Assessment of Blast Energy Usage and the Extent of Blast Damage in Hard Rock Open-pit Mines
Creators: Dotto, Magreth Sungwa
Publisher: name: University of Alberta Library
Year: 2024

<p>Blasting fragmentation influences the cost and productivity of downstream processes such as loading, hauling, crushing, and milling. Interaction between the rock mass and explosive energy determines blast outcomes. This research aims to enhance the understanding of the blasting process by examining key factors that influence blast outcomes specifically, the explosive strength, rock mass strength, and rock structures such as rock contacts and joints. The research estimates damage zones around blasthole and evaluates fragmentation patterns using the analytical approach and numerical simulation. These methods are applied to the data from an existing mining operation collected through laboratory tests and field measurements. The results are validated through comparisons with field measurements and estimations from other verified approaches.<br>
Key findings of the research indicate that explosive selection is a crucial aspect of blast design and should be based on the rock mass properties and the desired outcomes. The formation and size of damage zones around the blasthole are influenced by the explosive’s energy and rock mass strength. Stronger explosives are more effective in hard rock, creating longer and more extended fractures, while less powerful explosives result in a more uniform distribution of fractures in both soft and hard rocks, although with a reduced extent in hard rocks.<br>
Structural properties influence how explosive energy and fractures are distributed in the rock mass. The propagation of stress waves at the interface between different rock types depends on the impedance difference between the rocks, the intensity of the incident stress wave, and its direction. When a stress wave moves from soft to hard rock, it is enhanced and attenuates in the opposite direction. Despite this attenuation, cracks can easily propagate from hard rock to soft rock.<br>
In a jointed rock mass, the type and size of the infill material, along with fracture frequency, significantly impact how explosive energy and fractures are distributed. When comparing empty joints with clay-filled joints, the empty joints reflect most of the stress wave energy, whereas clay-filled joints allow better energy transfer. Energy transmission decreases as joint width and fracture frequency increase, leading to reduced fracturing: by 9% when fracture frequency increases from one to four and by 13% when joint width increases from 3 to 10 cm. Other factors like joint continuity, distance from the charge, and joint orientation and randomness further intensify these effects.<br>
Blast design is the art of carefully balancing explosive strength, energy propagation, and confinement to maximize rock fracturing. This is achieved by selecting explosives that match the rock&#39;s strength and adjusting the pattern design and initiation sequencing and delays to maximize energy usage. Understanding how rock mass properties influence blast outcomes allows for better design adjustments. Common adjustments include changing explosive strength properties, increasing the powder factor, changing the pattern size, and varying initiation sequences and delays. The study found that increasing the powder factor by increasing the blasthole diameter did not improve fracturing in jointed rock masses. However, altering delays and sequencing significantly enhanced fracturing: 10% in intact rock, 14% in jointed rock, and 7% at rock contacts.<br>
 The research is valuable to the industry as it provides practical guidelines for blast design, particularly in complex rock masses where standard approaches may be insufficient. The insights gained offer a basis for refining predictive models and exploring innovative blasting techniques.</p>