Investigating the effect of filling material thickness on blast-induced stress wave in plexiglass by numerical analysis

In blasting operations different proportions of the energy released are consumed in fragmenting the rock mass, heating up the surrounding and some portion of it is used in propagating blast waves, causing vibration of the particles in the nearby environment. However, reflections, transmissions and absorptions lead the attenuation of these waves. The influence of the extent of rock-mass discontinuities and different thickness of layers in fragmentation efficiency and stability of the mine steps are important for controlling the ground vibrations in the vicinity of blast holes. Hence, predicting the influence of blast-induced waves on the surrounding should be carried out prior to the operation for safety concerns. The main criterion for evaluating the damage caused by the blasting includes the particle displacement analysis, peak particle velocity, particle acceleration, changes in the applied stress waves and frequency contents. Research activities in this area could be classified into the followings: field studies, experimental investigations, analytical and numerical methods; the first two are expensive, and the analytical methods are often based on the unrealistic simplifications with respect to the rockmass behavior. However, the numerical methods by far offer a cost effective, speedy and reliable analysis. In this article, the effect of filling material thickness on the wave propagation of PETN blasting in a vertical discontinuity in plexiglass has been investigated using UDEC simulation. For this purpose, a uniform normal pressure dynamic of 11.0GPa was applied on a 5.08mm diameter blasthole wall. This dynamic pressure was obtained using a semi-empirical Liu and Tidman correlation. The shockwave caused by the blasting was assumed to be a triangular pulse, having a 0.05ms duration. A polymeric plexiglass, with dimensions of 230×114.3 mm was used for this study. Super glue was used as the filling material. Having considered the fixed geometrical involved, the influence of the filling material thickness (5, 10, 15, 20 and 25 mm) were investigated in a cylindrical blasthole vertically drilled (with large length to diameter ratio and complete decoupling). Peak particle horizontal displacement, peak particle velocity and induced stresses on either sides of the vertical discontinuity were measured in the elastic behaviour mode of the surrounding media. The energy reflection coefficient (ERC) and the energy transmission coefficient (ETC) obtained from the numerical analysis showed a good agreement with those of the analytical ones. However, the ETC in the numerical method decreased from 28% to 8.7% when filling material thickness increased from 5 to 25 mm, while this remained constant at 50 % for analytical method. An exponential correlation has been proposed showing the relationship between the ETC obtained from the numerical and analytical methods, with a correlation coefficient of 0.9962. Increased filling material thickness led to nonlinear reduction of both PPV and stress wave, and considering the constant reflection coefficient, it also led to the transmission coefficient being reduced exponentially while energy absorption increased exponentially, too.