Doing a PhD in data science / machine learning after a PhD in civil engineering
I will agree with the commentators who say that one PhD is enough. I am sure you can start applying ideas from machine learning in your field (civil engineering) and perhaps after a while even move to a completely different field. PhD is essentially a qualification that demonstrates your ability to engage in abstract learning and research. You don't need two of these degrees to doubly-certify yourself. If you feel that you are lacking in certain aspects of knowledge related to machine learning or data science, find collaborators in these fields -- that's way more efficient than taking classes (and more productive in terms of research output).