The Art of Siege - Mines - or the Episode

The tunnel progresses using a grid square system - and this should include at least two angles or changes of direction, which makes estimation of the range slightly more difficult.  The defender can of course 'counter mine'....

Mining and sapping is a serious business, and Captain Shandy called in an expert advisor...

The Mine explodes!  Gosh!

A suitable marker gives the effect of the blast...

 with the neighbouring defenders stunned. If the breach has been made, the attacker will attempt to storm the Ravelin from the forward Saps.