This is common when heavy clumpy soils are used to fill holes. When it is dry, the clods hold together with air spaaces between them. When they get wet, they sort of melt and fill the voids. Once the voids are filled, it won't settle any more.

I would suggest that you make someone aware of it in writing. Ask them to agree to fix the landscaping in the spring after the thaw. By then, it should be done settling if the circumstance that I described was the cause.

I had a similar problem with a client of mine and it turned out in their case that they hadn't blown out the sprinkler lines the previous Winter and ended up with splits in the underground poly pipe. Each time the system went on it washed away the soil around the split and started causing what looked like sinkholes.
Is it possible either the lines werent blown out or someone sliced through them?
Good luck,
marg

From looking at your photos it appears you are situated in a new subdivision likely involving previously undisturbed ground. This would rule out major sinkholes caused by subsurface disturbance as a cause. The gas line backfill issue would be a probable cause but the problem could also be from settlement of the backfill material within the other utility trenches across your lawn. Either of these causes would be the responsibilty of your General Contractor. This work should have all been properly completed by the Contractor prior to having the landscaping done.

It is difficult to compact clay in a trench after the trench has been filled right to the top and with the Contractor returning to the site and only "touching it up" served no purpose. The backfill material should have been excavated from the trench areas and replaced in approximately 12 inch lifts with each lift compacted with a mechanical compactor mounted on the excavation equipment or with hand operated machines such as a "jumping Jack". Utility trenches that are backfilled with loose clay material and then left alone will eventually become stable after several years when the clay backfill is compacted by its own weight & frost action and more importantly rain water seeping into the trench to assist the settling action.

At this time you should put the Contractor on notice that the work has not been completed in a satisfactory manner and you are holding him liable for the required repairs to be performed. This notice should be done through your lawyer if you expect to have any success.The repairs would be best done in the spring. I notice that a sidewalk was also poured across your yard and this could possibly also be susceptable to settle as a result of improper trench compaction but this will not show up for a couple of years.

Here is an update on this yard sinking situation.

The ground is now entirely covered with 8" of snow, so it's hard to tell what it looks like now. However just before the snow it did settle a little more and you can now clearly see a "line" where a "trench" was dug. I've been trying to find my plot plan to find out what it is that is dug underneath there but I suspect it could be city sewer.

Here is an updated pictures:
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I had the gas company out, and they tested the soil with a deep probe and said there is no gas leak. So that part is a relief. I haven't smelled gas out there for a long time.

I also had the landscaper out, who said as expected it's not their problem because I had my sprinkler lines blown out in October, and if it was a sprinkler line leak it would have shown itself earlier while I was running the sprinklers nearly every day for the last 8 months. He said when they installed the sprinklers they didn't dig any trenches as they have a special tool that just digs a tiny hole for the line and they pull it through the hole.

So he provided me with a letter to take to the builder to tell them that this is a problem with the grade from improper compaction in the soil on their part, and the builder should fix it.

I have notified the builder in writing, and not heard back from them. It does say right in our contract that the builder is not responsible for any "grade erosion" issues" after closing, so I think the only course I may have here is because I wrote the builder a letter before we closed on the home a year ago saying there are problems with the grade settling, so that's my only evidence that this has been ongoing for a long time since before we closed.

Thanks all for the comments and ideas.