Effects of Rock Mass Structure
While laboratory samples may test the intact rock, the actual strength of the rock mass and its resistance to blasting are usually far less than the intact rock values would indicate. This is due to the naturally occurring network of joints, bedding, faults, cavities, voids, and breaks within the rock. These flaws in the rock play an important role as they can create planes of weakness within the rock mass that will influence the fragmentation of the rock. Where a rock mass contains multiple rock types, or different facies, these too can influence the blast as different rock types may require different blasting design. Cavities and voids, which are a weathering feature, will be discussed below.
Structural discontinuities such as joints, faults, and bedding planes are all breaks that subdivide the rock. Their spacing, orientation, and persistence in the rock mass are the most important geologic consideration that will affect blast performance. Good mapping and site characterization is essential as the characteristics of these features will need to be communicated to the blast designer. The strike, dip, and spacing of these structural features should be well understood by the geologist before blasting design begins. The simple block diagram in Figure 39 shows how the terms are used.
The block size and fragmentation characteristics of the rock mass are heavily influenced by the spacing of these discontinuities (Figure 40. ). Explosive energy will not be well distributed through the rock mass when the borehole patterns are larger than the discontinuity spacing. When the borehole separation is 2 to 4 times the block size, much larger boulders with inadequate fragmentation that are difficult to handle can be expected. More effective fragmentation is accomplished where explosive charges lie within the solid blocks bounded by joints. This is typically adjusted by tightening the pattern and using a smaller blasthole diameter.
This can be particularly problematic for presplit faces. Where discontinuities are nearly vertical and strike parallel or within around 15 degrees of the direction of the final rock face, it can be extremely difficult to create a presplit face that does not follow these discontinuities.
The overall dip of the structural features present in the rock mass in relationship to the desired bench or final wall face can make a difference in the final wall produced. Blasting with the dip or against the dip can both leave rock slope stability vulnerabilities in the final rock wall. Blasting with the dip can allow for the use of lesser explosive charges, or use larger burdens as the rock moves more readily down the slope. However, this can produce much greater back- break at the top of the slope. Where the discontinuities may intersect the top of the slope or next higher bench behind the desired face, the rock may be removed along the dip, rather than at the design face location. Figure 41 shows backbreak where rock is removed at the top of the slope beyond the design face. Blasting against the dip generally requires more explosive charge as the blast must work against the overall rock mass structure. However, this can produce more overhangs.
Figures 41 to 43 show that the structure of the rock should be taken into account when planning a blast design because the structure can have a strong influence on the stability of the wall. Kinematic analysis of the rock structural features should be completed during the design phase of a project where rock excavation is planned to identify problem features that may develop in the design rock wall or excavation.
Figure 44 shows rock stability failure modes that can be created by rock removal and exposure of rock structural discontinuities. Design of the site should incorporate a thorough understanding of the rock structure and problems that can develop during construction. A review of the boreholes used for blasting as the project progresses should be used to check the original geological model for the site. A final wall should always be inspected after a blast to assess the rock slope stability and determine the need for any additional blasting, mitigation, or reinforcement.
Where a rock mass to be blasted contains more than one rock type, blasting can become more complicated as each material may require a different powder factor and design to achieve good results. The stratigraphy of the site should be well understood during the design process as the blasting techniques may need to be modified for each rock type present. Deck loading is often used to accommodate changes in stratigraphy.
Fault zones also present a problem, particularly where they enclose breccias. Blasting conducted near faults will often break to the fault surface. Venting of gases can also occur along permeable breccias or fault zones, causing a loss in the blasting energy and poor results unless deck loading is used. Porous faults and breccias constitute potentially weak zones that may be of utmost importance in stability considerations.
