What business can I do with it under the viaduct?

First of all, it is impossible to do business under the viaduct. The whole viaduct, including the open space below, belongs to the carriageway, so the government will not use it through your land acquisition.

But I suddenly felt that I was not allowed to do business, but I didn't say that I was not allowed to advertise. You can build several advertising signs and sell advertising space in that place, but you still have to contact the government.

Good luck.