If the roots surface a few years after you planting them... it is your fault
The depth of the hybrid poplar roots will depend mostly on you. The trees require oxygen, water, a way to get rid of that water, and a way to place the roots deep. These trees are capable to make roots as deep as 60 ft if the soil is soft enough, but If the soil is compacted, the roots will not be able to go too deep and will end up being very shallow. A tree planted in an urban area that has had traffic for many years will be about 3 ft. deep while a tree planted in the wilderness where there is very little traffic, will have much deeper roots due to the fact that the soil is not compacted.|
if you are interested in growing a tree that will last many years, and will have deep roots then you have to make sure the soil is permeable as deep as you can possibly make it.
Phytoremediation agencies have very long bits, they will drill sometimes as deep as 12' and plant 13-14ft. cuttings. You know those trees will have a very long life and the roots will be at least 15-16ft. deep.
I am not asking you to drill that deep, but if you can drill as deep as 6 ft. fill the hole with good fluffy soil and then plant a tree on top of that, your tree will have the chance to put roots at least 6 ft. deep, probably another 2 or 3 ft. deeper, the water will be able to drain quickly and the tree will not only be healthy but it will help by holding the soil in place and help prevent mud slides.