Effects of Weathering on Blasting Operations
Voids (openings, cavities, and caves in rock) can have a deleterious effect on blasting. These voids can be naturally occurring as in karst rocks such as limestone, or they may be manmade due to tunnels, shafts, pipes, or abandoned mines. Where any karst sensitive rock such as limestone, gypsum, anhydrite, or dolomite are expected at an excavation site, the geologist assessing the site should provide a description of the solutioning activity expected. Figures 47 and 48 show some problem karst features encountered during a blasting project.
Explosives can be lost into a void, particularly those in bulk or slurry form resulting in overloading. The air space can decouple the explosive and rock, decreasing the deficiency of the blast. Voids can cause a blast to have inadequate confinement, which can lead to additional fly rock. Where voids, either manmade or natural, are connected to the surface, unexpected air overpressure may be broadcast out of the void and onto the surface.
Where bulk explosives and slurries are used, the blaster should keep a careful account of the amount of explosive material that went into the hole. Drilling and blasting records are key to recognizing these conditions before loading of holes. Drillers will often record voids as “rod drops” that is a zone where there was little or no resistance to drilling. If loss of explosives into a void is suspected, the zone should be located and corrective measures taken to seal it. Special handling of the loading will be required where cavities are encountered; the loading may be plugged in the borehole above and below the cavity, or it may be sealed with sand, stemming material, or grouting. It may be necessary to prohibit use of bulk explosives on projects where there are numerous voids and cavities. This can be addressed through the plans and specifications.
Likewise, zones of intact but weathered rock can present difficulties to the blaster. Mud seams and weathered zones have different properties than the unaltered and intact rock. Weathered rock will blast more easily than a massive intact rock, so blasting techniques should be adjusted. Typically, the blaster may lower the powder factor when working in this type of material. Figure 391 shows some weathered limestone layers uncovered by excavation during a blasting project. The limestone in this photograph is substantially weathered, and numerous karst features are present.
One way of simplifying the handling of weathered material is to blast it separately from the intact material below. By partially excavating down to the lower limit of a weathered zone, the mass is simplified to one with fairly uniform properties. The type of explosives used for excavating weathered material will vary based on the extent and degree of weathering, but generally, the blaster may use cartridged rather than bulk explosives in this type of formation. The normal situation is that the weathered zone is at or near top of rock. The difficulty in producing a good final face is compounded here by the poorer quality of rock and the lower confining pressure. Therefore, the topmost presplit bench usually requires extra care in design and execution.
Mixed ground conditions, where rock “floaters” and intact rock are surrounded by soil or more weathered rock can present significant problems as these may be difficult to excavate with conventional equipment and problematic to blast. The depth and lateral extent of mixed conditions should be mapped before the blast design, and this information provided to the contractor in the specifications. Typically, all overburden will be removed from the rock before blasting starts in order to design and use appropriate blasting techniques. A blaster should not be allowed to blindly drill blast patterns in areas of mixed ground conditions.
These ground conditions will affect not only the design of the mass blasting, but also how the final walls are approached by the blaster. However, a few points are important to discuss here. Pre-splitting will typically be used to provide a relatively smooth finished wall. The loading of the presplit holes may need to be changed or eliminated based on the drilling and encountering voids. Typically, line drilling or precision pre-splitting will be employed at certain distances on either side of the void or other geologic feature. This will provide a finished surface.
Keeping the explosives away from the feature will prevent gases from escaping into the feature, thereby preventing unwanted effects.
