In most blasting operations, the first visible movement occurs when the face bows outward near the center. In other words, the center portion of the face is moving faster than the top or bottom of the burden.
This type of bowing or bending action does not always occur. One can find cases where instead of the center bowing outward, the top or bottom portion of the burden is cantilevering outward.
In either of these cases, the differential movement causes the burden to break in the third dimension. This breakage mechanism has been called flexural rupture or flexural failure. To properly discuss flexural failure, one must realize that these individual pie-shaped columns of rock caused by the radial cracking will also be influenced by a force perpendicular to the length of the column. This would be similar to beam loading conditions. When one discusses beam loading, the stiffness ratio is significant.
The stiffness ratio relates the thickness of the beam to its length. The effect of the stiffness can be explained by using, as an example, a full-length pencil. It is quite easy to break a pencil with the force exerted with one’s fingers. However, if the same force is exerted on a two-inch long pencil, it becomes more difficult to break.
The pencil’s diameter has not changed, the only thing that has changed is its length. A similar stiffness phenomenon also occurs in blasting. The burden rock is more difficult to break by flexural failure when bench heights approach the burden dimension in length. When bench heights are many times the burden in length, the burden rock is more easily broken.
Two general modes of flexural failure of the burden exist. In one case. the burden bends outward or bulges in the center more quickly than it does on the top or bottom. In the second case. the top or the bottom of the burden moves at a higher rate than the center. When the burden rock bulges at its center. tensile stresses result at the face and compression results pear the charge. Under this type of bending condition. the rock will break from the face back toward the hole. This mode of failure generally leads to desirable breakage.
In the second case. the rock is cantilevered outward, and the face is put into compression and the borehole walls are in tension. This second case is undesirable. This mechanism occurs when cracks between blastholes link before the burden is broken and is normally caused by insufficient blasthole pacing When the cracks between holes reach the surface, gases can be prematurely vented before they have accomplished all potential work. Air blast and flyrock can result along with potential bottom problems.
The bending mechanism or flexural failure is controlled by selecting the proper blasthole spacing and initiation time of adjacent holes. When blasthole timing results in charges being delayed from one another along a row of holes, the spacing must be less than that required if all the holes in a row were fired simultaneously. The selection of the proper spacing is further complicated by the stiffness ratio. As bench heights are reduced compared to the burden, one must also reduce the spacing between holes to overcome the problems of stiffness.
Reference
U.S. Department of Transportation, « Rock Blasting and Overbreak Control».