Helical Screw Pile Torque Pressure & Capacity

Question

Helical Screw Pile Torque Pressure & Capacity

mikealpha88 (Structural)

(OP)

7 Oct 21 13:42

Hello fellow engineers.

I have an interesting question for engineers who may be designing screw piles.

In one of the recent projects, screw piles were nominated to manufacturers specifications. The piles were supposed to be designed/installed to support 300 kN SWL.

After installation I have been provided with the below details from the piling contractor.

Shaft dia 168.3mm x 4.8mm walls
Single helix 16mm thick, 500mm dia
Depths of each pile to reach 3500 PSI pressure.

Usually, there is a torque reading and then Kt factor to multiply to torque reading to get the ultimate capacity of the pile. However, I have been provided with pressure reading.

If someone can please help me understand that if this pressure reading is the for the hydraulic fluid from the torque/pressure gauge or actual ultimate capacity of the pile in PSI?

Thanks

ATSE (Structural) · Answer 1 · 2021-10-07 15:39:08

Mike, I think you're on the right track. It appears you are missing one key piece of information: hydraulic line pressure vs torque relation, which of course just depends on your hydraulic motor specs.
You can get the Kt factor from AB Chance, and depends on the soil type.

PEinc (Geotechnical) · Answer 2 · 2021-10-08 12:57:27

Someone should have and provide a calibration chart for installation pressure to torque. Then, knowing the pressure and resulting torque, you can use the Kt factor that is appropriate for the type of helical anchor that was installed. Check with the helical anchor supplier for the correct Kt factor to use in your particular situation. The Kt factor is not always the same for each project.

www.PeirceEngineering.com

mikealpha88 (Structural) · Answer 3 · 2021-10-08 21:32:46

Thank you ATSE and PEinc for your reply. I have requested torque information from the piling company.

I have spoken to another operator who was installing 10 ton helical pile on other project. The operator said that to reach 100 kN pile capacity they use 89 dia shaft with 6 ton excavator to 1400 - 1800 psi pressure to achieve 4000 Nm torque.

It seems a bit right on border. According to AB Chance the Kt factor is around 25 1/m so the Qult = 100 KN with FOS 2 it will be 50 kN capacity which does not make sense.

Can you please help me understand what am I missing here.

PEinc (Geotechnical) · Answer 4 · 2021-10-11 16:36:07

Can't you give information in one system of units? You showed 10 tons, 1400-1800 psi, 4000 Nm, 25 1/mm. What is this, a science or physics test?
Chance usually says to estimate ultimate capacity as torque in ft-lbs x 10 = ultimate capacity in pounds force. Your torque is 4,000 N-m = 2,950 ft-lbs. 2,950 x 10 = 29,500 pounds force ultimate = 14.75 tons. 14.75 tons / 10 tons (required allowable/service load?) = 1.475 safety factor.
See Pages 6 and 9 of the attached PDF.

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Helical Screw Pile Torque Pressure & Capacity