We frequently fill weird holes with gravel. It's your call. I will say this, and I know this thread has some age to it, so it may be too late: MAKE SURE THE PROSPECTIVE KNOWS HOW YOU PLAN TO TACKLE THIS. This will make them feel more comfortable with you and will help you win the contract. When we do things like this (fill stump holes, post holes, etc) I state it in the proposal. Then I go one step further and I will put the sentence in blue or red font so it stands out. Little details make all the difference. .