slab-on grade over fill
slab-on grade over fill
(OP)
I have a client who is building a one-story dwelling on a site in Philadelphia. Had soil borings performed and soil is generally good, except for about 2 to 4 feet of fill. There had been a structure on the site that was demolished. The 4 feet of fill is in that area. I really will not know the exact extent of fill until we start excavating. The dwelling must be constructed on a slab-on-grade foundation (33x50).
I am looking for some input for options to deal with the fill. Excavate and replace with compacted granular fill; excavate and replace with flowable fill; one-way reinforced slab bearing on intermediate foundation walls; two-way slab on concrete piers; something else? I am planning on extending the foundation walls down to undisturbed soil, so this should not be an issue. The contractor is not experienced at dealing with these issues (no, we can't get another one). I do not have the experience to know which is most cost effective. I'm concerned about the problems the contractor may have with their lack of experience at compacting fill. The city will require testing.
I have a similar project where a homeowner has a family room (13x20) and a garage (20x20) that have settled an excessive amount. There are voids varying from 6 inches to 2 feet below the slab and I can push a 1/2 inch rebar at least 3 feet into the soil (silty clay) with relatively little effort. I'm going to need a similar design to repair this. Any thoughts would be greatly appreciated.
I am looking for some input for options to deal with the fill. Excavate and replace with compacted granular fill; excavate and replace with flowable fill; one-way reinforced slab bearing on intermediate foundation walls; two-way slab on concrete piers; something else? I am planning on extending the foundation walls down to undisturbed soil, so this should not be an issue. The contractor is not experienced at dealing with these issues (no, we can't get another one). I do not have the experience to know which is most cost effective. I'm concerned about the problems the contractor may have with their lack of experience at compacting fill. The city will require testing.
I have a similar project where a homeowner has a family room (13x20) and a garage (20x20) that have settled an excessive amount. There are voids varying from 6 inches to 2 feet below the slab and I can push a 1/2 inch rebar at least 3 feet into the soil (silty clay) with relatively little effort. I'm going to need a similar design to repair this. Any thoughts would be greatly appreciated.





RE: slab-on grade over fill
for something already in place (with similar conditions as mentioned above), look at extending the foundations to firm, residual soils.
RE: slab-on grade over fill
Maybe I should have stated this first: I'd just build my building on what's there if the "fill" is free of organics, debris, coarse-grained (i.e., "SM or better") and has a PI that's less than 30. Just how do the boring logs describe the existing fill?
f-d
¡papá gordo ain't no madre flaca!
RE: slab-on grade over fill
RE: slab-on grade over fill
RE: slab-on grade over fill
Last alternative you might want to consider is a mat foundation. The mat would need to be stiff, i.e. designed to span a void of about a 1/4 of the building size.
RE: slab-on grade over fill
You statd that the borings showed concrete, brick and asphalt, this is not a good sign.
If you client is willing to accept the risk of some movement within the slab, then you might extend your footings to native maerial throug the fill and perform a proofroll on the slab it self. You MUST make it clear to your client that some cosmetic problems might appear within the slab.
other solution would be to actually use a geopier system. Geopiers extend to anywhere between 8 to 15 feet, and they can densify your subgrade allowing you to place your slab!
But the safest bet is to completly remove the fill to 5 feet outside the premeter of the building and place engineered fill back in place.
I hope these suggestions help!
RE: slab-on grade over fill
Just some further thought. . .
f-d
¡papá gordo ain't no madre flaca!
RE: slab-on grade over fill
RE: slab-on grade over fill
As to the second question in your OP, are there any voids below the footings, if not, is it possible that voids may appear below the footings? For now, I am not assuming the footings are on natural soil. How much of the floor slab has been removed, or did you find this out by coring the slab? How old is the home.
A couple years back there was a similar issue with voids below a gymnasium floor. I was a drill helper at the time and would have to look to find out what recommendations were given on that project. It had something to do with a waterline leaking and slowling washed the sand out, creating voids below. The owners found there was something wrong when wooden gym floor started to swell (from the water seeping through cracks from below). The building itself was quite old.
Hope this helps.
RE: slab-on grade over fill
Regarding the second issure, the house is about 30 years old. No foundation settlement so I think the foundation is on undisturbed earth. The owners are original and say that the settlement started as soon as they moved in. They just did not do anything about it. We broke out two small sections of the garage slab that were cracked and took photos from the holes.