Bench blasting RDE Home  /  Applications  /  Bench blasting

Bench blasting is the most common rock blasting activity. By definition, bench blasting is blasting in a vertical or subvertical hole or a row of holes towards a free vertical surface. More than one row of holes can be blasted in the same round. A time delay in the detonation between the rows creates new free surfaces for each row. Theory of bench blasting. Rock breakage by explosives can be explained in three principle stages. In the first stage the detonating explosive crushes the rock in the proximity of the hole-wall due to high detonation pressure. In the second stage, compressive stress waves created by the blasting propagate in all directions with a velocity equal to the sonic wave velocity of the rock-material. When the compressive stress waves are reflected towards a free rock face, they return as tensile stress waves. This causes tensile stresses in the rock that will fail if the energy in the shock wave is large enough. The energy that is released from the detonating charge along with the distance between the hole/row and the free face have to satisfy a defined relation in order to generate the failure of the rock. (The tensile strength of the rock is approx. 1/10 of its compressive strength). In the third stage, the high pressure gas (approx. 1000 l/kg of explosives) from the detonation penetrates the cracks that are the consequence of the failure in stage 2 and widens them. The rock mass between the hole/row and the free face will then yield and be thrown forward by the gas pressure. Open pit mining and quarrying. In open pit operations, the bench height varies within relatively narrow limits. The intention is to mass produce blasted rock in a standardized manner in order to achieve lowest possible cost. The bench height is normally between 15 and 18 m but may occasionally go up to 40 m. Reasons for limiting the bench height are safety, risk of falling rock, and problems with precision in drilling. The hole deviation is more difficult to control in higher benches. Civil construction. In civil construction, the scope of the job is to create a space in the rock mass. It can be for a road cut, a pipe line trench or a foundation for a building. The bench height depends on the topography of the rock surface and varies from under 1,0 m to 20 25 m. To avoid excessive hole deviation the contractors prefer to limit the bench height to around 15 m.

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