Auger Boring Thrust Force and Drag Estimation

[pbc825]

I’ve been approached by a contrator to provide structural engineering services for an auger boring project. The project has been awarded and the prime contractor has asked the contractor to confirm the pipe size and grade is suitable for installation. Long term performance and operation is out of our scope. Although this scope does not include a geotechnical scope, I have a geotechnical masters degree and familiarity with piling techniques.

I spoke with a rep from the prime contractor. He’s described the scope as being to ensure the contractor is set up for success. The right machine, pipe strength and stability, etc. I’ll certainly do some pipe strength and stability calcs, but I think it would be ideal to estimate the drag and overall push force based on the soil bore hole logs. I have not yet reviewed the logs, but the contractor has described the souls as clayey.

That’s a big preamble, but the question is friction generation on the casing. How might I estimate it? The casing has a band on the front end equal to the thickness of the casing. They may choose to lubricate as may be required with water. Internet research has not been helpful. Any responses or shared experiences are appreciated.

 

[oldestguy]

As a geotech also, I'd not be asking this question until the field and lab work is done for the geotech portion. You may not have a question then.

 

[EireChch]

I agree with OG, why bother until you have too.

If the contractor is any way experienced I’m sure they know what equipment works for which ground conditions. Contractors tend to know a lot more that engineers when it comes to constructability, considering they do it day in day out.

If it’s clays with reasonable shear strength, do you even need to case? If groundwater is an issue just Tremmie the concrete to flush the water out.

But to attempt to answer your question, you could treat the problem as a tension pile. The resisting force would be frictional resistance around the shaft. Since it’s temporary casing, you could calculate the total frictional capacity. That would be alpha x Cu x area of your shaft.

 

[oldestguy]

I was going to edit above and add about what is in the last paragraph of FireChch, so now here.
Pipe piles probably are similar to this situation.

 

[pbc825]

Thanks fellas.

The contractor has much experience and was floored by the request for structural engineering support. I really have much less experience than they do. I will certainly rely on their experience for these road crossing casings.

I’ll likely approach based on pipe driving for an estimate, but I’ll also ask for a flat out guess on estimated thrust force from the contractor. I would think the extra thick band in the tip would tend to reduce friction along the majority of the pipe. Also, a big difference would be the tip resistance as the soil plug would be augered. If anyone has advice on these considerations it would be appreciated.

 

[fattdad]

design engineers don't often get into means and methods. Generally-accepted practice includes boring logs, SPT N-values, soil classification and strength testing. Contractors should be able to filter through that to make their equipment selections. Getting a design professional on that decision tree is likely a bad idea.

Then again, maybe I'm missing something?

f-d

ípapß gordo ainÆt no madre flaca!

 

[racookpe1978]

SO, Listen and learn. Get your feet (er, boots) and hard hat dirty.

True, its a construction-support job, but use it to learn more about the other end of the tools.

 

[fattdad]

maybe then, google, "Crowd force in drilling?" You will learn what the various rigs can deliver in vertical down-pressure. That and torque will get you something.

f-d

ípapß gordo ainÆt no madre flaca!