Effects of Rock Type
Rock type can have an effect on blasting operations in that different rock types have different strengths and densities, and thus require a different blast design to achieve good results. One very interesting property of rock is that, like concrete, it is stronger in compression than in tension. Therefore, a goal of efficient blast design will be to place the rock in tension rather than compression. However, with increasing confinement, the rock becomes very strong. Thus, to move the rock by blasting, it must have a small enough burden to be displaced by the explosive.
The intact rock describes the fundamental rock type and properties (e.g., limestone and compressive strength of 5500 psi). The rock mass includes the intact rock along with all discontinuities, joints, faults, bedding, voids, etc. that occur within a volume of rock to be studied or blasted. These breaks in the rock have a significant effect on blasting operations.
While the rock mass properties are often of much more importance, the intact rock values should also be taken into account during blast design. Samples of rock, taken by diamond core drilling without the discontinuities or structure of the rock mass are generally used to determine these properties. A word of caution on rock sampling of this type, as is described in many textbooks on the subject; there can be considerable variation in samples from location to location and these are very small samples when compared to the overall quantity of rock present at a site. Where limited testing is performed, more variability of results can be expected. Charts and tables for approximate values based on the rock type (e.g., granite, limestone, sandstone, shale etc.) are readily available, though the values can have a wide range and laboratory testing will better reflect actual site conditions. However, for initial estimation where laboratory tests have not yet been performed, these can be of considerable value to the designer as the first estimate of the properties. Table 12 lists some typical intact rock sample values. The following sections describe several testing methods to determine commonly used intact rock properties as prescribed by the American Society for Testing and Materials (ASTM 2008a,b).
Strength of the rock is usually described by the relatively simple and inexpensive unconfined compression test (ASTM D7012, Method C 2014). While the compressive strength of the rock is greater than in tension or shear, empirical values and ratios can be used to obtain the desired strength based on the rock type where only unconfined compression tests are performed. These are less accurate than actual laboratory tests.
Tensile strength is usually determined by the Brazilian Disk Tension test (ASTM 3967). Direct shear tests (ASTM 5607-08) can also be performed. However, the ease of rock breakage in tension is only partly due to rock being weaker in tension than in compression. It is also due to the fact that the rock is easier to fracture in tension as it is a brittle material. Explosive gases in a borehole, where there is a free face, load the rock mainly in tension, thus using far less energy than would be required if breaking the rock primarily in compression.
Tests to determine the elastic modulus (Young’s Modulus) and Poisson’s Ratio (ASTM D7012, Methods B and D 2014) are performed and have been used to determine the blastability of rock.