At-rest earth pressure 3

[dtsk]

Hi guys,

We have house piles that need to be designed for lateral loading.

How do you calculate at-rest earth pressures taking into account the sloping ground below the pile? Ground inclination is at 20 deg, soil friction angle at 32 deg.

Cheers

 

[EireChch]

I think the issue here is to fully understand the failure mechanism. If you can confirm that the mechanism is creep then you can design the piles to carry both axial load and lateral load (probably some bracing of piles beneath the house). I dont think a retaining wall is needed to stabilize the slope if it is creep. Unless the owner wants to have a level garden maybe.

 

[dtsk]

Apologies oldesguy/PEinc, it seems i may not have been clear enough and provided insufficient info.

It is only creep issue. No evidence of slips have been observed in the nearby area. The region is low risk of earthquake. Groundwater was not encountered and no frost issue. Trees are bow shaped at the toe which only suggests creep in this case. I am fairly certain that failure wont extend beyond 1.5m depth. I would've thought embedded of 2m into competent materials (highly weathered rock which can be drilled) would sufficient enough to resist slow movement within the upper 1.5m?

Piles will be braced as well. I agree with Eirechc that no retaining wall is needed if only creep issue. How would you quantify this creep load on to the piles EireChch?

 

[PEinc]

No apologies needed. I just don't understand why this known "creep" issue is of less concern. You never know when soil will get "tired" and decide to fail. Where is the fine line between measurable creep and instability? Isn't ignoring the creep a little bit like ignoring smoke before a fire?

http://www.PeirceEngineering.com

 

[Settingsun]

Hi fattdad,
I'm taking dtsk at his/her word that it's not a slope stability issue. The sleeve doesn't transfer load to the structure: I'm not trying to stop the creep, just stop lateral load being applied to the piles. The sleeve has a larger diameter than the pile and moves with the soil but the air gap between sleeve and pile means no load transfer and cheaper pile.

Predicated of course on stability of the slope...

 

[EireChch]

I will speak from limited experience here soils susceptible to creep - but I worked in an area that was prone to it for 2 years at the start of my career. I can tell you my two cents but it may only be applicable to my soils and I'd love to hear from someone who has years of experience with creep. It's always been a phenomenon for me

My senior was never too concerned about it, he use to hate putting the word "creep" in a report because it screams instability to a local authority when in fact it's not that dramatic. He would always word in a way to not use the word creep.

In Rennes of quantifying it dtsk I don't know, I think it's a bit of a phenomenon. Just be concervstive, I thought a Ko pressure times a certain width would have been a good place to start. I recall hearing that arching effects result in the pile taking 5D width of soil. I've never found anything to back that up though.

Fatdadd suggest using passive pressure and a Cp factor (what is a Cp factor...creep factor?).
Could you explain why you would use a Passice pressure? How would the pressure not be active or Ko since it's acting on the uphill side of the pile?

PEinc - if it is truely creep then it shouldn't be a catastrophic slip down the line...but famous last words maybe...

 

[fattdad]

It's all about the free-body diagram. If the house is standing still, but the soil is moving ("creeping") from uphill to downhill, where's the vector? It's pointing down the slope - in the direction of movement. If the pile is not moving, but the soil is "flowing" around the pile, that would have to mobilize passive pressure, eh?

Refer to Brinch-Hansen (1966) for more information on the term, "Cp." It's unrelated to creep.

f-d

p.s., as an aside, I work for the state DOT and we have all sorts of slope stability issues on our tens-of-thousands-of-mile road network. In response, we have some sites where we jam piles in the ground to stabilize moving slopes. I use the methods suggested above, 'cause it's right. . . Or so I think.


ípapß gordo ainÆt no madre flaca!

 

[fattdad]

I think Brinch-Hansen uses the tem Kq, rather than Cp.

f-d

ípapß gordo ainÆt no madre flaca!

 

[L0k]

Actually I prefer to install the retaining wall at the toe of the slope to reduce the soil moving ('creeping').