1970 - 1980: Bill Becker's Hanna Strip Mine Blasting Videos
Video and notes from Bill Becker
The following videos illustrate how 'overburden' was blasted off of a coal seam at an open pit mine. All of the open pit mines in the Hanna area from the early 1970's to the early 1980's used this method, or one very similar to it, to blast overburden. The process starts with the drilling of a series of large diameter holes (10-12 inch) on a 20 foot triangular pattern. The holes are drilled to the top of the coal seam or in lifts, if the coal was deeper than 100'. Once the holes are all drilled the powder crew comes in and loads the holes with explosives. These explosives consist of a Class A explosive (i.e. dynamite) and Prell (a ammonium nitrate mixed with diesel). For a hole of 100 foot depth you could easily have 1000 pounds of the Prell in each hole. The loaded holes are then all tied together with PRIMACORD. This cord consists of an explosive called PETN and burns at a rate of about 1 mile per second. Delays are now placed between each of the rows. These delays are on the order of 20-40 milli-seconds each. The sequence goes like this: A blasting cap ignites the PRIMACORD which in turn sets off the dynamite which ignites the Prell. Video #3 shows this process to good effect. The goal of a 'shot' is not just to break up the overburden, but to move as much of the overburden as possible. This is accomplished by initiating the 'shot' by placing the blasting cap at the end of the row closest to the last cut (the open pit). This area has already been mined and all the coal has been removed. When this first row explodes it throws overburden up and out into the open space of the last cut. The second row, after a short delay, explodes moving more overburden up and out. The process continues with each of the rows exploding successively. At least half of the overburden should be moved by the blast. The other half will be removed by machines (draglines, shovels etc.). Drilling and blasting is an expensive process. But not as expensive as mechanically moving overburden. So it is crucial that the 'shot' be effective. As you watch these videos you will see a couple of holes that did not fire correctly for some reason. You can tell by the orange smoke.
