Garbage used to fill up the costline and create new land

Large cities like mine (Buenos Aires) have two problems: an excess of garbage, and a shortage of space.
The solution would be to use the garbage to earn land space from the sea. (or river, in this case). I do not suggest that the garbage is thrown like it is. It should be compacted and covered with cement, to construct large blocks. Those blocks can be used to create new space within the water, useful for parks, airports or commercial building.
What is wrong with the idea? For now, it is only False News.