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.?

 

[GeoPaveTraffic]

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.

 

[PEinc]

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)?

 

[GeoPaveTraffic]

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.

 

[moe333]

Reference discussing use of passive pressure on a pile cap due to lateral spread.

http://cee.engr.ucdavis.edu/faculty/boulanger/PDFs/2004/Brandenberg_et_al_WCEE_2004.pdf

 

[DRC1]

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.

 

[moe333]

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

 

[PEinc]

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

 

[ckissick]

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.

 

[fattdad]

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!

 

[moe333]

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

 

[fattdad]

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!

 

[oldestguy]

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.

 

[PEinc]

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?

 

[fattdad]

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!

 

[nmdeveloper]

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.

 

[fattdad]

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!