Drilled Pier Foundations for 35' Mast Arms

[bucknell06]

Hi,

I am just out of school and am working on my first project associated with drilled pier foundations for mast arms and I am wondering what I need to include for parameters in the geotechnical report. The report will be submitted to a structural engineer for design of the foundations. Borings were performed and the typical soil profile is about 6 feet of granular fill (unit weight = 125 pcf, angle of internal friction, phi = 30) with N = 20 overlying dense glacial till with N > 50. Groundwater was encountered at approximately 7 feet bgs.

In my report I have provided the following:

1)unit weights and angles of internal friction for both the fill and glacial till

2)active, passive, and at-rest earth pressure coeff.

3) Ultimate Base Resistance

Is this enough information for a structural engineer to design the foundation?

If not, what else is a standard inclusion?

Thanks for your time,
Nate

 

[TDAA]

Talk to your boss about what is normally included for that type of work.

Also, try talking to the structural; they may want something specific, such as parameters for LPile, or they may just go with the baseline parameters listed in the code.

Although not always practiced, talking amongst the design team can be very educational, an should lead to a better project as a whole.

 

[cvg]

how about chemical properties of the soil / water which may affect the concrete or steel corrosion?

 

[mcutchi]

Totally concur with TDAA, and would add that required (or desired) parameters would depend on type of pile chosen, whether lateral loads present. e.g. if deflection is an issue, lateral soil modulus would be helpful. You dont always get everything you ask for, so ask for a little more.

 

[bucknell06]

Thank you all for your responses. I have spoken with my boss about this, but we don't do many reports for drilled piers. He is of the understanding that we have provided the required parameters. I spoke with the structural engineer who indicated that I need to give A) an allowable bearing capacity and B) a side resistance (in psf) due to the fact that the mast arm is controlled by lateral loads (she then continued to explain the torsional and shear forces). I am not sure how the side resistance will be used in design and want to make sure that it is not misused to add axial capacity to the drilled pier.

According to the FHWA Design Method for cohesionless soil, I have provided all of the required parameters.

Can anyone explain (and/or direct me to the proper resource) how the drilled pier is designed differently when lateral loading governs and what parameters are needed?

I am trying to understand what is needed and how it is to be applied to this problem.

 

[Ron]

Bucknell06...you might also check with your State Department of Transportation as they usually have this topic pretty well covered in terms of their expectations for the geotechnical report and for the parameters required for their approval.

 

[TDAA]

When designed for lateral loads, there are a variety of things that can be provided:

1. PSF values for lateral resistance, per the code.
2. You could provide information for the subgrade modulus (DAS covers this in his Foundations book =>for granular: k(sub z)=n(sub h)*z, where you provide the equation, and the value of n(sub h))
3. You could provide information for the development of p-y cures for use in LPILE. We normally supply the values that LPILE recommends, since they tweaked the program and their values, based on field tests, to correlate.

It sounds like the structural is fine with option 1. See table 1804.2 for the IBC (2003) allowable values. Of course, be sure these are applicable to your setting. The structural engineer should not confuse the lateral allowable bearing capacity with the skin friction.

The methods above get more complicated in the order shown. There are a variety of books that go into P-y curves. You could also look at the Ensoft (makes LPILE) web page to get a bit of an over view. They also have some books concerning design. Also, look around on the Deep Foundation Institute (DFI) page. They likely have publications listed.

 

[bucknell06]

TDAA thank you for your informative post. I think I will end up providing the lateral resistance per IBC as you have suggested.

After speaking with the structural engineer again, I have determined that when referring to torsional forces, she means actual turning of the drilled pier against the soil. I had not considered this since usually piers are in a group where this type of action would not come into play. These particular piers are for signal lights and require only one drilled shaft per mast arm.

So I would assume that skin friction does factor into her analysis, but not from an axial capacity standpoint. I assume that similar to designing against uplift, this torsional capacity is governed by the friction between the soil and the concrete drilled pier.

 

[oldrunner]

This sounds like a single pier that can rotate due to torsional loads on the upper arms and/or unbalanced loads on the wires.

You also need to provide the start of the passive pressure below grade. The pier most likely will be deeper than six feet (the fill depth?), therefore you should be providing the parameters for that material also.

 

[eric1037]

The Florida Department of Transportation has some excellent documents on torsional loading. You could also find some things from Dr. Christopher Byrum through the Transportation Research Board.

 

[jheidt2543]

I've noted the following paper a few times before, but it is a very interesting and informative look at the problem.

You might find the paper "Analysis and Design of Laterally Loaded Piles and Caissons in a Layered Soil System", by T.R. Naik and A.H. Peyrot. It was published in the 1970's in an ASCE journal. The paper is based on a modificaiton of Broms' method and it has a very practical approach with a good example problem and a pretty extensive list of references.