Sign Foundation

[Lutfi]

This is an interesting situation:

I am working on a project where the site engineer wants to excavate an area for retention pond. However, in the same area exists a big billboard sign. The sign MUST remain. It appears that the sign is supported via a drilled pier system. All data about the sing is not available.

My civil engineer wants to excavate 7 feet on one side and slope the banks of the pond on two sides. My company does not want the liability for the sign foundation integrity. However, I am recommending that the contractor drive sheet piles all around the drilled pier square in plan. I also intend to starting the sheet piles about 6 feet from the face of the sign pole.

My question,

1. If any one encountered a situation like this what would you do?
2. Is the six-foot enough to eliminate the pressure influence from the foundation? If not, is there a method to calculate the safe set back dimension?

By the way, I intend on contacting the sing owner and have them approve my approach and sign of on it. In other words, have them come back and address our approach.

Your input and thoughts would be appreciated.


Regards,
Lutfi

 

[dimjim]

There should be codes and standards for you to
follow in your State. Check with the Department of
Transportation for signs and billboards. There
might even be city codes that you must follow.

 

[Lutfi]

dimjim,

I live in Florida and I designed many overhead signs. However, I never faced a situation like the one I am describing.

I already contacted FDOT and I could not get anyone to respond since it is close to Thanksgiving holiday.

Do you have a reference that provides guidance as to how much earth needs to be at the pressure and the kick of a pole type foundation?




Regards,
Lutfi

 

[jimaitken]

I would first the engineer doing the retention pond design to adjust the pond configuration so that the sign foundation is not affected.

If you can't relocate the foundation then you need to get as much information about it that you can. Get the plans and design data. If that is unavailable hire a geotechnical engineer do a field investigation and testing to confirm the foundation type, depth and capacity. This is critical. It may cost some money, but compare that cost to the costs associated with the sign falling over and hurting someone, not to mention all of the other costs associated with a failure.

Sheeting around the foundation may be acceptable, but if it were me I would want more information about the foundation before making any recommendation.

 

[SlideRuleEra]

Lutfi - I would consider using a continuous top wale around the sheet pile perimeter (kind of a cofferdam turned inside-out). Sheet pile interlock is not very good at resisting the tension that the sign foundation would apply as it tried to overturn under wind load. A top wale might be conservative, but for the small size of the structure, would relatively inexpensive.

Whether 6 feet is a good distance or not...
I defer to the Geotechs. You could take a look at "Pole Building Design" on my website - I lot of the info in that (old) publication comes from the Outdoor Advertising Industry.

http://www.SlideRuleEra.net

 

[jimaitken]

Lufti

Let me add that you might want to get a geotechnical consultant involved anyway to help you negotiate FDOT geotechnical requirements. The geotech might be able to give you some advice that will help you on what has been done in the past for similar situations.

 

[dik]

If they are going to be driving the sheet piling, the vibration may affect the sign foundation.

Dik

 

[haynewp]

I see something similar to this done several times where they excavate around large transmission towers and leave like 15 feet or so around the base. I am not sure exactly how they come up with the amount to leave in place. It must surely depend on the soil characteristics, the amount of the pole that is left below the excavation line, and the loads.

 

[Lutfi]

Many thanks for responding. I am going to gather all I can on the sign before final plans are released.

SRE, Your thinking lines up with mine. I am going to add a whaler. I am intent on using concrete sheet piles for aesthetics. I am being told the pond will be dry.

I downloaded the pole file from your site a while back. It is a good one. I also have the outdoor advertising institute "old" handbook as well.

I already have geotech on the project and I will ask him. The time is bad because almost everyone is about to leave for the holidays.

I am just curious as to the cut off where the soil adds not value to the lateral load resistance. If I find it, I will be sharing it with all.


Regards,
Lutfi

 

[SlideRuleEra]

Lutfi - This is a very interesting problem. Perhaps one way to look at it is use the analogy of the interaction of piling in a pile group. In this case, you are looking for the pile spacing where interaction becomes negligible - each pile in a group approximates the behavior of an isolated pile.

Here is a paper that looks at interaction of piling at various spacings:

http://www.asce-sf.org/downloads/clay_analysis.pdf

See pages 1, 13 and 19.

Consensus seems to be that at a horizontal distance of 6.5 to 8 pile diameters the surrounding soil does not "know" that a pile is being laterally loaded.

http://www.SlideRuleEra.net

 

[miecz]

Lutfi-

It seems to me that this is an issue that the geotech is paid to solve. I would dump it on his/her lap.

Jim

 

[Ron]

Lutfi...taking 6 feet of soil away from a drilled pier could be risky. The pier is designed to have a certain depth to achieve moment fixity for the wind loads. Let's say you have a drilled pier with a depth of 36 feet. If you take 6 feet away, you take a significant percentage of the lateral load resistance away (not 1/6, since the resistance at the top is less than at the bottom, but could easily approach 10 percent), particularly in your soil conditions (assuming this one is near your location).

The lateral distance relationship is not the same as for a shallow foundation. You might consider having them place a ringwall around pier to provide some top stability. I designed one about 50 miles to the west of you and we HAD to have the ring around the top to get it to work for wind loads (it was a cell tower foundation). Your wind loads are even higher and your soil conditions probably not any better, except that you might have a competent clay layer down there.

 

[pba]

Lufti,
It might be worth thinking about re-profiling the pond to make it shallower near the sign. Obviously there is no chance of moving the pond completely or you wouldn't have posted, but shallower near the sign and deeper further away may help some.

One more to consider - While the pond may be dry most of the time it presumably is a design expectation that it will also be full sometimes. You may need to consider the effect of the water on the dry soils around the sign and then conversely saturated soils around the sign and an empty pond after drawdown. Clearly more work for the geotech boys...

 

[Lutfi]

SRE, many thanks for the paper. I had a conversation on Friday with a good friend of mine who is a geotechnical engineer. He recommended to me to leave earth equivalent to not less than 5 times the pile diameter. His rational made sense to me for the spacing. The ASCE paper in pages 13 and 19 corroborates what he told me.

The problem is the area where the sign is located is very limited. It is on a 5 acres site and the sign happens to be located in the tightest retention zone. The sign is owned by a third party and the property owner gave them easement that has been recorded for few years.

I cannot locate or gather any information on the sign. I made contact with FDOT and gave them the sign permit numbers. The guy who is in charge will be gone till Monday. I hope they have plans for the sign structure and foundation.

I spoke with other engineers in my office and other offices regarding this problem. No one seems to have the right answer. One suggestion was to project a line from the bottom of the pier at 45 degrees. In other words, if the pier is 20 foot deep, then retain 20 feet of earth all around the sign. This may sound excessive. However, if the sign has 3 foot diameter pier this will require 24 feet of earth all around using 8 times the pier diameter.

Jimiec, I agree about getting the geoteh involved. However, I do not feel comfortable on dumping on his/her lap for two reasons. The first is I want to see how this problem would be solved. Secondly, I think it is my duty to ensure that the solution is proper. I owe this to the owner.

Pba, the pond will be re-shaped. However, the civil engineer has his work cut out for him to achieve his volume as well.

I will keep all posted. This problem is indeed challenging and the solution should be the best technically and economically.




Regards,
Lutfi

 

[SlideRuleEra]

Lutfi - Perhaps one solution would be to use pile group interaction to your advantage. Working with your Geotech, calculate a pile type, size, depth, spacing, etc. that creates a traditional pile group with the sign foundation at the center.

A hypothetical example could be a ring of concrete piling (say each being 16" square, spaced at 4' on center). These surround the sign foundation, at a reasonable radius. Then, as the sign receives lateral load, the soil transfers some of this force to these surrounding piling - no pile cap or anything else, just driven piling.

This should allow the "island" to be somewhat smaller. Cost for a few spaced driven piling should be much lower than a sheet pile enclosure. Also you mentioned the aesthetics - the installed pile butts could be just below ground level - not visible.

http://www.SlideRuleEra.net

 

[apsix]

As I see it the solution must be based on the depth of excavation ie. a simple 'width = x times dia.' can not be right, although it could give a minimum limit.
Wouldn't a line projected down from the top of pier (at GL) make more sense?
I favour the sheet piling tied with waler solution.

 

[dimjim]

Until you get the sign details
you cannot get a decision that
would be meaningful. What a way
to spend the Holiday. You can
calculate the wind loads, etc.,
but you have to know how it is
sunk into the substructure and
what that is. Why hire a
Geotechnical Engineer if he does
not become responsible for his
analysis.

 

[pba]

Lufti,

I've thought again about your proposed sheet pile and waling solution - If you don't get the information you need on the existing sign foundation could you consider a modification of the sheet piling design so that it acts as a new foundation?

 

[Lutfi]

Well, one radical solution would be to fully take the sign down and design a new foundation for it that is flush with the bottom of the pond.

This whole matter is going to boil down to economics! The most economical solution will win. I want to make this least painful to the client.

I am not sure about the rest of you but I think this situation calls for intuitive thinking and demands engineering problem solution. Not the usual beam or column design. I kind of like it!

I hope all of you have a great holiday. Do not eat to much turkey.


Regards,
Lutfi

 

[dimjim]

Lufti,
I wonder if pooring a pad would help stabilized
the sign and give you a great assurance that it
would not topple and also help to assure water
run off.