The basic plan is to create a system that evenly supports the fairly rigid foundation. Things like organic materials and expansive clays usually don't figure into this idea; the organics change volume over time due to decay and expansive clays change volume when water is added or removed. Natural state materials are not consistent nor exist in suitable compaction to handle the building weight without the potential for deleterious differential settlement that can not be withstood by the reinforced concrete.
Guidelines:
Remove all organics from area.
Properly Plan adjacent area to preclude ponding (ie a driveway can create a shallow surface drainage problem, other forgotten items such as roof downspouts brought to a pipe to take rainwater away from building area etc.
Determine soil type either from copious experience working in an area, excavating a 'slice', or drilling or boring (geo tech's 'industry standard') and then having soil lab classify soil and do some tests. A compaction curve is an example of a typical test. This curve provides information about percent moisture and is vital to the calibration of the density meter utilized by the soils technician.
In this locale:
We have sandy soils, they are inert which means they dont change volume appreciably when water comes and goes.
We also have some clays: some are quite expansive and others not so horrid.
The overexcavation process mentioned in my previous post basically takes out all the soil in the building area plus five or so feet greater in plan view, to a depth which is about five feet below the bottom of the foundation....we don't have deep foundations for frost so we dig a hole around six and a half feet. The soil either gets reused, or it gets taken away.
Whatever goes back into the hole is pre-conditioned to the percent moisture on the curve, then compacted in even lifts.
The compaction machine varies as to the soil type: in sandy soil, a rubber tired loader or a scraper will get you there (proper moisture in soil as stated). You can use a vibrating pad drum which I do mostly because I have one. I like that machine with a push blade but some don't want the mass on the drum ... but the compactor is faster than the fill process so I can use the blade to strike off and make adjustments to the lift. It isnt really a fuel hog and the operator needs a patient hand and a good memory.
Same machine is necessary to my fleet so to speak as rubber tired wont nearly as well if a medium to low expansive clay is involved.
Another contrivance is pad or sheep foot wheel on a hoe arm ...it can work in either of these soil situations which is a consideration if vibration is a no no such as for an addition to an existing building.
Although a lot of things are done to varying degrees of success being defined differently by many, the idea of having varying thickness of compacted fill would never be accepted by any geo firm. Having said that, I have seen where certain geo tech firms have allowed expansive clays to be removed, moisture conditioned, and placed back and long term detrimental changes began manifesting well before completion of the building. I have also seen where owners were adamant no money would be spent for complete soil removal and put back and those jobs were done in bad clay - but those jobs managed to pull it off by changing the grade to avoid ponding & provide good drainage and they put the entire concrete system (foundation & slab) entirely in cut. Nervous is an understatement and guaranteed they kept the rebar in the slab.
That could be done with geo tech approval a few years ago - these days they are more likely to be conservative.
Another overall plan that can be investigated is the trench and slurry pre-footing. This involves some astute manipulation of the foundation .... after the scheme for final grade & drainage is figured, the organics are removed. Stepping is done but sometimes not if the slope is minimal. The layout for the footings is done and the material in the footing area only is removed well into sound material, sometimes this can be a few feet. A 3 sack slurry is placed in the trench (like next day so no water or dry out or fall in ditch etc etc) up to the bottom of the concrete foundation. The foundation is figured with extra rebar to handle side thrust from the surrounding soil, etc. The foundation is completely accomplished and there is no slab yet (hope that was getting across). Then the inside of the foundation is filled and the slab doweled to the foundation. These jobs portray the slab as non structural (which most standard 4, 5, and 6 inch slabs with center reinforcement are, anyway, but perhaps these words help when the cracks come).
Any method involving clay would be well advised to additionally consider the effects of underground water migration and drains are a good suggestion. Bench and trench drains can be installed without using pipe and gravel and filter fabrics, but they obviously need daylight so gravity keeps the water out. Example: last clay job two years ago involved a custom home with raised floor and a attached garage slab on grade ... site was expansive clay. Pulled it out with a 20 or 30 ton excavator to minus 6 ish feet. No inadvertent over excavating with a clean cut tidy bottom, same as the slurry in the trench method. Put back a medium expansive clay with some shale rocks using pre moistened materials to the exact number on the curve. Geo didnt call for it, but when asked did allow for it (ie you bet you can!) but a drain was added to the bottom of the hole and taken a good 150 feet to daylight. During construction, rains came as well as any number of times thereafter. The water runs out of the daylighted pipe not right away but some time thereafter>they said they feel a lot better keeping that water from saturating that hole. If we cant get a gravity drain, we put in a drywell sleeve for inspection and subsequent pumping if deemed necessary. Water movement under the surface can be a pistol.
Sorry to be lengthy, but I have found this subject to be quite a study and since I do concrete and the dirt work (in a different economy I should add), I am sometimes able to accomplish one trick or technique for or with another, and address the owners desires. Some folks aren't too upset about a couple cracks in the slab, while others want polished granite.
All things being considered since we have concrete with some of the shrinkiest aggregate in the country, I have much better results with reducing cracks in concrete by skipping monolithic concrete on all but the smaller slabs with the smaller foundations. Red iron buildings (system or prengineered metal buildings) have point loads and thrusts and sometimes are sold with wind poles which transfer moments .... regardless of this, the big foundation "has its way" with the slab so we generally suggest separate placements of foundation and slabs.
These are my experiences for this area ONLY, please continue your investigation into what method or combination works well in your locale considering cost schedule and expected results.
