Use of active earth pressure for retaining wall design.

[livewire9]

I recently reviewed a cantilever retaining wall design. It occurred to me that the use of active soil pressure behind the wall is less conservative than using the at-rest soil pressure. Is this common practice? A model of the wall calculates a deflection of 1/16". It doesn't seem like this amount of movement really transitions the soil pressure from at-rest to active. Any thoughts are appreciated.

 

[Lomarandil]

It may be common practice, but it can be erroneous. Whenever checking earth pressures, noting the deflection (as you have done) is important. Typically, the deflection required to mobilize a pressure state is defined as a rotation (deflection/height). Check out NAVFAC DM7_02 Figure 1 on page 60 for some rough guidelines depending on your soil type. Sands typically require less movement than clays.

 

[Signious]

As long as the wall has is ductile enough to deflect without yielding rebar / crushing concrete the wall will either:
a) Adequately resist the at-rest earth pressures without undergoing enough deflection to consider active earth pressures
or
b) Deflect to the point where active earth pressures are achieved

For the strength of the wall itself I think using active pressures is fine, but for overturning and global stability using at-rest pressures would be prudent to make sure the wall deflects at the steam and does not rotate around the toe.

 

[Lomarandil]

That's a good point. I often don't think of retaining walls as ductile, but they certainly can be.

 

[FixedEarth]

Near the corners it can be argued that the earth pressure is at rest. That is possible, however, cantilever retaining walls designed for active earth pressure with eccentricity in the middle third and where the allowable bearing capacities are not exceeded, can withstand the at rest earth pressures safely.

There are situations when you should consider earth pressure between Active and At Rest states. These include when the retaining wall is on closely spaced piles(soil arching is restricted), when you are on rock foundation(rotation about the toe is limited) and when you are designing hydraulic and U framed structures (they are more rigid). I have yet to come across a situation where you needed to use At rest earth pressure for cantilever retaining wall design.

http://www.soilstructure.com/

 

[oldestguy]

With questions like this I think designers would be wise to get out there and measure wall pressures with varying methods of backfilling. It will open your eyes as to how far off design assumptions can be.

 

[livewire9]

oldestguy,

What have been some of your experiences? Have you found pressures to be higher or lower than expected? Or both?

 

[oldestguy]

The mode of backfilling has a heck of a lot to do with the pressures first developed. My measurements were on walls that could not rotate. For instance, compacting near the wall can build up pressures exceeding an equivalent fluid pressure coefficient (Kh)of the unit density of the backfill and more, like 1.0+. That compaction effect has even been found some 8 feet down from the elevation of the compactor on the surface. My further measurements have shown that keeping the compactors out 24 inches will end up with a cushion effect (some movement within that 24") so as to duplicate what a rotating wall would do, leaving active as the usual Kv, with sand as the backfill material. That "looser" zone is of no consequence when it comes to the surface soil settling. I call that the "silo effect", hanging up on both the wall and the compacted material farther out. So, I'd design for active, but specify no compactors within 24 inches of the wall. A little codification would be to hit the final surface next to the wall if you wish, recognizing the looser zone will not transmit that effect much down the wall.

 

[livewire9]

oldestguy,

Good info. Thanks!

 

[oldestguy]

Damn spell checker. Codification ain't the word I started with. Was clarification.