Vertical component of active pressure in retaining walls 2

[petrosoft]

This was discussed once on thread507-128311, but I'm not sure what to conclude about it. Enercalc gives the option of using the vertical component of active pressure for retaining wall stability calculations. Their structural engineer says there is some disagreement about using it in the literature. My plan checker won't allow using it unless I can justify it. Does anyone have an insight to whether to use the Enercalc option or not?

Thanks

 

[cap4000]

If your using Rankines formula I would not but if your using Coulombs formula I would. What are the dimensions of your footing and wall height. Typically a gravity wall/no footing or a large footing with a small "heel" dimension justifies Coulombs formula. I don't have the Enercalc. Good Luck.

 

[BigH]

See the old standby: Terzaghi and Peck (1967) - they have a few simplistic charts (4 cases) - which shows this I believe.

 

[petrosoft]

Thanks BigH. I have the 1967 edition of Terzaghi and Peck but can't find the charts you mention. Do you have the page number? I've got to brush up on lateral earth pressure theory.

 

[minorchord2000]

We totally ignore the vertical component of the active pressure, so we do not select that Enercal option.

 

[BigH]

I don't have the '67 version with me; the '95 version is in the office. However, if you have NAVFAC's DM-7.2, go to pages 7.2-86 and 7.2-87. If you don't have the manual, go to VulcanHammer's web site - you can download it from there.

 

[petrosoft]

Thanks BigH for the reference to VulcanHammer.

I'm trying to picture the location of the active vertical component. It seems to be acting on the stem, pushing it downwards. Is that where it is?

 

[jdonville]

petrosoft,

Yes, on the back face of the stem.

Jeff

 

[petrosoft]

I've researched the issue, reading Coduto, Terzaghi and Peck and talked to Enercalc. There is a vertical component of active pressure in the Coulomb theory. Enercalc uses a coefficient of friction on the wall equal to 2/3 the internal soil friction. That makes sense if the wall is concrete (rough) and the backfill is granular.

I'm not sure when the Coulomb formulation is justified and when it isn't. Maybe that is the reason for the option in Enercalc.

Any thoughts welcome.

 

[UcfSE]

Coulomb takes into account the friction between the soil and the wall; Rankine does not. You can use Coulomb when it is appropriate to account for the wall friction and Rankine when it isn't.

 

[petrosoft]

I guess that when using a concrete or masonry wall and granular backfill, Coulomb is OK.


Thanks---Peter

 

[moe333]

When designing a cantilever wall (cantilever on soil side), the portion of soil above the cantilever is used as a resisting force. The forces from the retained soil beyond the cantilever are assummed to react along a vertical section at the end of the cantilever. Therefore, there is no wall friction and Rankine would be applicible.

Coulomb could be used where there was no cantilever, or where the cantilever was on the exposed side of the wall. In this case, the soil forces would be acting against the wall and friction would be applicible.

You do get a component of vertical force from Rankine where there is sloping backfill.

 

[petrosoft]

Hi moe333,

Cudoto's Foundation Design text (2001) page 797 states that the active component of vertical pressure is used to resist OTM in cantilevered retaining walls. The development active pressure assumes a slight outward wall movement so the soil wedge falls slightly while clinging to wall with friction. The only requirements are for outward movement of the wall and a rough wall surface. I don't know if clay backfill would also develop long term friction force.

 

[moe333]

Clay soils tend to creep over time such that active pressures revert to at-rest pressures over time, even with wall movement. Therefore, I would use at-rest pressure for clayey backfill.