If you have not had a good soaking rain in the past couple of days, I would not recommend using poison. It is also dangerous to use the bombs near tree roots. For if the root becomes inflammed then it could burn for days before finally erupting into a fire above ground. This is yet another reason for wanting a good soaking rain before igniting the bombs. If poison gas is appropriate for your situation, you must make sure you have found all the entrances to the dens.
Woodchucks can dig up to three entrances to one den.
The holes can be 20 feet or more apart. Don't be surprised if you only find one hole. Often younger chucks won't dig more than one. But understand that one hole will be easier to find then the second. The reason for this lies in the way chucks dig their dens.