Foundation questions

[skidoo]

My contractor wanted me to get about three feet below the existing basement floor into "a firm virgin soil base". Well, finally, some good weather and I get the basic floor at about the right level, then I go back to dig the space 3ft deep for the concrete forms. About immediately the soil changes to a very fine sand! Looking at my foundation, it looks like there is not much of a footer and it is sitting on a base of sand as well.

Now what? How does a sand soil base change the approach to a footing and retaining wall?

 

[ScottK]

Sand drains well so it's not bad. Just difficult to work with (ie collapsing in before you're ready to pour). I just discovered that my footings were apparently back-filled with sand. Not a problem, but I've got to be careful not to dig too deep too close as I don't want sand coming out from under any footings.

 

[CurlyDave]

Have you probed to see how thick the sand layer is?

I would do this by getting a piece of 1/4" steel tubing (brake line--auto parts store) and hook one end up to my biggest air compressor. See how deep I can push it with the air flowing.

 

[skidoo]

Haven't done that. I dug only about 2 ft down across the back wall. Seems the same all the way across. I checked the well report for a well about 150 ft away. It says in the first 81 ft the soil was sand and gravel. So, what then if it is all sand for at least the next 10 feet down?

 

[rimshot]

Haven't done that. I dug only about 2 ft down across the back wall. Seems the same all the way across. I checked the well report for a well about 150 ft away. It says in the first 81 ft the soil was sand and gravel. So, what then if it is all sand for at least the next 10 feet down?

============

The sand can be a good thing as usuallly any moisture can move through it. In addition to this your report suggests the basement is dry as is the excavated soil. If conditions are right you can dispense with any need for tile or sump. The thing to watch with dry sand is to not let it come out from beneath the existing footing because this is a bearing surface. Sometimes when tying into existing walls under these circumstances it is a good idea to start a bit higher with the level of the added footing and step the footing down provided that can work with the floor grade or elevation.

rimshot

 

[skidoo]

Makes sense. The basement has never been damp. There is no sump, etc... So maybe it can be a good thing. I did stop the digging there until I can consult with the concrete contractor... I'm just a little concerned with how to get the new retaining wall stable because the sand is so loose.

 

[CurlyDave]

I agree that you have to be certain the sand does not come out from under the existing footing.

This is important enough that I would pay a soils engineer to come out and give an opinion. I think you can still build on it, but you need good advice.

This situation has happened before. In fact, crack open your Bible and read Matthew 7:24-27. I don't think your result will be quite that bad, but you need an engineer.

 

[Wis Bang]

I have seen monolithic slabs for a tractor trailer shop building placed over pilings along the NJ coast. They sclupt the sand and used a plate compactor to fill the areas between the rows of pilings so the edges & over the pilings was 2.5' thick. while the in between areas it ws only a foot. don't let the sand come out from your existing footer.

When they drove the piles the first two of three in each spot dispaeared w/ one hit of the driver so the whole building stands on the piles.

 

[skidoo]

Been working at it... There sure was a lot of dirt that came out of this project!! I was really impressed with the 110TLB. It was the perfect size so far. I used every inch of the extra reach over a smaller green BH. If the tractor was any bigger, it would not have fit in the area I had to dig and maneuver in. If it was any smaller, it would not have been able to lift the concrete forms off the trailer. It took every bit of lift to get them off, and then it felt tippy forward. I was real careful...

Anyway, the footer areas compacted well and had them poured. The contractor had me dig below the existing foundation where new walls and footers will tie in. Sand did not fall out. I guess there is still some combination of sandy dirt that compacts well. The wall forms are almost complete.