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.?
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?
RE: How to counteract soil creep?
f-d
¡papá gordo ain't no madre flaca!
RE: How to counteract soil creep?
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?
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?
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?
RE: How to counteract soil creep?
RE: How to counteract soil creep?
RE: How to counteract soil creep?
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?
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?
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?
RE: How to counteract soil creep?
RE: How to counteract soil creep?
f-d
¡papá gordo ain't no madre flaca!
RE: How to counteract soil creep?
RE: How to counteract soil creep?
RE: How to counteract soil creep?
RE: How to counteract soil creep?
RE: How to counteract soil creep?
f-d
¡papá gordo ain't no madre flaca!
RE: How to counteract soil creep?
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?
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?
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?
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RE: How to counteract soil creep?
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?
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?
RE: How to counteract soil creep?
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?
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?
RE: How to counteract soil creep?
¡papá gordo ain't no madre flaca!
RE: How to counteract soil creep?
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?
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?
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?
RE: How to counteract soil creep?
f-d
¡papá gordo ain't no madre flaca!