(Good filk, there, BTW...)
The way I understand the OP's predicament (and, although I'm probably wrong, I'm going to run with this) is that he's set a column to be designated as channels. Miner(s?) jumped in, channelled their way to the bottom. While that was happening, masons (or other relevant material-handlers) were asked to build stairways in the shaft[1]. They will have been built by the particular artisans from the materials available at the top of the shaft.
At the very bottom, when the miner(s) halted in the downward dig, the designation may suffer from several concurrent problems:
- Materials at the bottom of the shaft (distance of zero) will be designated, only they aren't accessible by the masons that infilled the rest of the shaft.
- If you chose materials that the masons/whatever can access, there'd be the miners in the way,
- Possibly with a proper ground-access, there may be job cancellations when the mason/whatever tries to get to the bottom
To solve the first, I'd try either expand the list and navigate to
specific rocks/whatever that are non-zero distance away, or don't bother expanding but choose the first rock-entry on the list whose first distance is not zero.
In the second instance, the miner needs to dig sideways to give them somewhere else to be so they don't get in the way of the building.
In the third... well, if this applies then I would do what I should have done right from the beginning and set the miner(s) to not only go sideways, but dig more stairwells to either get back to the top or reach up a level or two and then back into the existing stairwells.
And, in the future, don't (d)esignate (c)hannels to be dug, but instead the up/down stairwells. No building is needed, the miners give you the structures you seek.
I
do sometimes designate a column of channels, for an aesthetic form of well-shaft, but at the lowest level that the miner can be reasonably expected to dig sideways from I have him do exactly that, to meet some pre-existing shaft (probably one which originally discovered the water-source I'm looking for), or at least some up-down column that I have pre-designated as their escape route. This is a specialised mining operation (with some more minor details in its execution, not explained) that anyone
can do, but I wouldn't expect anyone to do unless they were after much the same effect as me. If you were wanting to put stairs in (or even have stairs along the entire height, in an adjacent cell), there'd be no need to do this.
But I may not have understood your predicament.
[1] Because, I just found out in another thread, you can actually designate to build stairs in 'open space' below other stairs.