Active vs Earth Pressure

[jakeh76]

Can someone give a me an explanation of the differences and when to use each of the two earth pressures. I've read allot about it it seems kinda vauge about when to use each. If you know of any good references that would be great also. Thanks.

 

[gandersen]

Jakeh76:

The earthpressures that are used for design of retaining systems are selected based upon the expected movement of the retaining wall. The actual lateral earth pressures that are acting behind a retaining wall are highly nonlinear and can range as much as an order of magnitude. In order to make the design of retaining walls tractable, we commonly make one of three assumptions: 1) either the wall does not move under the action of the earth forces (At-rest condition); or 2) the wall moves away from the backfill sufficiently to fully mobilize the strength of the soil (Active Condition); or 3) the wall is pushed into the soil mass with sufficient force to fully mobilize the strength of the soil (Passive Condition).

The selection of the design case for a particular set of loadings should be referred to a geotechnical engineer.

In the case of tied-back retaining systems, the lateral earth pressures do not generally correspond with any of the above assumptions and the simplified calculation procedures are based upon measured earth pressures and are highly empirical.

 

[jimbeck]

The active pressure is when the wall is allowed to move - agreed; but then in long term retaining walls the soil consolidates and the pressure builds. For walls with a life of less than 1 year (shoring for excavations that will be backfilled), the active pressure can be used. Longer durations require at rest pressures (Ko).

 

[Focht3]

Nope.

Active earth pressures mean just that - wall moves until soil pressures change from "at rest" to "active." If the pressures build back up (although I don't see a mechanism for it), the wall moves again.

If the wall is somehow restrained, you can get an effect that looks like the "active" pressures are changing to "at rest"; but what is really happening is that the cohesion is decreasing over time as negative pore pressures dissipate.

 

[solutioninc]

The problem in reality is a little more complex because hardly ever does a retaining wall and backfill exist as we find in text books "before" the wall moves dropping atrest pressures to active or raising to passive, (again) as in text books. The wall will be normally backfilled in the field in layers and so the layers in the lower reaches will be subjected to larger strains and the top layers to smaller strains and that too, IF the wall has been designed and constructed using active presure coefficients and the factor of safety is 1.0. Higher earthpressure coefficients or higher factor of safety in the design will both reduce the wall movements and so the pressure on the wall is most likely to be near atrest conditions. The atrest pressure is dependent on the compaction method as well as amount. If the retaining wall is rigid, i.e., moves only due to the strains in the foundation soil, use only active pressure coefficients for design. If the wall is not free to be displaced or the backfill subjected to strains except by strain in a concrete element, use of appropriate atrest pressure coefficients (taking into account the method of compaction) is prudent especially if cracking of the element is undesirable. For cantilever retaining walls, actual eart pressures behind the wall will be more than active pressures but the high factor of safety in the structural designs will take care of that.

 

[BigH]

Remember to keep heavy compactors away from the wall - the horizontal pressures developed in compaction are not in the ko relationship with vertical loading pressures. With deeper walls compacted in layers, there would, I think, be some relaxation of the horizontal pressures - where there may not be in the upper reaches. Still, with time and wall movement, the likely scenario is the active (for this case).

Our earth pressure calculations really don't treat "locked-in" horizontal forces, do they?

So - best to use small compactors within, say, 1.5m of the wall.

 

[mrj1983]

The estimated earth pressures for use in retaining wall design are based on the expected movment of the given structure. When placing backfill behind a rigid, immobile retaining structure, the soil remains "at rest", meaning it has not fully mobilized and developed it's full strength. If a retaining wall structure is non rigid and will experience some outward deflection during backfilling(geogrid reinforced walls, modular walls, sheet piling, H-Pile/Lagging, etc.) the soil mobilizes. This means the soil particles are propelled by gravity to densify, thereby increasing the strength of the soil mass. By increasing the strength the soil mass, the translational earth pressure is thus reduced.
The soil cannot simply become "at rest" again. It has been densified and would need to be put back into an uncompacted state again to do so.