights and contributions.
Thanks to PEinc for helpful hints
DocDirt

 

[DocDirt]

I was wondering if there is anyway to estimate the actual socket friction in a drilled shaft from the torque put out by the rig?
DocDirt

 

[PSlem]

No. Rate of cutting will affect torque.

 

[DocDirt]

Pslem
Would not the rate of cutting affect the torque in the case of helical piles too?
DocDirt

 

[PEinc]

Table of A. B. Chance helix areas:

http://www.abchance.com/helical/square-shaft.asp

 

[DocDirt]

thanks PEinc
DocDirt

 

[PSlem]

Helix piles should screw in at 3" per revolution, the pitch. If they spin on hard material, then the torque drops drastically.

 

[DocDirt]

PSlem
Thanks again. Let me admit that I had an ulterior motive in starting this forum. We are working on a drilled friction socket caisson project. Rock is blocky and jointed shale. Designs call for an allowable socket friction of 25 psi. I represent the field inspection group. We are using a SOILMEC 4 in dia auger for the job. We are now using SID observe the rock and approve it. I was looking for another independent way to verify the friction capacity.
DocDirt