I live in a four-family building of townhomes, less than five years old. The yard in front of the building has been sinking for some time, two or three years. The sinking runs a line from one sewer clean-out valve at the side of the building, down that side, turning a corner under the driveway (which has cracked) into the yard to the next sewer cleanout valve, and out from there. About three feet out from there is a small plum tree that his now sitting in a sinkhole, it's probably 8 to 10" lower than the ground around it. Besides this sinking, there is an area between that and the homes, right in front of a decorative rockbed, which seems to be sinking in the middle, forming almost a saucer-look to it. That part is almost exactly behind the tree and about four or five feet back.
This has been noticed with concern by the lawn people, and most recently two sewer companies who came to fix a backup, because they said it can be indicative of a break in the sewer line. However, they dropped a camera down the lines and found no breaks.
Both my neighbor and myself are getting concerned because it continues to get worse. There are no basements, and when my carpet was pulled up recently, I found a crack in the concrete extending from one end of the unit to the other -- the builder tells me this is not a big deal, all concrete buckles, and as long as the crack is going in one direction and not spidering off, and as long as the crack is less than 1/4" wide and both sides are level, it's nothing to be concerned about. The sidewalk has also formed a horizontal crack, and the slabs are becoming quite uneven.
I realize that normal settling occurs after building, but this seems extreme. Other than a sewer line break which has been ruled out, what could cause ground sinking like this?