Study On The Failed Mechanism And Experiment Of Spiral Notch-holes Blasting

Notched blasting is employed to control the cracking direction by centralizing the blasting energy in the direction of the notch, which results in the asymmetry of radial crack development. Spiral V-notch loosen blasting has broadened the application of notched blasting from a single blasting control technique to a rock fragmenting one by changing the conventional straight notch along the axis into two spiral notches around the axis. The spiral V-notches vary their directions between the axial and radial directions, which makes it possible to axially regulate the notch density to control the sizes of blasted fragments. Compared with ordinary loosen blasting, blasting with this technique results in a smaller powdered section in a bigger cracked area, a lower energy consumption, a lower cost, a shorter construction time, less vibration, and less effect of dust, noise and vibration of blast on the environment, which implies a possibly wide application. Fracture mechanics and Westergaard Stress Function are adopted to build a complex stress function to derive the plane stress and strain fields at one tip of the crack under a Quasi-static pressure. An expression is formulated to define the stress intensity factor of spiral V-notch loosen blasting. Factors that have effects on the stress intensity factor are studied. It is demonstrated that spiral V-notch loosen blasting is an extension of vertical V-notch blasting, straight cracking, and like theories. Rock fracture mechanics and the quasi-static theory of blasted gas are employed to derive the rules of initiation, extension and termination of cracks. Calculating formulae for extending speed, width and extended length of cracks in spiral V-notch blasting are studied. Effects of the dimensions of a spiral V-notch on crack extension and those of blasted stress wave on the stress distribution at the tip of a spiral V-notch are investigated, so that a restrained area of crack development with the expansion of the blasted gas in spiral V-notch blasting is found. A mortar model is adopted with the combination of hydraulic pressure on notches cut by a die and expansion agents in a laboratory experiment to simulate the quasi-static course of blasting, which indicates that With the spiral angle unchanged, a bigger compression strength of the model results in a higher pressure for crack initiation; Under the same compression strength, the pressure for crack initiation in a spiral V-notch model is lower than that in a circular hole model; Cracks always initiate and develop along the tip of the spiral V-notch Expressions of major parameters are obtained by studying the blast parameters in the case of uncoupled explosive loading, on the basis of research on the mechanism of spiral V-notch blasting. ANSYS program is applied to the numerical analysis on the space flexibility and elastoplasticity of a spiral V-notch, which suggests that The stress distribution is uneven inside the spiral V-notch as tensile stress is focused near the tip of the notch; Tensile stress and compressive stress areas are located alternatively inside the whole notch, the stress changes slower at a point farther from a crack tip, and much faster at a nearer point; Under the action of a large tangential tensile stress, radial cracks initiate first at the tip of the crack; Elastoplasticity study finds that there exists at the tip of the crack a plastic zone that expand and extend to the direction of the tip with increase load so that cracks initiate at the tip of the notch and extend in the direction of the tip; Comparison between the theoretical study and ANSYS numerical calculation proves that the expression of stress field at a tip of the crack is correct by and large, and the assumptions and approximations for the theoretical analysis are reasonable. In-situ check experiments testify that the spiral V-notch blasting is applicable to engineering projects and have a good advantage in controlling the size of blasted blocks and widening blasted range. It also lowers t...