Pile Supported Retaining Wall
Pile Supported Retaining Wall
(OP)
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?





RE: Pile Supported Retaining Wall
RE: Pile Supported Retaining Wall
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.
RE: Pile Supported Retaining Wall
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?
RE: Pile Supported Retaining Wall
RE: Pile Supported Retaining Wall
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.
RE: Pile Supported Retaining Wall
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.
RE: Pile Supported Retaining Wall
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.
RE: Pile Supported Retaining Wall
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.
RE: Pile Supported Retaining Wall
RE: Pile Supported Retaining Wall
RE: Pile Supported Retaining Wall
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.
RE: Pile Supported Retaining Wall
RE: Pile Supported Retaining Wall
RE: Pile Supported Retaining Wall
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.
RE: Pile Supported Retaining Wall
RE: Pile Supported Retaining Wall
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.
RE: Pile Supported Retaining Wall
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.
htt
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.
RE: Pile Supported Retaining Wall
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.
RE: Pile Supported Retaining Wall
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.
RE: Pile Supported Retaining Wall
The safety factors are called for in Section 5.5.5 under Service Load Design Method. Thanks again for all the advice.
RE: Pile Supported Retaining Wall
I'm a little late to this . . . I am now in the process of designing a pile supported cantilevered retaining wall, and hit this thread on a google search. Wish I had tuned in 6 months ago.
I believe that the factor of safety against overturning, FS=RM/OM, figured by summing moments about the toe pile, should be limited to 1.5 if the heel piles are ignored. If the heel piles are included in the stability check via uplift resistance, a mechanical connection to the base footing is required. The pile uplifting resistance is the unknown to be solved for and the FS can be 1.0, because the safety factor is built in to the skin friction, etcetera.
Stumpy