After the old buildings were removed and all the holes filled in, the workers came back and made new holes in the ground called trenches .
Some trenches are where the water pipes go.
They will be underground after the dirt is put back in. Other trenches are dug so that steel bars for reinforcing can be put in before the cement is poured. After the cement dries it becomes hard concrete . This will be part of the foundation that holds up the building, and the walls, and makes the floor. Sidewalks are concrete and are also made of cement.