Backfill for Residential Construction

[instadog]

I am building a one story residential 20'x20' addition in Northern Virginia. The soil is very poor, very expansive clay. The permit office is requiring a 4' footing depth in lieu of the 2' typical in these areas. My footing will be poured reinforce concrete (8"x16&quot and I will have a reinforced 48" OC #4 bar CMU foundation wall. I will place a bond beam on the second course from the top. My questions:

1. Should I provide a second bond beam, even though the wall is only 6' high (2' above grade--crawl space beneath structure, 4'-footing depth of 8" below grade)? What would be additional reinforcing recommended? The CABO does not require any reinforcing at all, the steel is overdesign on my part...

2. Can I use the excavated soil/clay for backfill without any problems? Any suggestions on use of an alterate or tips for backfilling?

Keep in mind this is residential, not commercial. Thanks.

 

[Sean2]

Instadog,
I wonder why you have not had any replies until now?

I found your question a little difficult to understand, and this may be partly because we work in different countries, and your terminology is not all familiar to me.

I find myself particularly unsure of your construction and the proposed construction sequence. I didn't follow your description.

So, you are working in expansive clay. This may explain the reluctance of members of this forum to provide a definitive reply.

You must (as I'm sure you are aware) found your structure at a depth beyond any possible effects of expansion. You must also ensure that your development does not cause ANY expansion of the clays, either during or after construction. You also need to ensure that your design allows for the unavoidable expansion which will occur, hence the space under the floor. So far, I think you have not learned anything new from me.

Where do you plan to backfill using the excavated expansive clay? Have you tested the clay? You are right to ask if it can be used, and if it CAN be used, then you probably need to implement certain measures to allow it to be used. Some detail here would be useful, if you have not already had an answer.

Perhaps the absence of concrete advice in this reply will encourage another "eng-tipper" to reply more substantially!

 

[boo1]

I don't see the need for the additional bond beam. The first four feet of fill is balanced and last two is above grade.

I think the reason the building department is requesting the extra two feet stem wall is to normalize the clay movement. So the soils can be reused for back fill.

 

[Focht3]

Be careful here - the on-site clays can exert high lateral earth pressures on your buried wall. This problem becomes more pronounced as the clay is compacted more, and is drier when placed. How much lateral pressure can your wall take?

The most suitable backfill would be a lean clay - PI in the range of 7 to 20, at least 40% passing the No. 200 sieve. It has a low swell potential, but does not pass water readily.

 

[instadog]

Gentlemen, thanks for the comments. Since last posted, about 10 cubic yards of concrete was poured into a 20" wide, 48" deep trench (footing height of about 30&quot reinforced with a ladder consisting of #4 bars about 3" off the bottom. A visit by a soils engineer was required by the county and reported a 2500 psf capacity throughout most of the hole, except up against the house. The trench depth hole went past the layer of marine clay onto some cinder looking material that appeared quite hard. The soil up against the house, though, I had concerns about. The engineer pulled out an NIST traced #4 rod and shoved it into this area with his body weight, getting only about 3/4" deep. This passed his test of suitability. He stated he would be concerned if the rod went in 3 or 4 inches. My question is, about how much psf is this soil capability of supporting? I have a 24"x24"x36" pier sitting in this area with about a 4,000 design load. Should I be concerned?

 

[Focht3]

I like the rebar probe method. It's actually a good technique - when the engineer has sufficient experience to know what he's doing, that is.

If you have 4,000 pounds acting on a short column that hase a 2' x 2' base, then it's exerting a pressure of about 1,000 pounds per square foot. This is not a big pressure at a depth of 4 feet. While your description is vague, it doesn't seem like you have a problem.

 

[boo1]

Check link

http://www.co.san-diego.ca.us/cnty/cntydepts/landuse/planning/BuildingForms/dplu065.pdf