Shallow Pier Foundation Rotating Down On Hillside
Shallow Pier Foundation Rotating Down On Hillside
(OP)
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!
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!





RE: Shallow Pier Foundation Rotating Down On Hillside
RE: Shallow Pier Foundation Rotating Down On Hillside
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
www.HowToEngineer.com
RE: Shallow Pier Foundation Rotating Down On Hillside
RE: Shallow Pier Foundation Rotating Down On Hillside
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.
RE: Shallow Pier Foundation Rotating Down On Hillside
www.PeirceEngineering.com
RE: Shallow Pier Foundation Rotating Down On Hillside
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
www.HowToEngineer.com
RE: Shallow Pier Foundation Rotating Down On Hillside
www.PeirceEngineering.com
RE: Shallow Pier Foundation Rotating Down On Hillside
RE: Shallow Pier Foundation Rotating Down On Hillside
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
RE: Shallow Pier Foundation Rotating Down On Hillside
EIT
www.HowToEngineer.com
RE: Shallow Pier Foundation Rotating Down On Hillside
RE: Shallow Pier Foundation Rotating Down On Hillside
We recommended they/we hire a geotech, we'll see what happens.
EIT
www.HowToEngineer.com
RE: Shallow Pier Foundation Rotating Down On Hillside
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
RE: Shallow Pier Foundation Rotating Down On Hillside
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
www.HowToEngineer.com
RE: Shallow Pier Foundation Rotating Down On Hillside
RE: Shallow Pier Foundation Rotating Down On Hillside
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?
RE: Shallow Pier Foundation Rotating Down On Hillside
RW pic attached.
EIT
www.HowToEngineer.com
RE: Shallow Pier Foundation Rotating Down On Hillside
EIT
www.HowToEngineer.com
RE: Shallow Pier Foundation Rotating Down On Hillside
EIT
www.HowToEngineer.com
RE: Shallow Pier Foundation Rotating Down On Hillside
EIT
www.HowToEngineer.com
RE: Shallow Pier Foundation Rotating Down On Hillside
RE: Shallow Pier Foundation Rotating Down On Hillside