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At-rest earth pressure
3

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

RE: At-rest earth pressure

A sketch would be helpful. How far is the pile toe to the slope surface?

RE: At-rest earth pressure

for sloping ground, I'd use a chart solution. I think the NAVAC DM 7.1 or 7.2 has such chart solutions for active or passive earth pressure. To obtain the at-rest earth pressure, that's a bit of a different question. There is no chart solution, so we have to use, "Common sense!" In pure Rankine conditions, the coefficient of at-rest earth pressure is usually 1.5 times the coefficient of active earth pressure (i.e., (tan^2(45-phi/2))/(1-sin phi)=1.5.

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

If the slope (1V to 2.7H)is stable, the lateral load transferred to the piles is expected to be minimal. Any lateral load from the building to the piles in the downslope direction?

RE: At-rest earth pressure

(OP)
The existing slope where the piles are situated, is not stable and with evidence of instability noted and assessed to be within the top 1.5m of soil stratum. Hence the lateral load on the piles. No lateral loading from the house to the piles.

RE: At-rest earth pressure

Since the slope is moving the at rest situation is not present. For design, this chart from Navy NAVFAC manual would be the low end. I's say you should look at passive pressures. which might be many times this chart. There may be other charts like this for passive pressures.



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

Here is anther chart, but mainly for granular soil. The lower part is Coef. of active pressure and upper is coef. ofppassive pressure

RE: At-rest earth pressure

If this is an unstable slope, the job needs slope stability and LPile analyses where the piles acts as vertical, stabilizer elements subjected to lateral loads. The driving pressures may be much higher than at-rest pressure. The piles will need to be designed for the lateral load needed to stabilize the slope with the required safety factor.

www.PeirceEngineering.com

RE: At-rest earth pressure

yes, for slope reinforcement, the earth pressures at failure (i.e., if the slope were to actually start sliding) would fully mobilize passive pressure and the arching effects of the non-plane-strain condition would mobilize C-sub-p, which for a frictional soil would amplify the passive pressure by a factor of phi/10 (roughly).

This is not an at-rest earth pressure problem!

f-d

ípapß gordo ainÆt no madre flaca!

RE: At-rest earth pressure

(OP)
Thanks everyone for the response and info!

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

the thickness that's creeping will impart more than 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

It seems to me that, if the piles are vertically supporting the house, you don't want them to also be supporting a moving landslide. If When 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

It seems this is a house yet to be built. May options are possible. However a slope stability evaluation is needed before proceeding with anything. For instance how's come the instability is only 1.5 meters deep? A detailed slope stability evaluation is needed to show that, if it is really right, etc. Designing piles to resist a slope movement is a whole lot more complicated than to just assume some pressure, such as "at rest" or "passive". What boring and test data is available?

RE: At-rest earth pressure

(OP)
Will consider passive 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

This is another example of not giving enough information in the original post and expecting some help. Chances are most of the comments above don't apply to the site. It would appear that the piles will b founded on a sloping bedrock surface with very little embedment in "good stuff" to create stability, regardless of the presence or absence of a moving upper layer. Assuming it will move, those piles will just bend over and go with it.

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

He's not listening, oldestguy. He just wants to apply passive pressure to the piles and have us agree with him.

www.PeirceEngineering.com

RE: At-rest earth pressure

IMHO, for your problem, you can make an analogy to the effects of liquefaction-induced lateral spreading in piles.

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

Can you sleeve the piles and let the ground move around them?

RE: At-rest earth pressure

how does the sleeve transfer load to the structure though?

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

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.

RE: At-rest earth pressure

(OP)
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?

RE: At-rest earth pressure

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?

www.PeirceEngineering.com

RE: At-rest earth pressure

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...

RE: At-rest earth pressure

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...

RE: At-rest earth pressure

2
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!

RE: At-rest earth pressure

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

f-d

ípapß gordo ainÆt no madre flaca!

RE: At-rest earth pressure

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

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