When the at-rest condition should it be applied?

[pietro82]

Hi all,

I read on booke the at rest condition should be applied in case of deep foundations. When is a wall foundation considered deep? Is there any rule of thumb?

Thanks

 

[JedClampett]

At rest pressures should be applied when the wall won't rotate or otherwise move enough to use the Active case.
As the movements are defined as a percentage of the wall height and the rotation would cause a large undesirable movement at the top of a tall structure, maybe that's why they recommend using at rest.

 

[pietro82]

Hi Jed,

I know it, but how can I understand it? Is the allowable movement a kind of design constraint that it should be decided before the design?

 

[msquared48]

Definitely. It is inherent in the type of wall you are designing.

Mike McCann, PE, SE (WA)

 

[JedClampett]

For instance:
A basement wall, supported by the first floor, is restrained from moving and at rest pressures are applicable. But a cantilever retaining wall that can slide, deflect or rotate, would be subject to active soil pressures.

 

[DougHole]

Pretend you are trying to push a timber cabinet along on the carpet. If the cabinet is restrained by say someone holding it (i.e, Ko at rest condition), then when you push on it, there will be more stress on the timber of the cabinet than if the cabinet was able to slide along a bit on the carpet with you (like a Ka active condition).

Pressures induced on a wall in an at-rest condition are often about 50% of the vertical effective stress. However active pressures are a lot less i.e. about 30% of the vertical effective stress. this means that earth pressures will be a lot bigger in an at-rest condition.

it is generally conservative to design a wall for at-rest conditions. I always suggest asking the structural engineer how much the wall can move before deciding what you are going to design for ka or k0.

Doug Hole
Junior Geotechnical Engineer

 

[pietro82]

Thanks to all for the replies.

msquared48, where may I find some "threshold" values?

 

[Lomarandil]

Pietro --

Take a look at pages 59-61 of NAVFAC 7.2

http://www.vulcanhammer.net/geotechnical/foundations.php

 

[pietro82]

Thanks it is a valuable resource.