Study On Hydraulic Blasting Stress Wave Propagation And Coal Broken Mechanism

In recent years, hudraulic blasting technology has been applied in the engineering blasting field to a certain extent, such as controlled blasting for removal, shaft and drift excavation blasting, tunnel smooth blasting and open pit mining blasting.etc. Hudraulic blasting can transfer energy well and its wedging effect is obvious, with the advantages of a well rock-breaking ability and high explosive energy efficiency. Based on the above characteristics of hydraulic blasting, this paper proposes a new method of hydraulic blasting to improve coal seam efficiently, which can create a wide range of fractures in coal seam to achieve the purposes of improving permeability. This paper mainly studies on water pressure blasting stress wave propagation and coal broken mechanism, which providing theoretical basis for the field application of this method.In this paper, theoretical analysis, similarity simulation experiment and numerical simulation are combined to study the water pressure blasting stress wave propagation and coal broken mechanism. The main results are as follows:① Based on the explosive detonation theory and water shockwave theory, the formation and propagation law of water shockwave of water pressure blasting was researched. Furthermore, the formulas of the initial pressure of shockwave and the incident pressure on blasthole wall are deduced. In addition, using the elastic wave theory, the initial stress in coal is deduced and calculated. Study shows that explosive performance parameters, charging structure parameters, physical and mechanical properties of coal are the main factors influencing the water pressure blasting stress of coal. The initial pressure of shockwave increases with the increase of explosive detonation velocity and detonation pressure. The incident pressure on blasthole wall and the initial stress in coal increase with the increase of density of explosive and heat of explosion, decrease with the increase of decoupling index of charge. Under the condition of the same charge, the initial stress in coal increases with the increase the wave impedance of coal.② According to the stress wave theory, the propagation and attenuation law of blasting stress wave in coal was studied. Moreover, the failure criterion of coal is established according to Mises strength criterion and maximum tensile stress criterion. Initially, the damage form of coal is plastic compression deformation when the effective stress exceeds the dynamic compressive strength of coal. The crush area appears around the blast hole. The coal then presents brittle tensile rupture and the much-crack area appears when the tangential produced by the explosive stress wave exceeds the dynamic tensile strength of coal.③ The similarity simulation experiment on coal blasting was conducted. The effect of coal blasting with water and air charge non-coupling medium are analyzed comparatively. Super dynamic strain measurement systems and data analysis software were used to collect strain produced in coal seam, and the crack shape, crack surface quality and other blasting results were comprehensive evaluated. The results shows that: while hydraulic blasting(uncoupled charge blasting in water medium) on coal, the attenuation index of explosion stress wave was 3-[0.8μo/(1-0.8μo)]. The coal is firstly formed a pulverizing zone around the blast hole whose radius is about 1.5 times the radius of the blast hole, then is formed a wide range of fracture zone whose radius is about 13 times the radius of the blast hole, and finally is formed a blasting loose area. Under the same experimental conditions, the radial and tangential peak stress which produced by water pressure blasting are 2.15 times and 2.4 times of those produced in air medium. After water pressure blasting, some interconnected cracks with large length and width are found in simulated coal body and damage fracture phenomenon is serious. However, after air medium blasting, the expansion chamber is found and compression zone is formed in blasthole peripheral wall a nd bottom, and there are no significant cracks, which prove the advantages of hydraulic fracturing.④ According to the theoretical model, the numerical simulation on hydraulic blasting process in coal seam was carried out by using nonlinear dynamic analysis program ANSYS / LS-DYNA and ALE algorithm. The results show that: explosion stress wave in coal seam is spreaded around in means of ellipse with the initiation blasting point as the center. The attenuation rate of stress wave is fast at first but then slows down. In near to boreholes, the compression stress is primary, but far away from borehole, the tensile stress is primary. The coal crack extension is with a certain direction, with divergent distribution, but crack bifurcation has no direction, with random distribution. The dynamic impedance of water is similar to the impedance of detonation product. The deformation of water is very small when shock wave propagetes in the water. The energy transmission efficiency of water medium is very high and obvious.