ROCK PROPERTIES - Blasting results are usually affected more by rock properties than by the properties of he explosive used to blast the rock mass. Layers of different materials, weathering, bedding planes, joints and other discontinuities can present considerable challenges in blast design. In some instances these geological features allow significant amounts of explosion energy to be wasted rather than working on the rock. Closely jointed rock needs a relatively low powder factor to achieve satisfactory fragmentation, displacement and muck pile looseness. Considerably higher powder or energy factors are required in tough, massive rock, In this case, explosion energy can wedge open and extend few pre-existing fractures, and a large number of new fractures must to created to achieve the required fragmentation. An explosive effectiveness varies with rock type. In sedimentary strata, such as shale or sandstone, a low energy explosive (eg ANFO) usually produces sufficient strain to cause adequate fragmentation. A higher energy explosive is required in stronger, denier rocks. Rock properties affect many aspects of the mining operation:
Blast hole expansion phase - Shock energy is delivered during expansion of the blast hole up to the point of blast hole equilibrium. Fractures are created by the high compressive stress wave generated by the detonation.
Gas Expansion Phase - Gas energy is delivered during the gas expansion phase. This energy contributes to further fragmentation by extending fractures, and behave he broken rock.
Shock Energy - When an explosive charge detonates inside a blast hole, an intense compressive stress wave is transmitted into the rock mass. As the compressive stress wave generated by the explosive, passes through the rock mass, the redial compressive stress causes a complementary tensile stress to be generated. A dense crack network will be produced around the blast hole by the tensile stress and new cracks will start to form wherever the tensile stress exceeds the tensile strength of the rock. Spalling of the rock will also when the compressive stress wave strikes a free face or an open discontinuity in the rock and a tensile stress wave is reflected back towards the blast hole. Shock energy should not be mistaken for fragmentation Energy. Fragmentation also occurs during subsequent stages of rock breakage and is not solely a result of tensile failure of the rock structure.
Gas Energy Cracks started by the tensile stress in the rock mass are extended by the thigh pressure explosion gases which flow into cracks and joints. Cracks open further and explosion gases start to heave the rock mass forward. The gas energy used during this phase of rock breakage is called Heave Energy. High pressure explosion gases eventually escape to the atmospheres at around 100 MPa (1000 atmospheres pressure an effective work on the total energy liberated to this point is called the Fragmentation Energy and is equal to the Relative Effective.
Shock Energy + Gas Energy = Fragmentation Energy and. Gas Energy = Heave Energy
Influence of Free Faces - Free faces and open joints play a major role in the rock breakage process. A boulder (with many free faces being popped using a small blast hole and a part-cartridge of Power- gel may satisfactorily broken at a powder factor of 0.05 kg/m3. In comparison, a tight narrow trench or ramp, with one free face, in the same rock may require a powder factor of over 0.7 kg/cm3 to give an adequate result. Free faces located at optimum distance from the blast hole enable the explosives energy to perform the greatest amount of work on the rock mass. A blast will be more efficient if it has two free faces rather than one. Blast Hole Open Stopes , Blasting to Improve Fragmentation in Mines Coal Mining Vs. Metalliferous Mining CUT AND FILL MINING Development of an underground mine Inspection Mine Plan and Section Lead and Zinc Under Ground Mining Methods Mine Planning Engineer - Malanjkhand Copper Mines, Raising methods in metal Mines Typical Sub Level Stopes (54mm dia holes) UNDERGROUND MINING METHODS AT KOLAR GOLD FIELDS