Assessing sliding of foundation, how to determine active lateral earth pressure?

[StructureMan44]

I'm determining if a foundation for small tank requires piles or if a mat foundation is sufficient. I have a geotec report in the vicinity that provides the allowable soil bearing pressure to stay under a specified settlement. This allows me to check the overturning resistance for wind and seismic. The report does not provide the lateral earth pressure coefficients (at rest, active, passive) nor does it provide an angle of friction. Is it possible to determine the active lateral earth pressure coefficient with only knowing the various soil types and how far below the surface they are? Are there conservative values that are acceptable for pressure coefficients? If the foundation is 14'wide x 14'long x 5'high can I compare the pressure on one side due shear from wind against the allowable soil pressure?

 

[TehMightyEngineer]

If it's a buried tank you probably want the at-rest pressure coefficient. Ko = 0.5 is usually conservative and it's what I usually design to if I have little to no info about the soils.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[StructureMan44]

TehMightyEngineer: Thanks for your help. Do you mean is the foundation buried? It is and the tank will be at grade.

I should clarify that the tank could better be called a silo.

 

[TehMightyEngineer]

Basically, is the tank foundation surrounded on all sides by soil or is it more like a retaining wall? You mention sliding so if you have sliding then perhaps you'll be developing the active soil pressure. A picture of the foundation would be helpful.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[StructureMan44]

TehMightyEngineer: It is surrounded on all sides by soil. The closest structure is a building foundation that is <20ft away. I don't have a picture because it isn't built yet. The project manager just wanted a yea/nay on whether piles are needed. I was thinking active soil pressure is more appropriate which would be lower than the at-rest Ko = 0.5 you recommended.

 

[TehMightyEngineer]

If sliding is actually a concern then I would say that, yes, you're more likely to develop active soil pressures. Also, the Ko = 0.5 is quite conservative and you could lower it if you have a reasonable idea of the over-consolidation of the backfill.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[BadgerPE]

If you have full soil backfill all around, what are you worried about? If you are using at-rest pressures on both sides, the forces will balance out. To deal with your global wind forces on the silo, use the friction between the base of the slab and the subgrade. I have designed dozens of 12' diameter 90' tall tanks (silos) on mat footings and sliding is never close to controlling (in wind controlled areas) when just using V*tan(2/3*phi).

 

[oldestguy]

Go back to your Geo tech and get the answer from them. That's their job.

 

[FoxSE14]

If you are determining whether piles or a mat are more appropriate, you'll be primarily concerned with the allowable bearing pressures of each system. Consult with the geotech and inquire about allowable bearing pressure for a mat foundation (and the elevation this allowable can be assumed at). Likewise, if piles are a possibility, ask your geotech for allowable capacities for end bearing (and associated depth) and skin friction, if any.

As for sliding: Agreed with Badger, for wind on a silo sliding typically will not be your primary concern; overturning will likely control. Passive pressure is typically associated with the 'bulldozing' resistance that you are hoping for - you should also obtain this value, and any friction factor, from the geotech.

 

[StructureMan44]

FoxSE14: thank you for your advice. Unfortunately in this case we don't have a geotec, we're relying on a geotec report done 20 years ago.

TehMightlyEngineer: You mention that over-consolidation of backfill can lower the at-rest pressure coefficient. Is this because the vertical effective stress increases or because the effective horizontal stress decreases?

 

[oldestguy]

Evey if the report is old, the data should be there. It is not likely a report for a building giving bearing pressure for a given settlement will apply to a tank. However, you may be able to accept a higher settlement. Bounce it off another geotech.