Minimum Embedment Depth for Drilled Shafts 4

[geotechgal]

I am using the FHWA approved DRIVEN program to calculate the drilled shaft foundation lengths for several overhead sign structures. My shaft lengths are coming out to be very small (5 to 8 feet). I think they are too low possibly due to bigger diameters being used by the SE (36-inch). Are there any guidelines for Minimum Shaft lengths that I can use?

 

[Panars]

Ummm, the program DRIVEN is not for the analysis and design of drilled shafts. It is for the analysis and design of driven pile foundations (hence the name "DRIVEN"). If you selected "concrete pile", you are getting results for a 3-foot square prestressed concrete pile that is driven into the ground with a pile driving hammer (not that this is realistic).

I am not aware of a free software program to design drilled shafts, but the design methods can be put into a spreadsheet without too much difficulty. There is an FHWA manual called "Drilled Shafts: Construction Procedures and Design Methods" that tells you how to design drilled shafts. You can download it at this website:

http://www.fhwa.dot.gov/bridge/geopub.htm#drilledshafts

 

[jdonville]

Geotechgal,

Also be aware that you may need a different shaft length, depending on restrictions to the shaft diameter, for lateral load resistance. The FHWA reference quoted above covers lateral loads, as well. You will also want to check the applicable articles of AASHTO's Standard Specifications for Highway Bridges, 17th/2002 edition (ASD), or the LRFD version (Available from AASHTrg $$$). The ASD edition is based on the FHWA notes and can be readily programmed into a spreadsheet for axial loads. I am still saving my pennies for a copy of the LFRD version, but the FHWA notes contain examples of LFRD design of drilled shafts.

FHWA also has lateral load software available (COM624P) in the public domain. It may be too involved for this particular application, but it can be downloaded, along with the user's manual, from the FHWA website (you can Google "FHWA COM624P" to find the link).

Finally, Ensoft (

http://www.ensoftinc.com)

offers computer programs called SHAFT for drilled shaft analysis under axial loads, and LPILE for lateral load analysis. I haven't used these myself.

Regards,

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[Panars]

If you are designing laterally loaded drilled shafts, then I would highly recommend LPile Plus. COM624P was a good start, but it has not been updated with the advancements in laterally loaded drilled shaft design. For instance, it does not allow you to model group effects (by using p-y multipliers), and it does not include some of the more recently developed soil and rock models (I am thinking of Reese's weak rock criteria).

To jdonville, I would suggest waiting for the next version (should be fourth edition) of the AASHTO LRFD specs before you buy it. The section on foundations (Section 10) is undergoing some major revisions.

 

[UcfSE]

Vulcan Hammer has a lot of free downloads, including the FHWA documents referenced by Panars. These downloads include drilled piers, driven piles, and other information.

http://www.vulcanhammer.net/download/

 

[geotechgal]

Everyone,
Thank you for the valuable posts.
Panars,
I agree with you about DRIVEN program. I am also using the FHWA-IF-99-025 (Drilled Shafts: Construction Procedures and Design Methods). Table 13.1, P333 gives the Min Shaft penetrations for lateral loading. I am getting 42 feet length. I am not convinced that it is going to be so high especially since the loads on my section are low (Vertical-20 kips, moment 25 kip-ft).
Comments anyone?

 

[Panars]

geotechgal,

I agree. A 42-foot long drilled shaft seems really long. The minimum shaft penetrations you refer to are for the characteristic load method of analysis to be accurate. This is not the minimum required shaft penetration to support your load. You said in your initial question that the drilled shaft was for the support of an overhead sign. The minimum penetration for the characteristic load method is the depth at which there is no displacement of the shaft under the applied loading. This is not usually required for sign foundations, and the manual says this on page 331.

For sign foundations, I would suggest that if your primary concern is the lateral capacity of the drilled shaft and not deflections, then an analysis using the Brom's method is sufficient (with an appropriate safety factor of course). If deflections do control your design, then I think you (or your employer) needs to invest in some software to do p-y analyses.

 

[BigH]

Why don't you check some of the other threads - we've talked about sign posts in a number of them. You should check out your analyses with ROT procedures (rules of thumb). Sorry but don't have access to mine right now. Focht3???

 

[geotechgal]

Panars,

I used the Brom's method, I only get about 8 feet length. That is when I started looking for min embedment depth criteria.

BigH,
Thanks, I am looking through the other posts.

 

[VAD]

I am not sure of your soils conditions but 8 ft while possible from a calculation perspective needs to be tempered by judgement. Somethings to consider:

Depending on climate you may be subject to frost jacking of piles - In some parts this can be over a length of 6 ft. You can mitigate this as well but may be expensive. Hence piles should be deeper irrespective of which method you use to get numbers. Same concern for swell/shrinking soil where depth may be deeper than for frost jacking.

I would say you need a shaft length of about 16 to 20 ft in "well behaved" soil.

The thing to remember is that one should first examine soils characteristics/ ground conditions, the prevailing practice within the jurisdiction etc concieve a solution, use experience and judgement, and then lastly go to the theories/calculations. The latter should also be well understood as the factors discussed previously are not often taken into consideration, it is up to the user to decide.

 

[DRC1]

I would also expect to see a length of 15-20 feet in a nomal soil. With Broms method, the passive width is multiplied by 3 giving you a 9 ft wide passive zone. Obviously a certian amount of penetration is nessesary to develop that width. I would solve it as a cantilevered pile that was limited to the width of the cassion. This will be some what conservative, But I dont think it is over canservative and gives you a rational basis for your number. I realize the sign post will provide some lateral support, but probabaly not enough to consider it a brace.
Active and passive pressures will develop across the diameter of the cassion and with the low moment, using the cassion diameter for the passive width should yield reasonable answers

 

[broekie]

Check the publication: Standard Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals. I believe they use Brom's Method.

 

[SlideRuleEra]

You may find the information in "Pole Building Design" by Patterson useful. It is available (.pdf download) at the link below.

http://www.SlideRuleEra.net

 

[Polecat]

GeotechGal:

Being involved with transmission power poles and cell sites, I'm faced with this problem every week. A good check for your work can be found in the 2003 edition of the International Bldg Code,pages 370-371. The formulas are based on Broms and can be easily adapted to a spreadsheet. Suggested passive soil pressures can be found on page 363.

As mentioned by Panars, with signs and free standing poles, don't be overly concerned by groundline deflection or shaft rotation ---- your primary concern is to get enough depth to develop the maximum lateral pressure. In fact, the IBC indicates that the pressures suggested in Table 1804.2, p.363, can be doubled for isolated poles.

Based on an installation in sandy soil with an N-value of only 2 blows (pretty loose stuff with a Kp of only 2.7) and under submerged conditions, I get a depth of only 10 ft (with a 3 ft auger and SF of 2.0) to resist 25 ft-k.

Now, all that being said, I would also have to agree with VAD about tempering your solution with some judgment. Frost heave is certainly a factor to throw in. Also, you need to be sure that the end bearing pressure, coupled with skin friction (if in sand) is sufficient for the axial load.

In any case, I believe that the 42 feet is absolute overkill (unless perhaps you are in the middle of wetlands somewhere). All else being equal, I've seen 120 ft power poles resisting 2000 ft-k that don't require that much depth.

http://www.spiraleng.com