What considerations should I make when demolishing a driveway?

Question

I have a really long concrete driveway, and we'd like to turn the back half of it into part of the backyard. From this:

To this:

I'm hiring a demolition contractor for this task, but I'd like to ask here first to be well-informed about potential issues. The only 2 I can think of are:

1. We have a sewer line coming out of the side of the house and out to the street under the driveway concrete. You can see 2 circular entry ports leading to it (does this mean 2 pipes?). Is it safe to remove the concrete and keep this sewer pipe buried under the grass? Or would I need to add some sort of protective layer in between? Can I put down a lawn on top of this pipe?

2. We have a retaining wall with the neighboring driveway that's leaning out a little bit. Does it need the driveway concrete to stay upright? Is it likely to collapse if the concrete is removed and replaced with grass? It's very short, from 6" to about 18" towards the back.

3. Are there any other issues or concerns I should take into consideration before hiring a contractor to demolish the driveway? Foundation damage, gas lines, etc.? This is my first big project, and I don't want to rush into this without asking around for things to consider that I haven't thought of. This property is in Portland OR, and the house was built in 1918. Are there any city regulations, permits, laws to worry about?

Answer 1

1. The sewer and storm lines are usually deeper than few feet from the grade. If they are buried too shallow, you might need to drop them lower to receive adequate topsoil, and place thermal protection over them to act as a protection for frost. Unless you intend to place heavy items directly over the buried drain lines, no special protection is required.

2. Similar to the above, for a properly constructed earth retaining structure, the bottom of the footing should be placed at or below the local frost line. Also, no retaining structure shall rely on the toe side soil to maintain stability. If you are the owner of the retaining wall, you can do whatever you wish. If not, don't touch it, and use hand excavation near the wall instead of using a machine. Be prepared, you might need to stabilize the wall before putting the soil back. However, I doubt you'll have such need because your excavation shouldn't be more than 2'-3' deep (the thickness of the slab included).

My suggestion is to dig a few exploration holes to verify the depth of the drain lines and the retaining wall foundation. Also, most importantly, check with the local utilities to make sure there is no utility lines buried below, or ask them to mark the locations if there are such installations.