Concrete will keep it from settling.
Here's a trick Loren Smart taught me regarding sheeted tubs and Alleys. Loren is a pretty big Select sires guy out of Wallsburg Utah.
Out of the tub, put in about 4-5 foot of open rail to start the alley. I usually design in a cutout gate that's 1/2 sheeted so you can go to a loading ramp or calf table for instance.
If the tub is sheeted and so is the alley, cattle often can't see the contrast to find the alley or escape. Having the open rail lets them see daylight and they will go to check it our. That one trick will really improve the flow of cattle from the tub to the Alley.
BTW, that tub and alley you bought... I've seen a lot of them that have lasted a long while. Good buy!