slab-on grade over fill

[mjr6550]

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.

 

[msucog]

with only 4 feet of old fill, hire a geotech to perform some hand auger boring and/or density test in test pits. acknowledge that their results are at specific locations (i.e. there's still some risks involved with leaving the old fill in place) and move on. if they happen to find "something", remove it or fix it and move on.

for something already in place (with similar conditions as mentioned above), look at extending the foundations to firm, residual soils.

 

[fattdad]

Proofroll the site and that'll get it prepared to either receive fill or prepared for slab-on-grade construction (i.e., if you repair any areas that fail proofrolling). For foundation construction, I'd likely undercut the fill from below the bearing grade and restore the bearing grade using #57 open-graded aggregate.

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!

 

[mjr6550]

The boring logs show silt/fine-medium sand/misc. rock/brick/concrete/asphalt/some clay. N values averaged about 10. When I checked the site with a rod, I was able to push the rod 3-4 feet into the ground with little effort at several areas. Most areas it appears that I hit large debris and could not get far with the rod. If this site is similar to others I have seen in the area, there will probably be quite a bit of masonry debris mixed in with the soil, but not much organic stuff. I'll know more when excavation is performed, but I would like to have an ideal about what would probably work best from a cost and ease of installation standpoint before excavation begins.

 

[cvg]

I'm a bit confused. If your contractor can construct foundation walls, piers and reinforced concrete slabs - why is he unable to haul away the debris, haul in good fill and compact it? If he can't handle earthwork, have a sub do it.

 

[GeoPaveTraffic]

Given the relatively small size of the building, I would remove all of the fill from beneath and at least 3 feet beyond the sides of the structure. If the existing fill is free of organics and trash, then have the contractor compact it back into place. If the existing fill is trashy, haul it off and bring in clean soil or crushed rock. Flowable fill would work, but will be expensive; however, flowable fill will be much less expensive than a structural floor system and deep foundation.

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.

 

[hebrone]

Fill, specially with debris like on your ite, can be tricky.
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!

 

[fattdad]

Considering the presence of voids in the spaces between large diameter debris, I'd think some measure of site improvement is required for the entire building pad. Complete remove and replace would seemingly be the best option as the work effort and cost would be defined. You could also consider the complete removal of all soils below the bearing grade (or the use of rammed aggregate piers) and the possible construction of a soil raft, which would include the partial removal of fill below the slab-on-grade area, the use of a geogrid and the reconstruction using fist-sized aggregate, 57-sized aggregate, 21A-sized (i.e., dense-graded) aggregate and then soil fill (or just completing the pad with the 21A material).

Just some further thought. . .

f-d



¡papá gordo ain’t no madre flaca!

 

[mjr6550]

Thanks for the replies. cvg, The contractor did a reinforced concrete slab for another job built with grade beams and foundation walls on helical piers. They were not experienced at this, but I was able to inspect the rebar installation before the pour, so I had confidence in the results. Here, I assume they will have to hire an inspection firm and have then on-site to do Nuclear Density Testing. I assume they would need to be on site about two day? I have not looked into the cost for this. I though about rammed aggregate piers, but for small jobs I do not know if the there are any contractors around who would do this work, especially for a reasonable price.

 

[lovethecold]

If you could push a rebar 3-4 feet with little effort, I would be concerned about voids and soft fill. The N values could be falsly elevated if the sampler was pushing against some debris. I would go with others and recommend complete removal and replacement. If the contractor is willing to learn, then a good geotech firm should be able to help him obtain the needed compaction. An experienced tech should know how to obtain the desired results within the specifications. An option to make obtaining proper compaction easier would be to use clean sand. Most contractors around here tend to use clean sand for that reason. You had soil borings done, did you also get a complete soil report? This should be addressed within a full soil report.

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.

 

[mjr6550]

lovethecold, Thanks. We did get a soils report. The recommendation was to excavate and replace with granular fill. It seems like thats the way to go. I talked to a testing firm. They quoted $500/day for nuclear density testing (plus $250 for the Proctor) and thought it would take about two days. I think sand may be a good idea. Should be less of a concern maintaining optimal moisture content.

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.