Mehanics of Rock Fragmentation Static and Dynamic Laboratory Testing Applied to Aggregate Production

Doctoral thesis, 2001

Knowing the strength properties of rocks is essential for calculating the stability of natural slopes, constructions in and on rock, excavations, blasting geometry, crushing, and drilling. When rock and rock masses are subjected to either dynamic or static loads, the tensile strength is determined by tests appropriate to the load type. Static tests include both indirect tests, such as the Brazilian test, and direct tests like the dumbbell-shaped direct pull. Conventional dynamic tests are the Hopkinson pressure bar and ultrasonic methods. This work has two objectives: (1) to characterize diverse rocks in terms of dynamic and static tensile failure behavior, and (2) to conduct problem-based research for minimizing the generation of fines by-product in rock fragmentation and comminution. Two major processes of aggregate production are blasting and crushing operations. The rock is subjected mainly to tensile stresses and failure in both processes. A method for measuring the quantity of rock fines was suggested for the Brazilian test. A working hypothesis for reducing the amount of rock fines in the Brazilian test was examined. Moreover, an analytical solution for discs subjected to the Brazilian test was also presented; this facilitates an accurate analysis of the Brazilian test for anisotropic rocks. Also, a Hopkinson pressure bar was constructed and then modified to measure the dynamic uniaxial tensile strength of rock cores in which rock fails in pure tension. Results showed that, for a specific type of rock, generation of fines depends on the tensile strength of the rock. The higher the tensile strength, the higher is the percentage of fines produced. The working hypothesis that reduction of the strength of rock through water saturation should reduce fines generation was examined and shown to be true for the rocks tested. The dynamic uniaxial tensile strength of gneiss and granodiorite was approximately the same as the static strength from the Brazilian test. Moreover, analytical solution showed that significant errors can be avoided by using the solution for determining the indirect tensile strength of anisotropic rocks.