A few years ago the large tree in our parking strip had to be removed.  When the stump and roots were ground out, the guy hit the sewer cleanout pipe and took a big chip out of it, which if not fixed would allow soil into the pipe and ultimately clog it.

Here's a picture of a much newer version of the type of vent cap the cleanout had originally, but the 1927-vintage one had a larger bronze mushroom cap.  My dad replaced it with a flat insert for easier lawn mowing, but that was wrecked by the stump grinder too.

My solution was to place a rubber sleeve around the pipe to cover the chipped area.  I found a black PVC sleeve with a threaded end, which would slip inside the rubber sleeve, and a threaded cap to screw into it.

That repair lasted for a while, but the gardeners hit it with their mower more than once, and when they apparently destroyed the plastic sleeve, they got rid of the evidence, placing the threaded cap over the rubber section, leaving the cleanout vulnerable to foreign matter again.

A picture of the chipped section of the pipe:

Through a mixture of components, I came up with a better fix than the original, which allows for overflow to escape.

The protective surround is meant to be used with irrigation valves, but was perfect for this application.

The cover has a hole in it for overflow to escape.  The size of the surround isn't compatible with a standard sewer cleanout cover, so this was my best option.

Finished product.  Flush with the lawn surface and safely out of harm's way where the gardeners are concerned.  I hope this repair will last longer than the first.