As already mentioned, it's perfectly doable. Soil layer, and even sand, does not slump, and is immutable (without actually triggering cave-ins by dis-attaching layers) to degrees an Earthly engineer would already have been hospitalised with heart palpitations, just getting to that stage.
A rock-hewn fortress, disconnected from most of the sides and underneath and attached to land by merely a length of natural-sand floor (not even a tile thick, just what remains if you don't channel between layers) is viable. Or build a floor of ice (if you have it in winter) and it'll survive the summer, though natural and uncut ice does not.
Back before the current "three cavern-layers, magma sea and semi-molten rock" era, I dug out a desert down to the then natural flat bottom of the world (almost, I left a couple of layers, IIRC, for access tunnels and room for water-features that I never got around to filling) leaving behind 'natural rock' pyramids and obelisks, within a large area sunk into the embark, with vertical sides (which I found needed walls built atop of, because enemies that arrived had 'overlook' advantage).
These days, I tend to dig wide deep ditches down almost breaking into the uppermost caverns, as part of my enclosing fortress design, leaving strategic pillars to act as bridge-supports that let me govern access to my 'internal plateau(s)' from the undiggable edge.
There are basically two rules I use for channelling out like this:
1) Clear trees (and harvestable plants) beforehand, channelling away that top layer in strips once that is accomplished for any given area)
2) From then on, switch to doing ramp-designations on the layer-below, everywhere I know that I'm now not undermining anything, and complete a whole layer (or a defined sector) before designating its next ramps on the layer below.
(Though, added to the second, do leave a ramp, per layer, for access/egress. For each subsequent layer from the second downwards do not ramp out at least one tile, at the edge, which supports a ramp out, and which gives access to the layer-above's designated exit. When you're finished and perhaps then have designed in an alternate exit* from the bottom (which you might or might not also want to mop up the spare, but mostly useless, ramps), or are calously happy to leave a sacrificial miner trapped, you can then chip away at your access ramps from the top down to leave the originally-intended verticality.
But you can develop your own style through practice, and discover some of the (sometimes literal!) pitfalls you can encounter.
* - I favour leaving an 'island' of one undug tile on the lowest layer, individual ramp(s) left to access its top, placed such that a lowered drawbridge out of a tunnel in the excavation wall at the given Z+1 gives controlled passage when lowered but keeps things sealed when raised. You can pre-plan this such that it is accessible at all times before losing the connection out of the top, but I rarely go quite that far.