Lateral earth pressure from partial height backfill

[PMR06]

I am working on a temporary wall shoring project in which the contractor needs to brace a basement wall until 2nd floor structure is installed (they need to backfill now so that other work can proceed)

Step 1: With the basement floor slab in place, the EOR has given them permission to partially backfill the wall without the need for bracing. The lateral pressure would be active coefficient Ka x unit weight x height, as the wall would be free to rotate some small amount.
Step 2: We install braces so they can backfill remainder. Lateral pressure would be at rest K0 x unit weight x height, but what would the design pressure look like? It would be conservative to take the full height at rest pressure all the way down, but can a reduced amount be used since part of that backfill was done previously in an active condition?

The backfill material is all granular, free draining, with design coefficients provided by the Geotech.

 

[Ron]

In my opinion, it would be Ko for the entire height as the mobilization of the wall will have occurred for the partial fill (Ka condition), but the wall will likely not be pushed back and braced, just braced with the floor at the top of the wall, constructed after the wall has moved. At that point, Ko prevails for the full depth.

 

[oldestguy]

PMR:

Give yourself a big pat on the back. Doing it right! How many times have there been situations like this and no bracing is used. Some fail, but an amazing number seem to squeak by.

 

[PMR06]

Thanks oldestguy. This wall is 24ft tall so yes, we're doing it right!

@Ron, I am proceeding with full height K0. I still have a gut feeling the design could be less, I just can't find reference to justify it.

 

[jayrod12]

If H1<<H2 then it would likely only result in a minor savings to the braces so it seems like a waste of time to put a whole ton of thought into it.

 

[PMR06]

@jayrod, in my case H1 is about 1/3 total wall height, so it seemed plausible? I agree with your philosophy though, the "savings" couldn't amount to double the brace spacing or something so we've moved on.