At-rest earth pressure
At-rest earth pressure
(OP)
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
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





RE: At-rest earth pressure
RE: At-rest earth pressure
So, take the active earth pressure from the chart solution for sloping ground and multiply it by 1.5.
Now, how does that help? You see, under lateral loading, you will mobilize much more strength that suggested by at-rest earth pressure. Firstly, it's a pile, so plane-strain conditons do not apply. To account for arching effects, we use a term called C-sub-p, which is approximated by phi/10. To consider lateral capacity, we'd also consider passive pressure, which is also derived from chart solution for sloping ground.
Next topic should then be, "What are the deflections?" For that you'd need to develop your p-y curve, based on some sense of subgrade modulus.
f-d
ípapß gordo ainÆt no madre flaca!
RE: At-rest earth pressure
Thanks fattdad, that is very helpful.
dtsk
RE: At-rest earth pressure
RE: At-rest earth pressure
RE: At-rest earth pressure
I would use an area on the surface of each pile of twice the diameter to account for side friction load..
RE: At-rest earth pressure
RE: At-rest earth pressure
www.PeirceEngineering.com
RE: At-rest earth pressure
This is not an at-rest earth pressure problem!
f-d
ípapß gordo ainÆt no madre flaca!
RE: At-rest earth pressure
Just to clarify, the instability observed is 'soil creep', whereas the soil stratum is moving very slowly downhill. We are not envisaging immediate soil evacuation i.e slips. The load acting on the piles are minimal and the aim is just to address serviceability state of the structure. But we still need to quantify this pressure on the piles somehow.
My idea is to calculate the magnitude by 1.5m x 3D (D=pile dia), and thinking at-rest conditions would be more conservative rather than active?
dtsk
RE: At-rest earth pressure
So, your claim is correct. At rest will be more conservative than active. However, they are both incorrect, in my opinion. Additionally, owing to arching effects, the pile will take on more than the face area would suggest. So, for me, I'd start considering passive pressure and a Cp factor of 3 or so. I'd apply that force only on the surface that's likely to creep.
Carry on. . .
f-d
ípapß gordo ainÆt no madre flaca!
RE: At-rest earth pressure
IfWhen the landslide bends the piles, the house will also move, probably differentially, and be damaged. I suggest you look at designing a tiedback retaining wall at or near the toe of slope. The wall would need to be tall enough to flatten and stabilize the upper portion of the slope. The tiedback wall would laterally support the landslide. The bearing piles would only support the house.www.PeirceEngineering.com
RE: At-rest earth pressure
RE: At-rest earth pressure
Core logs have been taken at the site, with highly weathered rock at shallow depths (1m to 2m), with SPT 30+ at 2m. Based on the stability evaluation, movement is limited to the upper 1.5m, but still with satisfactory factors of safety. This is a precautionary measure after all.
Another idea i had was to model a slip at the toe of the piles, and through back analysis calculate support forces required and integrate into the pile design. This would also take into account groundwater or seismic conditions.
RE: At-rest earth pressure
At this point, if piles are to remain in the plan, they had better be socketed into bedrock. That may take a churn drill or other rig suitable for drilling into rock. How deep in rock, and what did the boring information come up with as to rock condition? Then comes the design of the piles themselves. How far into rock and can they withstand the slicing bending loads on them? Can you get a rock drill on that slope? Are rock beds inclined?
It is pretty obvious a lot of evaluating and stability calcs are needed, not just a simple answer for at-rest earth pressure, if it even applies.
Also are there any other factors, even though not considered, and not mentioned yet ] that may be of value for those trying to help???? Those might be: earthquake potential, building code limits, ground water, frost conditions in colder weather, historical slip evidence on the slope nearby, tree growth showing no sliding, etc.
RE: At-rest earth pressure
www.PeirceEngineering.com
RE: At-rest earth pressure
But, I also like PEinc's idea to stabilize the slope first (placing a retaining wall) and then use the piles only to support the building. I feel that this is the best approach for this problem.
RE: At-rest earth pressure
RE: At-rest earth pressure
Sleeves work to mitigate vertical movement, but would only amplify the horizontal stresses.
f-d
ípapß gordo ainÆt no madre flaca!
RE: At-rest earth pressure
RE: At-rest earth pressure
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?
RE: At-rest earth pressure
www.PeirceEngineering.com
RE: At-rest earth pressure
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...
RE: At-rest earth pressure
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...
RE: At-rest earth pressure
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!
RE: At-rest earth pressure
f-d
ípapß gordo ainÆt no madre flaca!
RE: At-rest earth pressure