water, water and water again. things may look dry to you but when it rains heavily some streets fill with water in a space of minutes. some areas get constantly flooded, others occasionally. water carries debris and mud so sewages block quickly adding to the disaster.
in addition to that there are underground streams in some areas. you can see it clearly on a road from santiago to POP. underground water "eats up" road surface.
it would be hard to create basement with proper drainage. dominican engineers are not familiar with this concept: when we were still thinking about constructing a house i wanted underground garage - common solution in my country and a great idea to save space if your land is not big. our engineer kept on saying "it cannot be done"
it can but it is expensive.
to illustrate this problem:
my parents in law have a house on the hillside. because of the position (slope) of the house they have half basement used as a garage (supported partially by the hill and partially by pillars). they had two gates on two sides of the property. few years ago we had mega rain that has flooded parts of the city in less than 20 minutes. water was so strong and so filled with debris it broke one of the gates and rushed through the garage down the hill. the stream was so powerful it took down entire section of wall before gushing further down.