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How to counteract soil creep?

How to counteract soil creep?

How to counteract soil creep?

(OP)
Problem:  50-year-old house on a steep (~40 degrees) slope, 5 feet of medium stiff clay over bedrock.  Clay has  PI=34, SPT blow counts ~10.  House is on shallow spread footings, and the lowermost footing that runs parallel to the slope is rotating, and the deck piers below the house are rotating.  It looks like a clear case of soil creep.

I'm recommending underpinning the spread footing with drilled concrete piers 10 feet into the bedrock, and replacing the deck footings with piers, also 10-feet into the bedrock.

My question is, how do I calculate the lateral forces acting against the upper 5 feet of the future piers (due to the soil creep) so that the structural engineer can evaluate bending moments, etc.?

RE: How to counteract soil creep?

Use passive pressure on the up-slope side of the pier, and no resistance on the down-slope side since the soil creep will eventually pull the soil away from the down-slope side of the pier.

RE: How to counteract soil creep?

Use residual friction angle and discount cohesion.  This is likley a case of a stiff fissured clay releasing its cohesion and having relic fissures that are otherwise affecting the strength.  My initial reaction is to also consider the potential benefit of an anchored bulkhead or tiebacks, if applicable.  Hard to tell across cyberland. . .

f-d

¡papá gordo ain't no madre flaca!

RE: How to counteract soil creep?

moe333, how do you get passive pressure on the up-slope side of the piers?  Isn't that the soil that is moving toward the slope?

ckissick, how do you propose putting drilled piers, socketed 10' into rock, under the foundation?  Why don't you just underpin the foundation conventionally to the top of bedrock with dowels connecting the underpinning piers to the bedrock?  

RE: How to counteract soil creep?

PEinc: Passive pressure would be the limiting pressure that would act on the piers.  It does not matter if the soil moves into the pier or the pier moves into the soil.

Pressures higher than passive would cause the soil to shear up-slope and the pressures would then be reduced.  This is the same pressure you would use to calculate forces on piers from lateral spreading ground du to an earthquake.   

RE: How to counteract soil creep?

(OP)
Thanks for the response, moe333.  Two questions:  How conservative is it to use passive?  It seems like not all the passive would be mobilized in such a situation.

Also, what would be the shape of the pressure distribution?   A normal triangle for conventional passive?  And should I neglect the upper foot or so?

To answer PEinc, it is common practice to drill adjacent to a footing, not under it, and dowel into the footing.  Also, the bedrock is sheared and weak.  It can be drilled fairly easily and can't be doweled into.  It's still good enough to remain stable, however.

RE: How to counteract soil creep?

I agree with moe333, use full passive pressure with a normal triangular distribution.  Eventually, something very close to full passive will mobilize.

RE: How to counteract soil creep?

(OP)
My other question is:  Why don't any of my texts discuss the quantitative aspects of this issue?  It would be a simple thing to describe.  Is there a good text book that actually provides solutions to real-world problems in the realm of  geotechnical work on houses?   

RE: How to counteract soil creep?

I would use full passive over the full height of soil, normal triangular distribution.  The friction angle you use to calculate the passive will determine how conservative you want to be.

RE: How to counteract soil creep?

Passive pressure acting on a pile is increased by the coefficient Cp, which is typically approximated as phi/10. So, if you have a pile embedded in a soil (clay) with a friction angle of 24 degrees, you'd calculate the Rankine passive pressure (2.37) and then multiply it by 2.4 for a passive pressure acting on the pile of 5.7. Then draw your triangle.

It's important to note that Rankine earth pressures are for the plane-strain condition.  Earth pressures acting on a pile are not plane strain and there is arching in the third dimension (i.e., the dimension that's not shown on the paper drawing).  Cp is a way to account for this.

f-d

¡papá gordo ain't no madre flaca!

RE: How to counteract soil creep?

I have to agree with PE Inc that I do not see how passive pressure would develop in this situation, or how even any limit state woul apply. Soil creep is generaly a global failure issue which has no bearing on limit states and should be investigated. Depending on conditions using passive pressure (esp. for clay on a slope) may be unconservative.
Not that we are a tag team, but drilled piers for underpinning do not seem as effective and considerabley more costly than tied back pit underpinning.

RE: How to counteract soil creep?

passive pressures would develop on the portion of the pile that is above the failure plane if the pile was designed to stay put.  The counter reaction would have to be from below the failure plane - the depth to determine whether you have a long pile or a short pile stress condition.

I'm not saying I got it right, I'm just typing what I thought was originally proposed.

f-d

¡papá gordo ain't no madre flaca!

RE: How to counteract soil creep?

Passive pressure would develop if the pile were forcing the soil away from the pile which does not appear to be the case. Active pressure would occur if a localized wedge of soil was bending the pile, which does not sound like the case either. I agree that the original direction was using passive pressures to calculate loads, My point was that this appears to be a global stability issue and should be treated thus.

RE: How to counteract soil creep?

I agree with DRC1.  Drilled piers in front of the existing footings would need to have very rigid connections to the footings.  I don't see how this connection could be made economically or could perform satisfactorily.  For passive pressure to develop, the pile, pier, or footing would have to move up hill toward and under the structure.  Not likely.  This is probably a slope stability issue if the remedial structure is installed away from the building; or it is a combination slope stability/underpinning issue if the remedial structure is installed at or under the building foundation.  I have never, in may years and a few hundred underpinninng jobs, seen a building "underpinned" using a drilled shaft that was not under the foundation.  I've designed and built bracket pile underpinning and have drilled and set piles under a foundation,, but have never seen nor tried to attache a drilled pier to the side of a footing.  I would probably use conventional underpinning piers to rock with rock dowels at the base of the underpinning piers and then install some rock tieback anchors.

RE: How to counteract soil creep?

I need a drawing.

f-d

¡papá gordo ain't no madre flaca!

RE: How to counteract soil creep?

Passive pressure can develop if the soil creep causes the soil on the up-slope side of the pier to impact the pier to the point where the adjacent soil "flows" around the pier over time.  I don't know if this is the case in this situation, and I don't necessarily agree with the proposed fix.   

RE: How to counteract soil creep?

In my opinion, passive pressure is a reaction, not a driving force.  When pushed, soil either successfully resists enough to provide equilibrity, or it fails at its maximum available passive resistance.  It does not push on the structure.  You will have a hard time finding any geotechnical reference that shows passive pressure both driving and supporting a structure.

RE: How to counteract soil creep?

.....both driving and supporting a structure CONCURRENTLY, that is.

RE: How to counteract soil creep?

It is common geotechnical practise to model laterally spreading ground on piles with passive pressure. I find this analagous to this situation.  In this case there is no resisting force since the soil down-slope is also creeping.  

RE: How to counteract soil creep?

Moe333's reply after mine makes sense to me (at least how I was visualizing the topic.  I also agree this is highly academic as I'm not sure of the fix.

f-d

¡papá gordo ain't no madre flaca!

RE: How to counteract soil creep?

Given the orginal problem statement, I believe that the full passive pressure is the appropriate soil load to analize the piers.  Whether that is enough to stop the soil creek, I don't know.  The answer would depend on the reason the soil is moving, spacing of the piers, diameter of the piers, etc.

Given the relatively short nature of the piers, the required diameter and reinforcing should not change much of a pretty good range of soil loading.

RE: How to counteract soil creep?

I'd still like to see a reference backing up that passive pressure theory.

If the soil is "creeping," I assume there is a slope stability problem.  When anchored walls are designed to stabilize landslides (unstable slopes), they are designed using slope stability analyses, not passive pressures.  It just may be that a design using passive pressures may work because Kp is usually at least 10 times greater than Ka.  Could it be that using Kp is just a simplified, hopefully conservative method for designing the fix (instead of using a slope stability analysis)?

RE: How to counteract soil creep?

PEinc,

I agree that in general slope stability issues should be fixed using slope stability methods.  However, if you cannot for one reason or another get a good stability model, this would be a backup plan.  For individual piers (drilled shafts) the maximum load that the single shaft could ever see is Kp.  Any load, no matter the source of loading, greater than Kp would cause the saft to move relative to the soil with no futher load being transfered.

If looking at a site and you gut feeling is that 5 piers should fix the issue, then the question is how much steel.  The max would that be amount required to withstand full Kp.  

Would it be better to drill and really "know" what is going on? Sure. But for one reason or another that is not always possible.

RE: How to counteract soil creep?

The study is based on granular material which even for somewhat loose states has a fairly high Kp. For this post the material is a soft clay which may have a fairly low Kp. Given a long slope and close spacing of the piers, it may be possible for the sliding mass to impart a greater load than passive, esp at the base.
Either way, one needs to determine clay strength and run the numbers. Just different numbers to run.
Using SPT's to determine clay strength, esp. for stability issues is not a good idea. SPT's are designed for testing granular soil, not clays.

RE: How to counteract soil creep?

A few final comments before I am done with this thread:

The clay was stated to be medium stiff, not soft.  The reported blow counts tend to confirm this.  Even though I am not a fan of blow counts in clay soil, they do provide some basic information.

The passive pressure analysis for laterally spreading soil is often used for a non-liquefiable clay crust.  And they involve long failure surfaces.  Do a google search and I'm sure you can find many more references if you are interested.

I believe the passive pressure analysis is appropriate for this situation.   

Good Luck

RE: How to counteract soil creep?

Interesting discussion.  I'll read moe333's reference paper.

RE: How to counteract soil creep?

(OP)
Thanks for all the responses.  I never wanted to treat it as a slope stability issue.  The existing house is about 50 years old and the soil creep is very slow.  The footings and deck piers have moved 2-3 inches in 50 years.  It's just enough to warrant remedial measures.  It's a classic case of creep caused by expansive soils as they expand in winter and contract in summer.  In California, with our 6-month dry season, this is a common problem.

I think the passive pressure approach is a good one, and probably a little conservative for this case.  The structural engineer will be able to use my recommendations to design the steel and select the pier spacing.

RE: How to counteract soil creep?

At the risk of repeating myself, using Rankine passive pressures on a pile is not correct.  You have to increase the passive pressure by the value Cp.  If you discount this on the basis that using Kp in the first place is sufficiently conservative, then you are throwing a solution at a problem you don't understand.  If you understand the problem and Kp acting on pile is a part of the solution, then you need to include Cp.

In the absence of a meaningful design sketch, I have no opinion on the solution.  Other than wondering why you don't want to consider tie-backs that extend beyond the "creep zone".

f-d

¡papá gordo ain't no madre flaca!

RE: How to counteract soil creep?

Fattdad, do you have a reference for the Cp methodology?  

RE: How to counteract soil creep?

Hansen, J. B. "The Ultimate Resistance of Rigid Piles against Transversal Forces."  Danish Geotechnical Institued, Bulletin No. 12, Copenhagen 1961, pp 5-9

¡papá gordo ain't no madre flaca!

RE: How to counteract soil creep?

On this Cp factor, we, in the WisDOT years ago would just double the theoretical passive pressure to account for shearing along the sides of the pile.  It seemed to work OK.

Or one could use the ultimate  shearing strength of the soil on the sides.

Set these piles close enuff and you have a wall.

RE: How to counteract soil creep?

oldestguy,

It sounds to me like you are talking about the resistance that can be provided by the piles rather thasn the driving force that is produced by the "creeping" soil.  How does the theoretical passive resistance and side shear that can be provided by the piles have anything to do with the actual driving or creeping force?

RE: How to counteract soil creep?

Quote:

Set these piles close enuff and you have a wall.

My point exactly.  If you have 12 inch piles spaced on 30 inch centers, you will end up having the equivalent wall loads conveyed to 40 percent of the wall area (i.e., you better consider a Cp value of 2.5)!  Soil arching really will affect lateral loading on piles as the plane strain condition doesn't exist.

f-d

¡papá gordo ain't no madre flaca!

RE: How to counteract soil creep?

I'm investigating a retaining wall that was leaning 1/2" to 2" out of plumb in 10' vertically. The wall was leaning away from the retaining side. does anyone know of a published specification regarding the allowable tolerances for plumb? In my opinion its 0, however it's been difficult to find anything one way or the other beyond common sense.  

RE: How to counteract soil creep?

it takes 1 in in 10 ft to mobilize active earth pressures.  So that would be about in tolerance.  If you want 0-in tolerance, you have to design for at rest earth pressures.

f-d

¡papá gordo ain't no madre flaca!

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