Pile Supported Retaining Wall 1

[miecz]

I'm designing my first retaining wall on piles, (actually a bridge abutment) and I'm following some designs that I've collected from various sources (textbooks, old projects). What's curious is, I don't see a check for safety factor against overturning. For a wall on a spread footing, this check drives the design. Is there a safety factor check for overturning at service loads for walls on piles?

 

[kslee1000]

You have done that unknowingly during pile design - the OT moment is resisted by a couple in the piles (if you have more than two rows of piles).

 

[Qshake]

Overturning is critical when there is nothing to resist the uplift as is the case with spread footings or cantilever walls on soil or rock.

When walls or pile caps the piles can be connected to the pile cap to provide uplift resistance to the limit allowed by the Geotech or the structural connection capacity.



Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[miecz]

The Geotech report has the uplift resistance as the weight of the pile, approximately 5 kips. The compressive capacity is given as 200 kips.

It seems that one conventional method of design, is to space the piles to limit the pile force to the limits, and batter the front piles for the driving force.

However, if I then calculate a safety factor of resisting over driving moments about the front (battered) row of piles, I get a safety factor just over 1.

Seeing that the resisting moments come from the dead weight of the soil, concrete, and pile, there doesn't seem to be any hidden safety there. Am I missing something?

 

[UcfSE]

I would expect the number the geotech gave you to have a safety factor included, unless they noted it as a nominal load. In the case of nominal capacity instead of allowable you would, of course, need to apply a factor of safety or a resistance factor. When figuring the overturning, if you use the 0.6D+W combination you'll have some safety factor built-in there.

 

[MichSt]

What about the girder reactions? They will most likely be contributing to the resisting moments, as your centerline of bearings will probably be behind your front pile. Have you considered this?

I also second what kslee1000 has said…as long as your back pile does not exceed its up lift capacity, the abutment will be ok against overturning. Same with sliding…as long as the lateral capacity of your front pile is not exceeded, sliding will be ok.

 

[miecz]

Thanks for all the suggestions, but I don't feel as though we've answered the original question, i.e., does one need to check an overturning safety factor for retaining walls on piles?

UcfSE-

The uplift resistance from the geotech is simply the weight of the pile. It's not like he calculated a resistance and factored it down.

Applying the 0.6D+H combination, the overturning forces are generated by Ka times the unit weight of the soil. If I factor the dead weight by 0.6, then it seems to me that both the driving and resisting moments are factored by 0.6 (more or less) and its a wash.

I didn't mention in my original post, but I'm following (by contract requirements) AASHTO Standard Specifications, Service Load Design Method. AASHTO has thier own load combinations, and none corresponds to 0.6D+H.

MichST-

You're right, the girder reactions will help with the abutments. However, if I include the girder reactions, and dead loads, AASHTO 5.5.5 demands that the overturning safety factor be greater than 2, for footing on soil, and 1.5 for footings on rock. But I can't find a criteria for footings on piles. Further, no one seems to explicitely calculate the FS on overturning, and the common design approach for this case seems to result in a safety factor of 1.

 

[UcfSE]

The 0.6 would be applied to the resisting moment, but not the driving moment. The resisting moment comes from the dead weight of the soil, footing and wall. The driving moment comes from the active earth forces, which are not dead load and would not get a dead load factor.

Not to go on about the pile uplift, but how did the geotech calculate the uplift resistance of the pile? Did he or she actually use the weight of the pile only or does it just happen to come out to that?

I have the LRFD AASHTO code so I can't look up just what you see, but in mine the overturning criterion for spread footings is given by the location of the force resultant, not the ratio of resisting to overturning moment (11.6.3.3 LRFD 3rd ed.). Along that line, perhaps you can locate the force resultant for the strength limit state and see if you get a net uplift on the piles, and adjust accordingly.

You can try that out, or just put a FS of 1.5 or 2 and go from there if you're uncomfortable. I couldn't find a specific requirement for retaining walls on piles other than a note saying vertical wall elements shall be designed to resist all forces, etc. (11.8.5.1). Maybe it's just judgment time.

 

[miecz]

UcfSE

Not sure I agree about the 0.6 factor. ASCE7 applies that to the dead load, not the moments due to the dead load. The active pressure is defined as the dead load times the active pressure coefficient.

The geotech used the weight of the pile (We have a very conservative geotech).

You're right, the LRFD code has eliminated the Safety Factor Criterion for footings on soil. Only, I'm stuck using the Standard Specification. Curious that both codes are mum on pile supported foundations. I guess that reinforces the notion that AASHTO never required a SF ratio check for pile supported walls and abutments...

Thanks again for your help.

 

[UcfSE]

You're right about the way the active pressure is calculated. That makes it active pressure, not dead load. The building code separates earth pressures from dead load with its own variable in the load combinations, usually "H" re: ASCE 7 section 2.2.

 

[MSEMan]

The AASHTO assumption appears to be that piles (being a ground improvement) are useful for controlling settlement and improving bearing capacity, but not generally considered as part of the structural system of the abutment wall (except for the dead weight of the pile). So base dimensions etc are still governed by the geometry of the applied and resiting forces and abutment walls on piles may have similar base geometries to those bearing directly on the formation.

 

[miecz]

UcfSE-

You make a very good point. Honestly, I'm still not sure if ASCE 7 intended the .6 factor to be applied to the DL before or after applying the Ka factor. It's a moot point here, as AASHTO does not have this load case. For non AASHTO designs, our state building code follows IBC 2003. I see that IBC 1906 requires a safety factor of 1.5 on overturning for retaining walls, and it doesn't exclude pile supported walls from this requirement. It appears that IBC 1906 was changed from IBC 2000. I wonder what IBC 2000 required. I think that applying a 1.5 SF to overturning to your interpretation of ASCE 7 would result in a very conservative design, especially for walls on soil.

 

[DRC1]

In a spread footing the wall is designed for a FS of 1.5 and bearing is computed seperately. In a pile supported wall the overtuning is resisted by axial lod in the piles which is developed in bearing. So for a pile supported wall the bearing capcity of the piles provide the over turning resistance. The geotech gave you working or allowable loads for the piles. Depending on how the capacity of the pile is computed (by load test, capwap or analytical) the FS can vary from 2.0 to 3.0. Thus your "FS" is built in.

 

[BigH]

There are numerous references over the last several years (2003-2004)in ASCE Geo Journal for pile groups undergoing overturning moments. Might want to check them out.

 

[miecz]

DRC1-

Unfortunately, the geotech gave the uplift resistance as the "weight of the pile". I don't see that I have a useable safety factor of 2 or 3 there.

BigH-

Thanks for the reference. I really do need printed examples at this point. But I'm not a member of ASCE.

 

[BigH]

Yes - but doesn't your company get journals? This is always the problem with papers - in journals and conferences. It is very difficult to keep up especially if it is you paying the bill. You might want to go to a nearby university library on a Saturday or so and see if they can be of help.

 

[VAD]

One of the problems we all face initially is using examples from others re books, old designs etc. While this is good, very often there are assumptions made based on the experience of those designers that may not have been recorded i.e after a while you assume that these can be seen by others by inspection. Could be dangerous but as everything else some who have seen alot and done alot are comfortable making such decisions.

After a while when you have a design problem you examine all possible scenarios which you feel may be important irrespective if this is not indicated in the code etc. I see nothing wrong in determining the overturning of a wall on piles. How would the system react. What kind of soil is your piles founded within. Do not forget that your resultant of loads has an important part to play and you may stll have issues with piles depending on the conditions- soil type, topographic features etc.

Design is not intended to be analysis. If you are on piles you should not necessarily have to make your footing dimensions the same as a footing resting on ground. The pile cap pile system would be called upon to resist the forces re sliding, overturning etc. In school we do mostly analysis. Outside we are faced with design and conjuring up all possible modes of failure etc. You are the only one to decide based on the factors of the site etc.

Since this is your first design I would explore it based on what you feel would be the mechanism that you have to address to ensure that your system would function satisfactorily. The codes expect you to do that and not every situation is covered explicitly.

An old text Retaining Walls.... by Hutchinson - I think probably only available in the University Libraries give a complete treatise on retaining walls and their design and is worthwhile reading.

 

[MichSt]

Jmiec – I’m a bridge engineer and have designed numerous pile supported abutments, piers, and wingwalls and I would say DRC1’s post is a nice summary of how overturning is handled in pile supported foundations.

You say you’re getting some up lift on your back row of piles? If this is the case you should increase the distance between the front and back rows of piles enough to eliminate the up lift and put your back pile row in to compression. You could also play with the number of piles in the front and back rows.

Here is a link to an LRFD example of a pile supported foundation design that is published by the FHWA.

http://www.fhwa.dot.gov/bridge/lrfd/us_dsp.htm#DesignStepP11_10

While the analysis in this example is much more in-depth than I will typically do, you will see in this example that no FS for overturning is calculated, and that the stability of the foundation is checked by insuring the pile axial capacity is not exceeded.

Good luck.

 

[miecz]

VAD-

I appreciate your thoughts and suggestions. Books are sooo expensive...

MichSt-

Thanks for the link. As was noted above, the LRFD specs do not call for an overturning safety factor for any retaining structures, whether on soil, rock, or on piles. The Standard Specs do, at least for walls on soil or rock.

Do you typically try to keep your back row of piles in compression, or do you design your back row of piles to an uplift capacity prescribed in a geotech report? It sounds as though most designers utilize a significant uplift capacity, which I don't have here.

 

[MichSt]

Right after I posted I remembered reading in this tread that LRFD doesn’t have the overturning FS…oops. What section of the standard specs are you referring to for the overturning FS???

For typical designs I like the back row of piles just above zero loading. However, for long abutments or walls I may consider removing as many of the back row piles as I can to reduce foundation costs. This may lead to some up lift in the back pile row, which I will use the pile weight as a conservative assumption for the up lift capacity. Either way, I don’t like to rely much on the up lift capacity of the piles.

Hope this helps.