Page 444 - Moving the Earth_ The Workbook of Excavation

BLASTING AND TUNNELING

9.44 THE WORK

FIGURE 9.38 Hydraulic rock splitters. (Courtesy of Elco International, Inc.)

ROCK MECHANICS

Rock is broken or fractured by an explosion in three ways: compression, shear, and tension.
Compression is obtained by the direct, hammerlike blow of the explosion against an unyielding
rock mass. An explosive that is so deeply buried that it cannot break out to the surface breaks by
compression only. This is the least effective way to use it.
Shear is movement of pieces or blocks of rock along lines of weakness. Tension is produced
by reflection of the explosion back from an unconfined surface or face of the rock.
In a hard rock, maximum effectiveness of explosives is in tension. Tensile strength is only
about one-tenth of shear strength, and shear strength is only one-tenth of compressive strength.
This means that a blast that can break out to a free face at an efficient distance may produce 100
times the rock breakage of one that is completely confined.
The importance of tension has been demonstrated by setting off a tightly confined explosive
far enough below a horizontal surface that the explosion could not break out, but near enough that
surface rock was shattered in a cone-shaped crater, separated from the small explosion chamber
by solid rock.
It is very fortunate that rock breakage is not produced mostly by the outward movement of
gases from an explosion, as in this case rock throw would be many times greater than it is. In a
well-engineered and successful blast, most of the rock moves less by explosive force while being
broken than it does afterward by slumping under the pull of gravity.
The improved fragmentation obtained by using millisecond-delay caps is partly due to creation of
a series of free faces from which waves of succeeding explosions reflect to produce tension in the rock.

DAMAGE

One of the contractor’s problems in connection with blasting is the possibility of real or imaginary
damage being done to structures in the vicinity.