There are a lot of old flood control "watershed lakes" that were installed by the Corps of Engineers in my state. I saw guys up surveying one time when I was out fishing and stopped to chat with them. They said a benchmark was installed on the top of every one of those dams. They drive out every X years to inspect the dams and shoot the level of the bench marks. (I think "X" was 5 years.)
He said if the benchmark has not moved they just move on to the next dam. If the benchmark has moved they do a more thorough visual inspection. He said that in his experience, when the benchmark is going down, they always find a problem.
Your contractor may have messed up on something. On the other hand, he may have done everything exactly right and it still didn't work out properly. Soils can be highly variable beasts.
Mike Otto (one of our dam building experts) has stories in his book of doing everything right but some quirk of the earth caused a dam failure.
Good luck on your project!
