Drilled pier foundation depth

[GreginTexas]

I am moving a 35' light pole and I need to install a new foundation. I have done a couple of antennas up to 80' mast, and it seems there is a specfication and a 'rule of thumb' that goes with selecting the depth. I seem to remember it taking into account wind load, height, depth, and width of the foundation. (spec was a specification specific to the antenna industry, based on Broms rigid method I think) It seems that there was also a ratio of depth to height to use as a minimum. This light pole is square with a round foundation. A friend was supposed to have already posted this, but I could not find it after looking around.

Thanks in advance.

 

[Ron]

Gregintexas...your foundation depth will depend on the soil conditions and the moment induced on the foundation from the wind load on the pole.

There was a similar post in this forum a few days back about soil strength for a free-standing pole. Scroll down in the forum and you will see it. Post was by cjreeder. Similar comments applicable.

 

[Focht3]

GreginTexas:

Where is your site in Texas? (I'm in San Antonio.)

Ron pointed to cjreeder's message thread; other relevant threads are also available. Do a keyword search on 'pier', 'pile' and 'p-y' to find some of the others.

Light poles are usually quite different than cellular towers because of the significant eccentric dead load. In the electric utility business, the 'rule of thumb' is that the embedment is 10% of the pole height plus "___" feet; I've seen the 'blank' filled with values from 1 to 5 feet. This practice is losing ground, but still quite common among the electric co-ops and smaller utilities. A more realistic value is 15% to 20% of the pole height (for tangent structures.)

Having said all that, we need a few more details before attempting to answer your specific question. What are the soil & groundwater conditions? Nearby structures or roads? Consequences of failure? Consequences of excess tilt?

Let us hear from you!

 

[GreginTexas]

This site is on the Texas coast, and the site has been improved over the years. The original soil is a silty clay that starts at around 3-4 feet below the surface. The improved soil on top is a compacted clayey sand. I have dug around and found the spec I used for the prior antenna mast, it was EIA 222F. I also found in one of my old books the with the Broms method which takes bracketed lateral soil pressures for different soil classifications (e.g. Clay is 100#/ft2). I also found a company document (sorry, proprietary or I would post) that has a chart that holds depth constant but varies diameter of foundations for poles up to 50 feet high. My dilema is that I have an 18" auger on site and would rather vary depth. I am further hindered by the fact the with the extension, my auger will reach only 8 feet. At 8 feet and 18" diameter, the surface area available at any one time for lateral stresses would be 5429 sqin, besting my company's chart for the same pole at 4750 sqin. Since the company chart varies diameter, the volumetric mass is greater, but in dealing with overturn being principal and bearing being negligible, I think I am good in that comparison.
I am about to go through Broms method and use ANSI/ASCE 7-95 to determine my wind loads. Since I am on the Texas coast, I think that I have to use 130-140 mph. I also found a detail from a project I helped on near the Texas coast that has the same light pole that only went 7' deep at 18" diameter. That design was 50 miles inland from Corpus Christi and may have used 100-110 mph for wind load.
If you guys have any other pointers, let me know, also if you have the equations out of EIA 222F let me know, because I found the reference to that spec, but don't have the actual document anymore.
Thanks,

 

[geonet]

Greg. Several lateral pile analysis programs were developed in Texas and really are the current standard. L-pile is available through a company in Austin. I believe Com624 is free or close to it. Both are more realistic than the Broms approach. That being said, it is just a light pole so the broms method or one of the countless other methods may be good enough.

 

[Ron]

GreginTexas...you probably should look at using ASCE 7-98 instead of '95. Check your local code for reference.

L-Pile would give you the soil stresses, shear distribution in the pile, and moment, but you will have to "interpret" the point of fixity (thus, the depth of the pier). I believe L-Pile is public domain software.

 

[Focht3]

Ron:

LPILE is definitely not public domain software! COM624 may be; if so, you can get it from FHWA (I think the executable code can be downloaded from their web site - or so I've heard.) BUT - you'll get no tech support...

GreginTexas:
geonet is correct about LPILE being the current standard for lateral load analyses - now currently version 4 (i.e. LPILE4.) Contact Ensoft in Austin for purchasing information. It's a good program and is well supported. I use COM624 - from my days as a grad student at The University of Texas. (Go Horns!) I do my own support - have the source code, did my master's on laterally loaded pile groups, etc.

Having said that, I compared Brom's charts to COM624 for a limited number of problems in the early 1980's - got favorable results. Broms was a little conservative, which seems appropriate to me. It's a lot cheaper than learning to use COM624, or buying LPILE4 for a single problem... The clayey sand fill won't survive a tidal surge; the silty clay might. Broms ignores the first three pile diameters - or about 4.5 feet in your case. You may need to get a five foot auger rod extension for your drill rig - eight feet will probably be several pier diameters short of where your pier should bear, regardless of pier diameter (because of the "poor" properties of clayey sands when they aren't confined.) But beware of going too deep: depending on the soil profile, you will probably encounter groundater at depths of 7 to 12 feet...

Since your site is along the Texas Gulf coast, 130 to 140 mph wind speed is very appropriate. I grew up in Houston - am old enough to remember Hurricane Carla (1963), and was in Houston when Hurricane Alicia (1983) came through. My parents had a place on "west beach" in Galveston at the time; I was able to see the devastation first hand - and that was a weak one!

Good luck - and let us know what happens!

 

[GreginTexas]

Finally found the official geotechnical report done on the site in 1996. I was off a little on the soil types. The first 2 feet is finish gravel and soils. From 2' to 8' is "very stiff sandy clay (CL)." Below that is "dense silty sand." Bearing pressures with F.S.=2 is greater than 5000 psf to 6', then drops to 2000 psf from 6-8', then up to 12000 psf below 8'. And yes, groundwater is going to be an issue, this is in close proximity to a main tributary within 2 miles of Galveston Bay. I checked the geotech report for my Corpus installation, and that soil was CH to 12' deep.

 

[Focht3]

I don't know who did your 'official' geotech report - or whether the strengths you quoted from the boring logs, or from the report text. I grew up in Houston - parents had a place on west Galveston Island - practiced there since 1981. Sounds like Pleistocene soils - locally called the Beaumont formation. The strength drop from 6 to 8 feet is not realistic if the soils are confined - good for at least 4 ksf anyway.

I'm not sure which 'tributary' you're talking about - assume it's either the Trinity River or Buffalo Bayou / Houston Ship Channel. Or Channelview area in one of the petrochemical plants - hence "the site has been improved over the years" comment.

Plan on 140 mph winds and a 10 to 14 foot storm surge - upper two feet of soil will be lost, but the very stiff silty clay shouldn't scour too much.

 

[GreginTexas]

'Official' report is from a geotechnical company that was done when the original improvements to this site were done. Site was improved and a LOT of clay and stabilized sand has been added and compacted to the site over the years, creating the top layer(s).