Shallow Pier Foundation Rotating Down On Hillside

[RFreund]

This is similar to the hillside power pole situation you typically see while driving. The pole is embedded into the hill side and is now X degrees from vertical as it rotates down the hill.

We have a situation where an existing residence built on a hillside has exiting balcony piers that appear to be sliding/rotating down hill (home appears to be ok). They would like to add on to the balcony but the proposed piers must be designed so as to not rotate down hill.

My questions:

Are there any references that discuss this topic?
How can you design for this situation? I thought about installing tie back helical piers but I'm not sure what the force on the pier would be.
Is this a slope stability issue?
Any other comments would be appreciated.

Thanks!

EIT

http://www.HowToEngineer.com

 

[Riggly]

your topic said "Shallow Pier Foundation..". That I think is the problem - the they are too shallow. Design has to tak into account the slope. Embedment in a slope is typically deeper compared to embedment in a flat area, all other things being equal. The down-slope side of the pole/pier provides less lateral support, and that difference becomes more significant when the foundation is shallow.

 

[RFreund]

Riggly,

Correct I am thinking the same thing. However how would you design the foundation? Apply an active pressure due to the slope on one side and a reduced passive pressure on the other? Then determine embedment or possibly use a tie back?

EIT

http://www.HowToEngineer.com

 

[Riggly]

The pile-soil interaction is so complex that it can only be reasonably done with the aid of computer programs - I typically used LPile to perform the analysis, and there is an input for ground slope. However, I think you have the right theoretical concept.

 

[FixedEarth]

RF;

Yes this has to do with a slope stability issue. One of two things may be happening. Either the pier is not seated deep enough to get a F.S. of 1.5 against deep seated rotational failure & surficial slope failure (parallel to the slope surface or we have a slope creep issue. Slope creep is when the upper 3 to 5 ft soil is slowly moving downward parallel to the slope. Look at any short trees on the slope and see if they are tilting. One of the geotechnical books by Robert Day discusses this topic.

Simplest solution is to confine the piers. You can do this by excavating a continuous trench in front of the pier and filling it with concrete and some reinforcement. If the slope is steep (say greater than 30 degrees), you may provide drilled piers for this trench footing. Aim for daylight distance (horizontal distance from bottom of trench or new pier to the slope surface) of 15 ft or one third the slope height, which ever is greater.

 

[PEinc]

Is it possible that only the piers are moving but not the slope? I would not be surprised if the slope had been widened out with uncompacted or poorly compacted fill soil and the piers are bearing in this looser soil. RFreund said that the piers are sliding/rotating. Are they sliding and rotating along with the slope? Is the slope moving away from the house? If not, maybe the piers need to be founded at a deeper elevation , in virgin, competent soil.

http://www.PeirceEngineering.com

 

[RFreund]

Hmmm. Good questions.

I can't say for sure if they are sliding or if they are rotating or both or if the slope is falling away from the home. I believe in some cases the top couple feet may appear to be moving away from the home but this may just be due to erosion? It is possible that it is a settlement issue but the deck is small and the load to the piers is small. The reason I say they are moving down hill is because the piers support a balcony 14' above that is tied to the home. The balcony posts are bowed out of plumb and the balcony floor is sloped down at the posts (out of level) however it is level at the connection to the home at the re-entrant corner. (an interesting side note is that the wood posts are bowed as if the top connection is acting as a fixed connection. You wouldn't expect it in wood framing but there seems to be some restraint there). The home is in good condition and the owners have not noticed any cracks in brittle finishes and the foundation did not appear to have any significant cracks.

EIT

http://www.HowToEngineer.com

 

[PEinc]

When a slope gets widened, it should be benched so that the new fill can be placed on stable, level ground and be properly compacted. This rarely will happen unless it is a highway project with specs and field inspection. For a residential structure, if fill is placed on an existing slope without benching, it will probably not be compacted either and will slide down over the original slope, taking with it any load bearing structures that are founded in the new fill.

http://www.PeirceEngineering.com

 

[Riggly]

The underlying problem really seams to be slope movement. What is the embedment depth of the poles? How steep is the slope? How is the nature of the toe of the slope? Any vertical cuts at the toe? How hight is the slope? It appears that the embedment is insufficient, and you also seem to have a structural problem, where the poles are not strong enough to withstand the net lateral force. Do you know how the slope was constructed? Or whether it is natural deposit? Any information on the type of soil?

 

[RFreund]

Well before I was just looking for general information and would still be interested. FixedEarth - any idea on which Robert Day book? How is the forensic one that he authored?

Riggly - Attached is a sketch of the actual problem.

The house foundation appears to be OK however the existing piers are moving. Not sure how much laterally and how much vertically.

EIT
www.HowToEngineer.com

 

[RFreund]

Here is a picture trying to show the existing condition.

EIT
www.HowToEngineer.com

 

[FixedEarth]

My guess is the backhoe swang the buckets during the basement excavation to the descending slope side and therefore uncompacted fill was placed. Then the piers were dug with some eccentricty. Also when they connected the roof drains, the water comes straight to the pier and is not being diverted. I don't think it is slope creep problem any longer. It was not embedded deeply into competent soils. See what your project geotechnical engineer recommends.

 

[RFreund]

Thanks FE.
We recommended they/we hire a geotech, we'll see what happens.

EIT

http://www.HowToEngineer.com

 

[msquared48]

You've got a downspout dumping rainwater upstream if the outboard pier, the one showing the most distress. You need to tightline this downspout and take the runoff well downstream of the pier. This should stabilize the situation.

Additionally, but of far less impact, it appears that the house foundation may be oimpac tinf laterally on the pier, depending on the depth of the house foundation.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[RFreund]

Yes I agree, with both assessments of the water needing to be diverted. Also the segmental retaining wall could be moving some as well. However the proposed piers would be located 6' from the house along the back and up along the sides of the home where the slope is quite steep (approx 1:1). Any thoughts on the foundation at this location? My initial thoughts were:
1.) Helical pier system (vertical and tieback)
2.) Deep foundation (on the order of 10-15ft)
3.) Tie back into the house foundation. (Don't like this idea too much as we don't know anything about the original construction.)

Thanks again.

EIT

http://www.HowToEngineer.com

 

[dirtsqueezer]

I might think about clearing all of that hillside material and gravel from out behind your footings there. Seems like that would be the thing that's pushing things over. Maybe a nice-looking rockery there to block the hillside material from creeping into your structual elements.

 

[Riggly]

@dirtaqueezer. are you serious? I believe that is a thin layer of stone, and with that angle of the stop, I don't think is is doing anything significant to those piles.

How tall is that retaining wall? Could it be that the could it be some modes of failure of the retaing wall is casing the piles to move? The pile far beyond the wall seems ok and on a steep slope. I don't think that a shallow sliding of the slope is the problem. Tell us some more about that wall - it's construction, height etc. Is it MSE?

 

[RFreund]

Unfortunately I don't know much about the wall construction. It appears to be a Anchor Diamond SRW wall. It is 3' tall and it appears that there is no embedment (possibly due to erosion). The wall appears to be in pretty good shape so I suspect there is Grid otherwise I would expect overturning. However there may be some sliding. It seems as if the wall does not have grid it would be failing and definitely causing a problem. If it does have grid it should be performing adequately unless the grid is installed in the wrong direction (see this frequently).

RW pic attached.

EIT
www.HowToEngineer.com

 

[RFreund]

I should note that the new piers will not be at this location. The will be infront of the wall and on the slope behind the house (see above attachments).

EIT

http://www.HowToEngineer.com

 

[RFreund]

Soils pic.

EIT
www.HowToEngineer.com