Geologic Problems that Influence Blast Floors
Toe and Elevated Floor Problems
The problem of toes and elevated floors after a blast can be traced back centuries to when explosive were first used in mining. Why do they occur? What is commonly blamed as the cause of these toes is geology, but how does geology change that rapidly to where one shot has a fine floor and the next is elevated?
A better approach than attempting to blame factors out of the blasters control is to solve these toe problems and reducing excess blasting, uneven floors, and possibly mechanical removal by following a systematic approach to identifying what is causing the toe. In most cases the toe is not caused by geologic conditions, but instead is a direct cause of the explosive product, pattern design, or drilling inconsistencies. Furthermore, if a geologic condition is causing this issue it should not simply be written off as a given that the toe must occur; a good blaster or engineer will quickly modify the blast pattern to account for the geologic change and eliminate this problem.
Geologic Conditions and Blast Design Corrections
Geology seems to be the scapegoat of blasters and explosive companies worldwide, as the everchanging nature of the rock makes blasting a highly dynamic, slightly unpredictable environment. However, what sets a good blaster apart from the rest is the ability to understand and cope with geologic difficulties. Can geology cause problems with toes and elevated floor? Of course. Should a good blaster know how to change the blast pattern to eliminate these problems? Of course.
How do Hard Seams effect Blast Toes and Elevated Floors
The first problem that could cause a toe or elevated floor is a hard seam. A hard seam can be from a harder material such as a granite intrusion into a limestone mine, a reduction in the amount of bedding planes or jointing in a certain region, or even the grain size of certain sandstones. No matter what the cause of the seam they can all be treated equally from the viewpoint of blasting. Two types of hard seams can exist in at a mine, the first of these is a vertical seam such as an igneous intrusion or vertically dipping bed. Floor issues associated with this type of seam will often be isolated to this specific bed and will show up along the strike. The method to correct the floor problems with this type of geologic condition will depend on which direction the blasting is progressing. If the blasting is progressing along the strike (parallel to the bed) then the spacing between boreholes around this seam can be tightened up slightly. If the blasting is progressing up dip (perpendicular to the bed) then the burden should be reduced when blasting in and behind this bed.
The second type of hard seam that would influence the toe would be a hard, horizontal seam that is located at the toe of the blast. While typically the blast would have the burden reduced to account for this hard material, this case may have the rest of the material easily blasted at the current burden. The goal is then not to sacrifice the economics of the operation by reducing the burden just to deal with a toe, or to accept the toe. The ideal solution would be to put a heavier charge into this hard seam, such as emulsion if the borehole is normally loaded with ANFO. If packaged emulsion is used, this area could have a larger diameter emulsion. If the borehole is normally loaded with bulk emulsion, a cast booster could be placed into this area. In this manner the hard seam is dealt with by adding additional energy without adding significant cost to the operation.
Can Soft Seams effect Blast Toes and Elevated Floors?
Not only can a hard seam create a toe, but a soft seam can have the same effect for a completely different reason. If a soft seam is located in the blast and is not properly taken care of, the seam will blow out which will significantly reduce the energy in the borehole. This is similar to that of an overconfined charge with too large of a burden; which is one of the major causes of toe and elevated floor problems. For this case soft seams should always be decked across to ensure proper breakage to the full depth of the blast.
How can Bedding and Jointing Create Toes?
The next geologic situation that will be considered is that in which vertically oriented bedding or jointing exists between boreholes. This situation may have one or more bedding planes between boreholes. If a toe is forming in between boreholes, this is likely due to the bedding planes limiting the interaction of boreholes in the same row. To combat this situation, a slightly closer spacing should be used throughout the pattern.
The last geologic situation that will be analyzed is that with vertically oriented bedding or jointing existing between a borehole and the free face. In this situation the bedding planes are limiting the explosive energy near the free face and at the toe, creating areas in the rock that are receiving little to no explosive energy and are simply being pushed on as the remainder of the material breaks. This is especially prevalent if the rock is harder and stuck to the floor but the planes are lightly cemented together. In order to account for the burden of the blast should be reduced (with an accompanying decrease in spacing).