Retaining Walls

[Inlander]

If a retaining wall is backfilled prior to restraining the top, would the soil pressure be active? Specifically, I want to tie the top of the retaining wall to the slab. I assume the wall will tilt prior to restraint. So it seems it will be active. The cantilevered condition is the worst case here anyway even with the lower EFP.

Also, the CBC requires the EFP to extend to the bottom of the key. Is there any way to justify not applying this? Just from experience, the key depth required is unreasonable.

 

[jdgengineer]

If a retaining wall is backfilled prior to restraining the top, would the soil pressure be active? Specifically, I want to tie the top of the retaining wall to the slab. I assume the wall will tilt prior to restraint. So it seems it will be active. The cantilevered condition is the worst case here anyway even with the lower EFP.

Yes, the compaction of the backfill will deflect the wall and engage active pressure. If you apply a restraint at the top, any pressures applied to the wall after the slab (restraint) is in place would be likely want to be considered at-rest and designed as fixed-pin. For instance, surcharge loads, seismic loads, and fluid pressures.

Also, the CBC requires the EFP to extend to the bottom of the key. Is there any way to justify not applying this? Just from experience, the key depth required is unreasonable.

No legit manner that I am aware of but MANY engineers neglect it. However, from typical assumed soil loading profile it would be required. It's likely that your geotech gave you passive as "net passive" rather than "gross passive" so perhaps you can justify an increased passive pressure to help compensate.

 

[CBSE]

I agree with jdgengineer.

This is pretty much the exact reason I do everything I can to avoid a restrained top/bottom retaining wall. Contractors rarely pay attention to the shoring requirements in the documents and just start building. It's amazing more accidents haven't happened.

 

[JAE]

You might ask this in a geotech forum but after the active pressure is engaged and the wall moves slightly away from the soil, and you then cap off the top of wall with some kind of restraint, it seems to me that the future movement or consolidation of the soil would slowly begin to form at-rest pressures as time goes on.



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[GeoPaveTraffic]

@JAE,

Once the wall moves (tilts) and active pressures are developed, the pressure should not increase if the wall is then restrained at the top. That is the theory. How that would hold up in the specific situation being discussed/built, is anyone's guess. For example, what happens if part or all of the backfill is subsequently excavated sometime in the future and then backfilled?

My advice, if the top of the wall is restrained; design it for at-rest pressures.

Mike Lambert