Sinkholes are usually filled in first using some boulders, and then small rocks and then after that they usually fill the sinkholes in with gravel and sometimes concrete will be poured into the sinkhole after that.
So when you have a sinkhole filled in they usually fill the sinkhole with a layer of boulders, then a layer of smaller rocks, and, finally, a layer of gravel.
Then after that if it's still needed they may pour concrete into the hole to help fill it in.
The way that most engineers fill in the sinkholes use the boulders, rocks and gravel which is known as the graded-filter technique.
The graded filter technique is the most common and best way of filling in most sinkholes.
Having a professional fill in sinkholes can cost around $4,000.00 to $10,000.00 and sometimes more depending on how deep the sinkhole is.