Tiered cantilevered block retaining walls 1

[micks87]

Hi All,

I have two questions.

1) tiered retaining walls. Discussion in the office today yielded, the easiest way to analyse the tiered walls is to design the lower wall assuming it was retaining the full height. See sketch attached. This example has a lower wall height of 1.8m and a upper wall height of 0.9m, so design the lower wall for a retained height of 2.7m and design the upper wall for a retained height of 0.9m?
Is this overly conservative?

2) Same example as number 1, but what if the ground is falling away at 15 degrees. How would this change your design approach?

Thanks for your time.

 

[Agent666]

I'd agree with designing the front wall to be full height dependant, though it does depend on how far away it is. But given your proportions I can't see how you couldn't design for the entire height unless using something like wallap which most structural engineers don't have access to.

There was some NZ based advice on this published in a design example by MBIE a few years ago. I'll see if I can track it down for you.

 

[micks87]

Thanks Agent666.

That would be appreciated

 

[Agent666]

Ok, I found what I was thinking of, it was not by MBIE, and was unfortunately related to timber pole retaining walls! (damn memory fail). Same principles probably apply to ensure no loading within the failure wedge though. Seems to be some anecdotal evidence on the net relating to being at least 2 times the height of the first wall to the second wall to avoid significant loading on the first wall from the second. But I think the rotation and settlement of the second wall are of particular concern in your arrangement where the 2nd overlaps the first.

In the situations you note I tend to prefer the following arrangement, not only are you not putting a wall within the 'fill' of the lower wall and possibility of settlement and wall rotation (quite important if on the boundary and trying to retain neighbouring property), there is also possibly less excavation, just design the back wall for the full height retaining and walk away. If it doesn't work as masonry, you can have stiffening pilasters on the uphill side or pour some section at the bottom in concrete with block on top.

 

[micks87]

Brilliant Agent666, great idea. Thank you for the reference.