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

 

[HENRYZAU]

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

 

[fattdad]

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!

 

[dtsk]

A sketch attached.

Thanks fattdad, that is very helpful.

dtsk

 

[HENRYZAU]

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?

 

[dtsk]

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.

 

[oldestguy]

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..

 

[oldestguy]

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

 

[PEinc]

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.

http://www.PeirceEngineering.com

 

[fattdad]

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!

 

[dtsk]

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

 

[fattdad]

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!

 

[PEinc]

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.

http://www.PeirceEngineering.com

 

[oldestguy]

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?

 

[dtsk]

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.

 

[oldestguy]

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.

 

[PEinc]

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

http://www.PeirceEngineering.com

 

[Okiryu]

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.

 

[Settingsun]

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

 

[fattdad]

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!