Fix damaged sewer clean out.

I have two sewer cleanout access pipes that stick up vertically a few inches above the grade in my front yard. They appear to be 6" sdr 35 pipes. When I moved in, both had caps that fit over and around the top of the pipes. One of these recently got whacked by a lawn mower which broke off about a 2" section of half of the top of the pipe and cracked it down a few more inches. A replacement top mostly covers the hole in the pipe's sidewall, but I'd like to do better. (By contrast, the cap for the other pipe looks like it took a glancing blow at some time the past and is firmly stuck to the pipe).

First, I'd like to cut the pipes down to below grade. They are buried in most of my neighbors' yards. So long as I mark where they are (so I can find them when needed), are there any obvious problems with doing this? I saw a blade that will attach to a drill and let me cut the pipe internally. Is that a good method to avoid digging a large horizontal hole in my yard to make room for a saw? (Digging a small vertical hole around the pipe itself seems unavoidable).

Second, I was considering bonding a 'hub to FPT' adapter to the end of the cut pipe and then adding a threaded cover instead of the current slip-on caps. Is this worth doing if the result will be below the dirt? Is there a reason to stick with the current slip-on caps instead?

Any thoughts are appreciated (on the plan, method, tools, etc). This is my first post here and one of my first plumbing related projects, so I am sure there are issues I haven't contemplated yet.

I can provide pictures if it would be helpful. Thanks!

 

It looks to me as if this bushing could go on the SDR 35, and let you convert to the more-rugged schedule 40 PVC to come back up. I am not a pro.

http://www.thedrainagesource.com/6_PVC_DWV_X_6_SDR35_Reducer_Bushing_Sp_x_S_p/p1209.htm