Footer at edge of slope 1

I need to pour a footer that is close to the edge (2 feet) of a moderate slope. The footer will be supporting an existing two story addition.

Can someone suggest a design for the footer so that it will be anchored into the ground and what type of design will prevent future slippage as the soil may erode over the years.

 

[ishvaaag]

As you say the slope is moderate, you may choose to excavate the slope and retire the risky slope of your building. Then the slope may some day locally fail, but the fall-out won't affect your building.

Normally, a slope stability analyiss is required, and some retaining wall may also turn be convenient.

 

[72shortbox]

assuming a shallow slope and light load I would just embed your footing deeper and keep good vegitation on the slope

 

[ishvaaag]

In my previous answer I was assuming the slope was behind and above your building: the answer still could apply to downwards slopes but was not in the intent. By going deeper unto the soil some superficial loss won't affect as much your building.

If your addition comes close to a downwards going slope, you may use some centering beam solution to the foundation of the loads coming close to the edge. Such loads will be passed in cantilever to more inwards footings, and balanced by loads at the other end of the centering beam by part of the building weight placed there. This way you put your foundation load farther frome the edge, and make your building less prone to suffer damaging distortion if the slope is partially washed or shows some superficial instability. It might even loss support there and your building stnd on the centering beams' foundation. You could then simply re-fill.

On the other hand, except that the slope is feared to show slippage, as long it is less than 2 horizontal to 1 vertical it shouldn't give any special problem. The only precaution when one feels sure on the good nature of the stability of the slope is to use a reduced allowable pressure for the foundations.

And one can also analyze the stability of the slope, but normally being gentle it shouldn't show any problem except in extremely poor soils. Try to ascertain if the slope has been stable in all the time of which people remembers, and judge also if it is likely flood or slide conditions can occur. If not, the reduced pressure scheme should work well.

Don't forget in such case to add ties orthogonal to such edge, preferably somewhat stiff, imitating the centering beam action.

 

[BigH]

jiml66

If this isn't too late, given that you are not likely a specialist in geotech slopes as you asked the question, it is my suggestion to hire a local experienced professional geotechnical engineer to look at your situation. From your area, he would be well conversant with the normal solutions to such situations. Believe me, it will be a grand well spent. I doubt that formal analyses will be required if you have a good experienced and practical geotech review your condition and your situation is a locally typical one.

 

[ctmtwilliams]

Hey jim166!

I taotlly agree with BigH. The only thing I would add is that in most areas you would need a permit to construct the retaining wall. The design of the retaining wall will most likely need to have the stamp of a registered proffesional for the permit approval. If you were to design the retaining wall yourself, and didn't go through the permitting process you could be incurring a great deal of liability if you sold the property and the retaing wall failed.

 

[drt]

jiml66,

Check with the building/engineering department in your city. The city should have minimum top of slope and bottom of slope setback criteria which you must follow in order to get your permit. Your are going to get a permit, right?

If you are considering errosion, sounds like your at the top of slope. For a conservative guildline, use the criteria in the Uniform Building Code (UBC). Per the UBC the outside edge of the footing at the top of slope must be setback a minimum of one third the slope height. To determine this, sketch your slope in cross section and draw a horizontal line from the bottom outside edge of your footing to the slope face and this must be at least one third the slope height. THE UBC top of slope setback is a maximum of 40 feet. Methods to increase the your setback and meet the code include deepening the exterior footing depth (ie. if the slope is 2:1 or 26 degrees for every additional foot you go down you gain an extra 2 feet of setback), use caissions and a grade beam or modifying the slope. BOttom of slope setbacks (which are not as critical from a geotechnical standpoint) are one half the slope height with a maximum of 15 feet.

I'd agree with BigH. Hire an experienced local geotechnical consultant. They will be in tune which local problems and can you help you avoid them. Calculate their fees as a percentage of your total cost. It's well worth it.

As for reducing the bearing capacity, you'd be better off increasing the seback. Low bearing capacities will not help you if get a surifical instability, or local stability problem or if slope creep occurs.

2 feet is really close. It's even difficult to walk between your proposed extension and the top of slope.

 

[BigH]

With ref to Jiml66 - thanks for the info on the UBC - I haven't worked in US since 1981 and am a bit rusty. It sounds, though that, say you have a 20ft slope then 7 ft would be a bit excessive depending, of course, on the height of the wall. A few years back we were putting retaining walls on steep mountain slopes as part of mountain road reconstruction. I had wanted to use a form of grid reinforced earth wall to form wall and part of roadway to keep toe pressures low (it would tend to be more or less uniform) but this wasn't permitted by BOQ (and you can only guess how hard it is to "add" something new to a set in concrete contract). We ended up doing our walls with about 1.5m to 2m setback from edge of slope. Clincher was, though, that the slope extended down for something like another 200m or more! In such situations, try Meyerhoff's approach to bearing capacities on edge of slopes.

Definitely need to protect toe against erosion.

Best regards to all.

 

[Focht3]

Another good thread with lots of great recommendations!

jim166:
The advice to hire a local geotechnical engineer is the best advice we can give you. Follow it without fail. One statement in your original message concerns me:

The footer will be supporting an existing two story addition.

It sounds to me like you have already experienced some problems with the addition. Now is not the time to "go cheap" and try to fix it yourself by following our free advice. Hire a professional.