Everyone should expect some settling of the soil after digging and back filling a hole in the ground. This is no different when a new septic system is installed. Here are some pictures of settling that occurred after a new aerobic septic system was installed. The trick here is to fix the settling as soon as possible so that the sod can be rolled back without harming it to much. If you wait to long then you can only put soil in on top of the sod and wait for the grass to grow over it. This fix was done within one week of the install and the roots of the sod had only grown about 1.5 inches. Also the ground was kept wet by the owner so that the sod wouldn’t dry out which helped in the peeling back of the sod to add the sand.
Now the sod was peeled back and sand was added underneath. Sand was used because when it is dry it will sift down and fill the crevice which was left by the settling.
Here is a final photo after 1 cubic yard of sand was added and the sod was watered in again.
Some additional settling may occur but it looks much better and folks should not trip and fall in the crevices.