Structural backfill zone for wall design

[bagger]

Hello: I am looking for opinons for the location of the structural backfill influence zone for foundation wall or retaining wall design. I typically recommend eqivelent fluid pressures for open graded material within the influence zone of the wall, ie. 45+Phi/2. For cohesionless material this typically works out to be 30 degrees from vertical, from the base of the wall. I have seen the influence zone defined as 45 degrees from the edge of the footing. This requires a signiicantly large backfill zone. My question being, is using the 45 degree zone just being conservative or is there a technical justification for this size influence zone. Thanks in advance for any opiions.

Bagger

 

[fattdad]

you are mixing apples and grapes. Rankine earth pressures relate to plane strain stress conditions. The influence from an isolated load is calculated using Boussinesq or Westergaard equations and influence by the change in vertical stress and Poisson's ratio for the material. Rules of thumb may suggest such a 1:1 load spread, but even if you are beyond that "envelope" it does not mean there is no influence. Just run the numbers, it's not that difficult. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[bagger]

f-d

The influence zone was probably a bad choice of words. I am interested in the failure plane or failure envelope wrt lateral earth pressures on the wall. I believe the area behind the wall defined by this plane should be backfilled with cohesionless material. Text book references this plane using the Mohr's Circle based on the phi angle of the material, ie. 45 + phi/2. I am looking for a technical justification for using 45 dgrees from the heel of the wall footing and not 45 + phi/2 from the base of the wall.

Thanks for your response

Bagger

 

[Doctormo]

Some wedge theory comments for what its worth:

The Rankine "failure plane" is defined as 45-phi/2 from vertical for a vertical wall, level backslope, and no wall-soil interface friction. The Coulomb "failure plane" will lay back more or is flatter for the same situation due to wall friction but will result in a lower calculated pressure. Pick your religion at this point. Both require that the same phi angle material be on both sides of the "failure plane" or "wedge" to provide the assumed frictional resistance from the soil shear strength thus the zone has to be somewhat larger than the failure plane envelope.

Any change in wall batter, backslope, or additional surcharge loadings will tend to cause the "critical failure plane" to lay back further with an infinite backslope and Coulomb analysis approaching 45˚. In most cases, backfilling a 1:1 zone (45˚) is the safest method to limit pressure to the simple granular fluid pressure model although it could be less. In fact, one can just go a couple of feet behind a wall and then up at a 45˚ angle with granular material and satisfy the lower design pressure state criteria. Concrete walls commonly use granular material to the rear of the footing as that area is excavated and must be backfilled anyways.

Keep in mind that there are different measurement points, one for the wall stem (or internal pressure) and one for external stability pressure (back of footing). If one is just trying to reduce stem pressure, the zone can be smaller than if on is trying to reduce external pressures as well. This is why you may see a wall backfilled with granular material to the rear of the footing and then up at a 1:1 from that point back to keep all pressures low.

 

[bagger]

Doctormo

Thanks for the response and commentary. The 1:1 slope from the back of the footing appears to be the more accurate model for wall backfill regarding lateral pressures.