Retaining Wall with height variation

[MelStud]

I have to design a CIP cantilever retaining wall which height varies along with the terrain. How do I account for the different heights when designing the various components of the wall (stem, heel, toe)? Should I design for the worst case and keep that design through out the whole length of the wall?

 

[Dinosaur]

I can't help unless you give us an idea of the range of height you need to deal with. For example, if I were asked to design a RW that ranged from 10ft to 15ft, I may well just go with one design. If on the other hand I were asked to design a wall that ranged from 4ft to 32 ft, I would definitely provide different designs in different portions.

 

[MelStud]

Yes, the wall varies from 1ft to 20ft then down to 14ft

 

[molibden]

How long is the wall? If it's short I wouldn't bother with different designs, also it's easier for the contractor.

 

[KootK]

I don't think that I'd change it up anymore than once every 50 ft of wall.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[MelStud]

We don't have the exact measurements yet, but the wall will be about 300ft long

 

[structSU10]

I might make a design from 1' - 8' or so, then a different design for anything over 8' based on the max height. can play with the range to see what makes sense, but if half the wall can see a much smaller footing / rebar it is likely worth it to have different designs.

 

[jdgengineer]

Personally, I'd schedule it and let the contractor do what they prefer.

 

[bridgebuster]

1' isn't worth designing or building. Can you grade the slope so that you have a minimum stem of 4 feet? I would use 30' units, which is our typical contraction joint spacing.

 

[DaveAtkins]

I think I would design three walls:
* Short wall where the footing is only 12" wider than the wall (6" on each side of wall, no heel, no toe)
* Intermediate wall where short wall doesn't work, but tall wall is not required
* Tall wall for the worst case

DaveAtkins

 

[MelStud]

Not really, I believe the TW has been already set by the developer or civil engineer. The wall might start at 0ft (finish grade) or from 1ft, so what should I do from 0ft to 4-5ft wall height? can I design a gravity wall and then switch to cantilever CIP?

 

[mike20793]

I would design 3 walls and designate their heights as 5 feet max, 10 feet max, and 15 feet max (or other heights) and leave it up to the contractor to figure out which is easiest for them to construct.

 

[TehMightyEngineer]

Or you could do a precast MSE retaining wall. Easily accommodates stepped retaining walls. [nudge, nudge]

And, yes, you can design one part to work as a gravity wall and one part as a cantilevered wall. The wall wont care, just as long as the total combined wall can't fail in overturning or sliding. I'd keep local bearing pressures roughly the same down the length of the wall to avoid differential settlement issues.

As for the design; unless it's a fairly long wall I'd design it to be 100% typical. This way the contractor can get an assembly line setup going bending the bars and cutting them to the rough length. Then, when in the form or when placing them they can trim to the exact length. This will likely make things much faster and easier for them (thus cheaper) and will not cost too much more in rebar. Saves on your design time as well.

Edit: Just saw that you said this was a 300 ft wall. That's long enough in my mind to easily justify multiple sections. I'd clearly define the cutoff point for a wall detail (e.g. this wall detail good up to X ft in height of backfill), and leave the language and design open so that the contractor can choose when to carry a detail down the remaining length of the wall.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[MelStud]

thanks....have another issue we just learned...will address in a separate post