Why Explosives Work

©Dr. B. C. Paul 2000

Note – The information found in these slide represents the author’s view of information familiar to those who have studied the field and does not intentionally duplicate the work of any one author or source. Because of the historical nature of this material, the lecture draws on ideas and materials published over a period of

decades from a wide variety of sources that were summarized and interpreted by the author.

Why Do Explosives Work

• First Answer - Cause they blow up stupid– Energy burst pounds the crud out of the rock (just

like braking it up by hitting it with a hammer)

• Answer was accepted without question till World War II sensor technology became available– measured the overpressure in a blast and noted that

it was high enough to exceed the strength of the rock only right next to the charge

But They Do Work?

• 1950s studies of how something that lacked the energy to break rock masses in fact fragmented large areas

• Key was rock tensile strength– only about 1/10th the compressive strength

• Noted in high speed photo’s that rock mass broke up from the face - not the contact with the explosive

• Density Charge with the air causes a reflective wave that tears the rock mass up in tension

Going to Heck in a Hand-basket

• 1960s someone put sensors into the rock faces

• Noted that strength of the reflected tensile wave drops below rock tensile strength within a foot or so of the face

• Tensile waves reflected from the rock face could not possibly be breaking up the rock mass

The Mechanics Wizards to the Rescue

• Noted in high speed photo’s that burst of explosive fumes release from the face just before it flies apart

• Noted that explosives are unique in producing large volumes or hot gas quickly

• Noted that tension at the end of a crack is almost infinitely greater than across a mass in general– ever noted a crack in your wind shield?

The Intelligent Explanation

• Shatter energy of explosives start cracks next to charge

• Expanding gasses flow into and pressurize the cracks

• The cracks drive forward to the face• The gas pressure heaves the rock out and apart• Provided the first practical guidance - you need

good stemming to hold gas pressure in the hole

Nagging Doubts

• Drill cuttings hold in enough gas to blow up a 15 foot bench of solid rock?

• Oil Shale tests produced equally good fragmentation without stemming

• All the data came from blowing up plexi-glass in a lab– Sounds like fun - but is it sound science?

• Put drill holes in plexi-glass and they initiated fractures with no gas pressure

Coming Full Circle

• Rock strength at point defects is much less than average for rock mass

• Defects and fractures can cause reflected waves• Reflecting compression and tension waves act on

point defects initiating cracks that can be driven forward with relatively low levels of shock energy

• Explosives tear rock up by acting selectively on its point defects.

The Tri-modal Fragment Size Distribution• Fragmentation studies show three families of rock

fragments• Fine crushed powder - material comes from the crushing

zone around the explosive charge - 1950s story does apply

• Fragmentation zone - size is controlled by blast design - from zone where tensile and compressive waves can fail rock on point defects

• Boulder zone - heave and recoil separates rock along pre-existing fractures - distribution is a function of the rock mass