How to counteract soil creep?

[ckissick]

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.?

 

[moe333]

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.

 

[fattdad]

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!

 

[PEinc]

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?

 

[moe333]

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.

 

[ckissick]

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.

 

[GeoPaveTraffic]

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

 

[ckissick]

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?

 

[moe333]

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.

 

[fattdad]

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!

 

[DRC1]

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.

 

[fattdad]

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!

 

[DRC1]

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.

 

[PEinc]

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.

 

[fattdad]

I need a drawing.

f-d

¡papá gordo ain’t no madre flaca!

 

[moe333]

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.

 

[PEinc]

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.

 

[PEinc]

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

 

[moe333]

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.

 

[fattdad]

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!