-if you are crossing a roadway then you have the depth pretty much set up, +/- 1.5' that you can pick if you used a V.C, If the pipe has less than 3' of cover, encase the pipe in concrete class"A",
Will you be able to establish a ridge in the middle and convey the runoff by gutters on the sides? That way you can shorten the flow path and consequently the depth, it also prolong the pavement lifetime.
If the soil is Predominantly sandy, the situation will most likely not get any worse, and the settlement will not increase, the question is how far the columns are leaning out?