I want to get suggestions on this idea I had. I've attached a drawing of my design since it's difficult to express in words. I want to build an L-Shaped planter box large enough to construct an L-shape bench on the inside corner, attached to the planter box. I also want it to be bi-level. I'm thinking the length and width will be about 10-13 feet and the first level 4 feet high and second level 5.5 feet high. See attachment to help visualize.
So I need some tips and advice:
1) I was going to use pressure treated wood. Good idea?
2) Should I line the inside of the planter box since it will be filled with dirt to make it last longer? If so with what?
3) I live in Chicago so I'm worried about the frozen winter. Should I spend the time to dig and build a rock base to ensure the planter doesn't shift around? I would not like to do that since that will take a lot of time. Another alternative I was thinking about was digging and creating a base where the posts are at.
4) Same issue as above. What about the soil within the box. When the ground freezes and expands do I have to worry about the soil bursting out?
Any suggestions on materials, design or construction is appreciated!!