Without knowing the properties of the current soil, you can't know how the 3 to 6 feet of fill will behave. Could be fine, could settle a couple of inches, could trigger a landslide that causes problems for another structure.

Mike Lambert

Thanks Mike,

That's what I figured.

Cheers,
Chad

Your instincts are correct. However, why clay? Clay is difficult to compact even in easy access areas. Also, it can change volume, thus surface elevation if moisture content changes with time. I'd prefer a granular soil as the compacted fill if the support is critical. In addition, the cost of the compaction is likely to be higher if clay is used, due to control of moisture for optimum density. So, with the fill work you also should have a geotech engineer involved to be sure that is done correctly.

I cant imagine that a geotechnical analysis and recommendation for the 3 - 6 feet of clay fill would not also include analysis of the subgrade. however, 3 - 6 feet of fill is not much, so unless the subgrade is really lousy material, or your groundwater is high, or you have wet conditions, or you are in a high seismic area, or the slab will be very heavily loaded or small amounts of differential settlement cannot be tolerated or your clay is highly expansive, fat clay than you have no problems...

if you don't know what you are building on, the contractor will tell you during construction and it won't be good news.

f-d

¡papá gordo ain’t no madre flaca!

There could be expansive clay that will swell and heave the slab after it is covered with the fill. There could be collapsing soil that wil get wet and compress after it is covered with a granular fill. There could be deep compressible soils that will compress and cause the slab to settle several inches under the weight of the fill. Or there could be an old basement filled with trash that will cause many inches of localized settlement. Etc.

Here are a couple of approaches:
1. Ask the structural engineer to recommend a geotech. Ask the geotech company to do a tabletop study and site visit and provide recommendations on that basis if they are comfortable doing so, or present a proposal for a full geotechnical investigation and report if not. A senior geotechnical engineer with plenty of local experience and familiarity with the geology and local soils may be able to do this based on geologic maps, USDA soil maps, site inspection and his experience. For a fee, of course. Be sure to tell him what the intended use is and why it is critical.

2. Research the site yourself with geologic maps and the USDA soil maps (if in the USA), whatever you can learn about site history, and a site inspection. The soil maps can be quite helpful with tables of suitability of the soils for various engineering uses. Go to websoilsurvey.sc.egov.usda\....

The problem with #2 is, you have just taken on a lot of liability by working outside your area of expertise, and you probably can't do it any cheaper than the geotechnical engineer.

Either way, do some research to determine what kind of fill to use, compaction specifications, etc. Then have the geotech company provide inspection and testing of the subgrade preparation and fill placement. If these are not done properly, you can make a bad site out of a goood one.

6 feet of clayey soil backfill will provide about 700 psf over overburden load on the existing soil. If the existing soil is also clayey, then you will have a significant potential for additional settlement, just from the overburden....get the geotech involved. I would also question why you would want to fill with clayey soil....not the better choice for the reasons given.

and, in my opinion (oldestguy beat me to it) use granular if at all possible. Don't know your location (do you get winters?) but if depth of frost is important, properly graded cohesionless material will be non frost-susceptible.