Drilled Piers for Signboards

[CISGeotechnical]

Could anyone tell me or help me in getting the paper of Ivey, D.L., and D.L. Hawkins on "Signboard Footings to Resist Wind Loads", Civil Eng., pp. 34-35, December 1966.

I am the geotechnical consultant for a Billboard's company and would like to get the original paper shown above. I got the reference from the Woodward, Gardner, Greer's 1972 Drilled Pier Foundations book.

Any help will be appreciated.

CISGeotechnical CIS Geotechnical
jrodriguez@centralindustrialpr.com

 

[dlew]

CISGeotechnical,

I have a copy of that article, faded with the years but still legible. E-mail me your Fax phone number if you want it, and I would be glad to fax it to you.

AEFSr2002@aol.com

 

[CISGeotechnical]

After being the Billboard's company geotech consultant for a while, this company has asked me to analyze their existing old Billboards in order to know if they comply with current UBC code and the code back then when it was constructed. I have found that almost all of them are embedded in the soil less than required using the criteria of Ivey and Hawkings who's paper dlew kindly facilitated to me.

Does any one know any other retrofit method to make the existing drilled shaft comply with expected wind loads other than making an enlargement in the upper area of the shaft.

Any practical suggestion will be appreciated.

CISGeotechnical CIS Geotechnical
jrodriguez@centralindustrialpr.com

 

[Ron]

CISGeotechnical...
The most common remediation method is to place a "collar" on the shaft. If you have no room for this, your best bet is to remediate the soil (add strength to soil), by chemical or cementitious grouting.

 

[CISGeotechnical]

Ron:

By "collar", do you mean something like expanding the shaft contact area with the soil along the shaft length until the passive resistance requirements are met?

This is the rationale I've been using but usually expanding the upper area with something like an expanded upper footing, sometimes cubic. Typical shaft diameters are 54".

Could you tell me an example of what you have used, if so. Client is interested mostly in economical alternatives. In my country, Puerto Rico, chemcial or cementitious grouting is done by just a few contractors, and their prices for this type of work may exceed other alternatives. But yes, sometimes may be not enough room to fit any particular case.

Please repply with some kind of example and/or explanation of your experience.

Thanks. CIS Geotechnical
jrodriguez@centralindustrialpr.com

 

[dlew]

CSIGeotechnical,

Some utilities in sandy Florida, to reduce the length of embedment of their electrical transmission poles, make the hole larger than the pole and backfill the space around the pole with gravel. To calculate the depth of embedment they use an effective diameter for the foundation of approximately 65% of the gravel annular width. For example, for a 18" diameter pole and a 54" diameter hole, the effective diameter used is 41".

I would not use this system if the soil were clayey and/or impervious, since the water that would accumulate in the voids of the gravel could affect the properties of the cohesive soils.

Also in transmission poles construction, I have seen "bog shoes" used to increase the horizontal bearing area at the top of the embedded portion of the poles. "Bog shoes" are two pieces of wood or concrete poles placed horizontally and bolted to the pole about 3 feet below the groundline. Bog shoes are normally used to provide more bearing area to support the weight of pole and any wire tension vertical component on marshes or on sites with low bearing capacity soils.

Unless you have to follow by code an established criteria or formula (such as the OAAA or UBC), before proceeding with the retrofit of the sign footings, I would check other analytic methods to verify the capacity of the foundation. You may be interested on:

1) The book by Woodward, Gardner and Greer shows 3 or 4 different methods (see paras 3-4 and 3-5).

2) Bowles' "Foundation Analysis and Design" has also a section on laterally loaded piles based on the work of B.Broms.

3) The computer program "L-Pile", developed by the U.S Army Corps of Engineers, it is based on the work of Reese. This program is public domain, but I believe the COE does not support it any more.

Regards

AEF

 

[Ron]

CIS Geotechnical...
The process I have used in the past (for towers and high wind load single column structure, as dlew also noted), is to create an additional annular ring foundation at the top of the shaft (it doesn't have to be round... you can use any shape that fits your conditions). The size of the annular ring can be computed based on load response of the shaft. As dlew also noted, L-pile will help you with this, though you can do it by hand as well.

One large advantage of this approach is it gives you a great amount of surface area to push against the soil, thus reducing the unit stress on the soil. Further, you get a benefit from sliding friction.

The procedure is to excavate from the surface to some depth, usually 3 to 4 feet below grade, for some distance out from the shaft, usually on the order of 3 to 5 feet for multiple shaft signs, more for single shaft signs. This would give you an effective radius of about 6 feet or so at the top of the shaft, more for a single shaft, say 10 feet.

Now you have a "block of concrete" that has more mass, greater surface area, and is easy to construct. Please note that the annular ring should be doweled into the existing shaft.