Action of a system of linear ejection charges with short-delay detonation

Abstract

Linear ejection charges in trenches or tunnels in the form of a continuous extended mass of explosives are used in the construction of canals, in opening up mineral deposits, and for other purposes. In many cases the planned dimensions of the excavation require two or three charges. An effective method of controlling the action of a system of charges is short-delay blasting. However, this has not been thoroughly studied in connection with linear charges.From the theory of similarity it is clear that the principal parameters governing the development of the explosion of a linear ejection charge are the scale of the blast, which is usually assessed b~he length W of the line of least resistance (LLR), and the so-called reduced depth of the charge, W'= W/VQ/, where Ql is the mass per meter of charge. The quantity W, like the often-used index of action of the blast n, indicates the degree of loading of the charge.Rodionov and Adushkin [1] have demonstrated that for charges with ordinary LLR (up to 20-25-m) we can neglect the effect of gravity. In such cases, other conditions being constant, the time of development of the blast should be proportional to W, and its dependence on ~ is determined experimentally.We have studied how the times of development of explosions of linear charges depend on the blasting parameters in loams with a density of 1.5 g/cm 3 and a moisture content of 8-10%; we used high-speed cinematography. The camera was an SKS-1M operating at 400-800 frames per second. The frame frequency was calibrated using time marks printed on the film every 10 msee.Charges of ammonite No. 6-ZhV weighing 6.5-26 kg per meter were laid in trenches with LLR of 1.0 or 1.4 m. The depth of the charges was varied between 0~20 and 0.45 m -kg-~ the blast action index ranged between 1.5 and 4.8.From the motion-picture film data we plotted profiles of the development of the ejected burst and the time dependence of the upward velocity of the soil. Figures 1 and 2 show sample profiles and graphs. We can distinguish several stages in the motion of the burst. In the first stage, which lasts up to 10 msec, the soil was accelerated to velocities of hundreds of meters per second -much higher than those from blasts of concentrated charges with action indices of up to 1.5 [1].In the second stage of the motion of the burst the velocity gradually decreased, though remaining high (up to 100 m/sec), while the cross section of the burst was rounded in shape, with no marked projections. At this stage the height of the burst reached 10 m for a development time of the order of 100 msec.In the third stage the soil velocity continued to fall, and the motions of different parts of the burst became nonuniform -separate rock jets could be distinguished, and are clearly visible on the profiles. Between the jets the soil moved more slowly, and spaces appeared, i.e., the dome broke up at the transition between the second and third stages. Finally, in the fourth and last stage of development of the burst, the gas currents overtook the soil, while the u...