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Tiered Drilled Shaft Retaining Wall

Tiered Drilled Shaft Retaining Wall

(OP)
I am in need of some clarity. I have a retaining wall system that is composed of two cantilever drilled shaft walls, one behind the other, and my question is whether the top wall has any influence on the lower wall?

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The top wall is 14' high, the bottom wall is 22' high and there is a 20' horizontal setback. Is there any surcharge from the higher wall acting on the lower wall and if so, how is something like that computed?

RE: Tiered Drilled Shaft Retaining Wall

If they are cantilevered, of course the top wall will impose loading on the soil between the walls, and that loading has to be resisted by the lower wall.

Your picture of the lower wall shows no embedment.

RE: Tiered Drilled Shaft Retaining Wall

(OP)
Here is an updated picture. I drew the one yesterday in a hurry. Would you explain to me how the loading from the upper wall would exert a loading that the lower wall has to contain? I also included the failure wedge for active pressure behind the wall. I've been reading multiple places how if a loading falls after a distance equal to half the wall height, there would be no surcharge on the wall. My main issue is understanding surcharge and how it would effect a cantilever drilled shaft wall since I can't seem to find any examples on how it's been analyzed before (I can only seem to find tiered MSE walls).

RE: Tiered Drilled Shaft Retaining Wall

The upper wall will likely affect the lower wall. The amount of influence will depend on the spacing, the soil type(s), and the hieght of the slope you show above the upper wall. Calculatinging the soil pressure will be difficult. I suggest you look up some of the graphical methods, one that comes to mind (if I'm remembering correctly) is Cunningham's method.

By the way, the 1/2 the wall height is a rule of thumb for "simple" configurations that even then doesn't always work. What you have is far from simiple.

Mike Lambert

RE: Tiered Drilled Shaft Retaining Wall

(OP)
I'm just trying to get a better understanding because a lot of publications say many different things. What I don't understand is if you have a soil nail wall with the same configuration, if it's behind the bottom wall an offset distance equal to that wall height, you analyze both walls independently. The only grouping effect that you analyze is a global slip failure plane; the forces on the wall being two separate issues, one not effecting the other. I'm trying to see how to translate that from soil nail to cantilever drilled shaft walls in that soil nail walls stop at the base of excavation; does the fact that the drilled shaft is embedded play another role. Other documents mention that if there is any loading behind the failure plane, it need not be considered in wall design; the wall clearly falls outside of that plane. If I do end up doing a boussinesq equation, is that really practical?

RE: Tiered Drilled Shaft Retaining Wall

How do you know that the upper wall falls outside the failure plane of the lower wall? The active wedge that you included on you sketch is only the failure plane for simple geometeries. As noted previously, you do not have a simple geometery.

Mike Lambert

RE: Tiered Drilled Shaft Retaining Wall

(OP)
In geotechnical circular no. 7 provided by the fhwa, pages 120-121, "When the horizontal setback is larger than the height of the lower wall (H2), each individual wall will act independently and each wall must be analyzed and designed as two independent soil nail wall structures. If the horizontal setback is smaller than the height of the lower wall, the lower wall must be analyzed considering the upper wall as a surcharge." This is in regards to a soil nail wall though. I want to know how much of this is applicable to a cantilever drilled shaft wall.

RE: Tiered Drilled Shaft Retaining Wall

Do you want to analize the wall correctly, or do you just want an easy answer? If you want an easy answer, ASSUME that the statement in Circular No. 7 applies to your walls and go forth. Soil nail walls and cantilever drilled shaft walls are completely different structural solutions to, at least normally, completely different problems. ASSUMING that design guidence for one applies to the other is likely not going to get you a very good design.

If you want to analize the problem, look up earth pressures in a good graduate level soils mechanics book and get to work.

Mike Lambert

RE: Tiered Drilled Shaft Retaining Wall

(OP)
You mentioned nothing in your previous response that I didn't already know which is why I'm here in the first place to see if I'm on the right track, hence this being a forum. If you haven't had any previous design experience with this particular case or have any other useful ideas other than "get a graduate level soils book and get to work" why even reply? I asked earlier if using a boussinesq equation and converting the soil retained by the upper wall into an equivalent surcharge load to apply to the lower wall would be a valid reasoning but it seems like that fell on deaf ears. If it is indeed very different from simple geometry, please explain to me why. I'm just trying to understand the best way to approach the problem at hand.

RE: Tiered Drilled Shaft Retaining Wall

Your are explaining a plane strain condition that includes structural elements to stabilize a grade change. The entire system should be analyzed as one system. That would mean to me that the original question is moot. Is one wall influncing the other wall? Yes. I say this with confidence, you should return one safety factor for the structural design. The passive side of the top wall will influence the active side of the lower wall. The passive side of the lower wall will include forces from the upper wall and the slope above the upper wall.

Now you may also say that the drilled shafts have some center to center spacing, you may tell me about Cp values as determined from triaxial strength testing. It would not change my earlier statement. There is some free-body diagram that needs to be addressed and the problem is ONE problem.

Just my thoughts. . .

f-d

ípapß gordo ainÆt no madre flaca!

RE: Tiered Drilled Shaft Retaining Wall

Basics of Retaining Wall Design, 9th Edition, A Design Guide for Earth Retaining Structures by Hugh Brooks and John P. Nielsen discusses "cascading walls" on pages 34 and 35. It states, "If X > Y, probably little effect on lower wall." Now, this discussion deals with conventional walls, not sheeting walls. X is the setback distance between the two walls and Y is approximately the vertical distance between the bottoms of both walls. The sections on sheet pile and soldier beam walls do not address tiered or cascading walls.

www.PeirceEngineering.com

RE: Tiered Drilled Shaft Retaining Wall

(OP)
I had something similar drawn on another sheet of paper but I wasn't sure. I'm pretty confident now on being able to convert the influence based off of the embedded portion of the shafts than a wall; the last few comments were very helpful. Thank you.

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