Surcharge Caused by Tapered Retaining Walls 4

[IAmNotVerySmart]

Hi I am working on the analysis for a house cut into a plot of land. The walls against the dirt are retaining walls with the bottom of second story wall being 8 feet away from the top of the first story wall that is 8 feet in height. The top of the first story wall and footing of the second story wall are going to be connected making it all one large connected wall. My question is how would I calculate the surcharge created by the second story wall onto the first story wall? I Analysed each wall separately rather than being one large piece. I also assumed the first story wall has an axial load on the top caused by the total weight the second story wall and footing (since they are all connected) but I am also not sure if that is correct.

 

[JoelTXCive]

a simple sketch or diagram would probably help here.

 

[IAmNotVerySmart]

Sorry if it is unclear. Here is a rough image of what is happening.

 

[oldestguy]

One phase of the analysis should be looking at this as one wall and secure the two together. The inclined backfill surface at the upper wall adds significantly to the problem. My initial guess is that subsequent examination of soil pressures is that this is way too undersized a system.

 

[r13]

I think you need perform two analyses, one is addressed by oldestguy, the other one is similar to your thinking. - analyze the walls separately. I think it can be done by assuming the connection between the slab and the lower wall is broken. Thus the upper wall will produce a surcharge to the lower wall, the surcharge is simply the resultant pressures from the stability analysis of the upper wall. From here, just analyze the stability of the lower wall as usual.

By the way, I don't think the word "tapered" is of correct use.

 

[IAmNotVerySmart]

Yes tapered is not the right word to use, sorry. Thank you for the responses but I am still not sure how I would treat both walls as one system. Would it be both walls as one continuous wall? Or would it be as drawn and the footing connecting the two walls is there to just transfer the load to the bottom footing which is what I would care about.

Sorry I am young and inexperienced so I am just not sure how the footing connection between the two walls affects the overall analysis.

 

[r13]

Yes, in reality the walls may behave more close to your thinking. But, for safety, overall stability can't be ignored. Because if the slab to lower wall connection does not break as assumed, then the pressure behind the two walls should be close to a single L-shaped retaining wall (H=19', B=S1+S2). Hope this make sense to you.

Also, the things to watch for - the sloping grade & ground water.

 

[spieng89]

You'd have to analyze these walls separately due to constructability. The bottom wall would need to be formed, poured, and then back filled before the top wall would be in place. You'd have a unrestrained condition during construction and restrained after with different stiffness values for the wall (k value).

 

[oldestguy]

Regardless if there being two, an analysis assuming they are one unit is needed, in case you decide the other alternatives won't work, etc. I'd want to confident that the whole combination is safe as one unit. Fortunately the stepping back into the slope is to your advantage in that case, but how safe?

 

[BridgeSmith]

The correct terminology for this system would be a tiered wall system; that may help in finding outside resources.

As far as how you treat the design, that would be dictated by the restraint conditions. If the basement slab restrains the bottom wall from sliding, and is assumed to be permanent, then sliding of the bottom wall does not need to be considered. If the upper slab is connected to the top of basement wall, either for tension and shear, or a connection that can carry moment, then that needs to be considered in the design, both the forces and stresses. If it's connected, the top of the basement wall is restrained, making the moments on the bottom of the stem of that wall much lower, but it will increase the sliding force applied to upper slab and the footing of the upper wall.

Connecting the walls together makes it one system, which has significant advantages in efficiency and economy, but makes the analysis much more complex.

Just 'eyeballing' it without putting any numbers to it, as drawn, barring restraint from the floor system, I suspect the upper wall would slide and overturn the basement wall. Unless the foundation soil is very stiff, I'd also expect some significant vertical settlement. Again, much depends on the restraint conditions at each level (basement slab, floor slab, and top of upper wall).

Rod Smith, P.E., The artist formerly known as HotRod10

 

[msquared48]

I would argue that as the soil settles under the toe of the top wall, all the toe pressure from overturning will revert to the end of the toe, loading the top of the lower wall.

Consequently, I think you can ignore any surcharge loading to the lower wall other than the load from the upper toe. You can make this happen by underconsolidating the fill under the upper wall toe.

You will also have a horizontal force at the top of the lower wall from sliding of the upper wall.

Mike McCann, PE, SE (WA, HI)

 

[r13]

You will also have a horizontal force at the top of the lower wall from sliding of the upper wall.

Correct. My omission.

I also like the "tiered wall" description, and the staged construction thinking (very important).

 

[BridgeSmith]

I also like the "tiered wall" description...

That's what AASHTO calls them, anyhow, so I thought it might lead to more productive searches, at least.

I've designed a couple similar ones, except mine were MSE walls with block facing.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[IAmNotVerySmart]

Yes tiered is exactly the term I should have used. Thank you this is helping me with my search so much. I really appreciate all the help and advice from everybody.

 

[r13]

Two suggestions on design for you to consider:

1) As opposed to rigid upper slab - lower wall construction, you may consider to provide a expansion joint at the interface to allow modest movement of the upper retaining wall. At the joint, these two elements are connected with smooth dowels only (assuming the slab stops at the back face of the lower wall), and the joint is then filled with flexible joint filler. A water stop is essential to prevent seepage.

Another way to make a flexible connection is to provide a slab seat (similar to corbel) along the lower wall, and rest the slab on it. The joint details are similar to above.

2) The drains shown on your drawing may not be adequate for the size of your retaining wall. Clog is the concern. You might need to add weep holes to ensure the free relief of hydrostatic pressure behind the walls.

 

[dhengr]

IAmNotVerySmart:
You might gain some advantage in soil pressures and wall interactions (superimposed loads on the lower wall) by lowering the footing a few feet on the upper retaining wall, increasing its heel length and shortening its toe length. This lowers and moves back its soil pressure influence on the lower wall and below. Then, you have an 8’ wide horiz. beam (the mid height slab) which could be tied into the two found. side walls. This would provide a horiz. reaction several feet above the ftg. on the upper retaining wall, and it could also act as a tieback reaction at the top of the lower retaining wall.

 

[r13]

I am curious about the use of your tiered walls. Is it for the earth retention purpose only, or else?