Strip the topsoil and organics, cut or fill to grade, moisture-condition the clay, and compact in lifts, typically to a 95 percent Proctor spec on engineered work, then proof roll to find soft spots. Select fill goes in where the design calls for it, with a vapor barrier under conditioned space.
More detail
Base preparation is the part of a slab you never see and the reason slabs fail or last, especially on Blackland Prairie clay. The sequence is fixed. Strip: topsoil, roots, and organics come out of the footprint, because anything that can rot or compress under the slab eventually will. Cut and fill: the pad is shaped to design elevation, and on sloped lots that means engineered fill placed on purpose, not spoil pushed into the low corner. Moisture condition: expansive clay compacts properly only in a narrow moisture window, too dry and it will not densify, too wet and it pumps, so water is added or the soil is dried and worked until it is right. Compact in lifts: fill goes down in 6 to 8 inch layers, each compacted before the next, commonly specified at 95 percent of standard Proctor density on engineered projects, with density tests to prove it. Proof roll: a loaded truck crosses the pad and any soft spot that moves gets fixed before concrete hides it. Where the design calls for it, select fill (imported, controllable material) caps the native clay, and a vapor barrier goes under any slab that will be conditioned living or working space. None of this is visible in the finished floor, which is exactly why it belongs in the written scope by name: strip depth, fill type, lift thickness, compaction spec, and testing.