Screw Pile Design

[Newgrad AZ]

Hi. I have been tasked with specifying two light unidentical prefab buildings in a desolate region. Screw pile seems to be the most economical option for the foundations. The client is very much against doing any geotechnical investigation on the site. Though it is a common practice to supply the screw pile contractors with just the foundation reactions so that they would install the piles without the need for any geotechnical inputs. However, I am concerned about the possibility of difference in settlement between the buildings and its effects on the pipes that run between the buildings supported on their exterior walls on the either sides. Could any analysis be done on this issue without having geotechnical parameters of the site?

 

[dik]

You should have a geotech report... failing that, with cohesive soils, there is a pretty good correlation with installation torque and vertical load resistance... you have to dig up the appropriate value...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[WesternJeb]

All clients are against geotech reports because they are more money. I believe IBC (2015 version at least) requires a geotech report in section 1803.2 unless you have one from an adjacent property. Just point this out to them and tell them your hands are tied.

 

[Newgrad AZ]

Thank you @dik and @WesternJeb. I will push for a geotech report otherwise there will be no way for me to accept liability for this design even with the torque-capacity correlations. If the piling contractor is willing to do it, that's another matter.

 

[AskTooMuch]

Is there a concern on lateral capacity? We've done hundreds of screw/helical piles inside our plant. Even with geotech, their calculation for compression capacity doesn't always match once we start drilling, we either add more piles or stop once torque is reached. The torque from actual drilling will give you the actual compression capacity so I probably will be ok without geotech.

Lateral capacity is calculated in Lpile using the geotech report so you can't check the actual lateral capacity based torque. Unless you do pile load test.

 

[dik]

From literature, I know that for cohesive soils, there is a close correlation between installed torque and capacity and that this can vary between soils in different areas... I don't know by how much. When I designed my helical piles, I was working directly with the geotekkie and some piles had four or five helices (maybe 6?)...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[dik]

...and include time for re-design.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[phamENG]

Helical piles are great, and torque correlations are a great verification. They only tell you about the soil immediately surrounding the helix. In some areas, you can hit lenses of dense material or have hard layers over soft. You might drill down 20 feet, hit the "right" torque value, and be ignorant of the fact that there's some WOH material at 22 feet. Woops.

You need a geotechnical investigation to adequately interpret the torque values.

 

[Newgrad AZ]

Thank you @AskTooMuch. The buildings are small (10ft high) and light so Overturning is the major issue. I agree that the screw piles are often fully delegated to the piling contractors, and competent contractors do a decent job in delivering a qualified design and installation most of the times. My point is that I cannot take liability for the work if I am expected to go in blindly without any Geotech input.

 

[Newgrad AZ]

Thank you @dik. I would say several iterations might be necessary to arrive at a qualified design if all the base parameters including geotech are available.

 

[Newgrad AZ]

Thank you @phamENG. I absolutely agree. It is beyond point of doubt that geotechnical conditions can vary greatly in depth which could easily be left undetected by the piling contractor. All the more reason to go in with geotech support.

 

[jerseyshore]

We do a million helical piles in this area and for any real structures I require a geotechnical report. Smaller projects I may allow a test pile or two in place of a geotech report, but not for any big or commercial projects.

 

[AskTooMuch]

Shouldn't the helical pile engineer be the one asking what he needs? Unless this is a project where no helical piles engineer is involved.

In my previous projects, the helical pile engineer is fine with geotech report near the project but still within the plant. They are also fine doing their own SPT or CPT and not needing a full geotech report. This is a cheaper option.