What are the expectations of PhD program specifically for a 'sorta-soft-science' (PhD Urban Planning)

I'm interested in returning to study, and have a Masters in Urban Planning (yes, from an accredited institution).

After working 7 years as an Urban Planner in the Middle East (and 2 in USA), I'd like to return to and study Urban Economics, specifically how to strengthen economic relationships and partnerships between Arab-Islamic cities and American cities. There's a glaring gap in the understanding of the mechanics and relationships - human & economic.

But it's not really a hard science or humanities or economics subject alone: thus 'sort of soft' science. What in general will the expectations (deliverables) be from this program for this type of field? Apologies if it varies too highly between fields & institutions, rendering this too broad.