sign foundation

huangyhg

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