clear stone under residential footing

[pringles]

Hi,
I am building a 30' X 40' foot addition. I had to excavate the footing in a step since the land was sloping away from the original house and I had to get rid of the top soil etc. I basically dug out all deposits of clay, top soil and other questionable material until I reaches a hard almost rock like substance that could only be removed with a pick axe. ie: very stable and hard. Here is my question:

I backfilled the stepped excavation with 3/4 clear stone since I didn't want to have granular A delivered on top of clear stone since I have to use the clear stone under the basement slap anyway. I compacted it in 8" lifts with a 400 pound plate compactor and put in about 3-4 feet of clear stone and brought everything back up to level. I started to worry about putting my footing on compacted clear stone becuase a friend I talked to who builds houses says he always uses compacted granular A. The site is well drained and even after heavy rains for days the excavation never has any standing water at all. The addition is one story and the footing is 2 feet wide by 12 inches thick with 1/2 inch rebar in it. Will the clear stone support the load and remain stable? My initial gut feeling was that it is better than ganular A just by the feel of it. It will not compress at all. With granular A there is always the question of whether it has been compacted enough and will it still settle more.

Thanks very much for any comments.
Steve.

 

[JAE]

In my area, there is an abundance of loess soil (silt) where if we place open (uniformly graded) granular material under footings, it will act as a french drain, attract the flowage of moisture and begin to develop sink holes around and under the footing. You normally should not use uniformly graded material with lots of voids as this may create very large problems.

 

[pringles]

When you say it would develope sink holes, do you mean in the clear stone or would the water flow through the clear stone and start eroding the silt ground that the clear stone is sitting on. I don't understand how sink holes could ever form in the clear stone directly under the footing??

 

[cvg]

jae - it sounds like the fine material is being eroded and moving into the voids in the open / uniformly graded gravel. It is not forming sink holes.

you should place a well graded, granular material under footings which will filter the finer subgrade materials.
Granular A would probably be a better choice.

 

[SCGeo]

Did you ensure that the sides of the footing were firm when you over excavated? If weak soils are present along the sides, loading the clear stone could cause it to move laterally.

Another option I have seen that may help with excessive voids is to add a fine sand to the footing to fill the voids created between the stone.

 

[Wasdifferent]

pringles,
You are playing with fire. Get a geotechnical engineer on your job.
Best regards,
-Michael

-Mike

http://www.sewellconstruction.org

 

[pringles]

Hi,
Thanks for all the replies. I actually found by searching on this site where Geotechnical Engineers where discussing using clear stone under footings for bridges and other large stuctures where dranage was an issue. I can't remember where I found it.

I talked with two Geotechnical Engineers, one who works for the government and one that works for a Geotechnical company here in ottawa and they said there was nothing wrong with using clear stone under my footing. The only problem that can arrise are if I have unstabel silty type soils and a high water table. Even then they weren't concerned with the footing itself settling, just the ground around the building if there were adjacent structures. They said the adjacent soil could migrate into the voids and cause settling of soil around the structure which could effect buried pipes or buildings adjacent to the building in question. My soil is very stable. I found a large pile of clear stone when I excavated that has been covered for 20 years and it was still all clear with nothing in the voids. I also asked the engineer at the building code office and they said that there was nothing specifying that it had to be granular A, just as long as it was a granular material which includes the 3/4 clear stone that I used. There seems to be a lot of different opinions on the subject from what I can see. I guess the key is that I don't think there is a hard and fast rule that clear stone should not be used but that just be aware of some of the effects that can occur under certain consditions.

Thanks again,
Steve.