Seismic Waves Distribution from Blasting in a Rock Massif with One Crack or System of Parallel Fissures (original) (raw)

Vibrations induced by blasting in rock: a numerical approach

Blasting is a powerful excavation method in terms of both production efficiency and economical costs, but its high environmental impact due to noise, vibrations, and potential damage to surrounding structures may limit its extensive application. The use of explosives as excavation tool is usually ruled by the national codes of practice, in which tolerable limits for induced motion are given for the different structure classes. In order to predict the vibrations induced in the ground at a given distance from the blast centre, attenuation laws, derived from either in situ measurements or analytical solutions of simple elastic wave propagation problems, are adopted. As the attenuation laws usually refer to homogeneous continua, they may not be adequate as a predictive tool for complex geological sites. In such cases, numerical analysis provides a valuable alternative, as the whole propagation history of stress waves could be simulated in principle, irrespective of the geological complexity of the specific site. To describe the progressive effects of underground blasting on the surrounding site, a finite element approach is presented. The explosion energy is translated in a time history of pressure at the boundary of the blast hole. Cracking of the nearby rock mass is modelled according to a cohesive crack model, while elastic behaviour is assumed for the non cracked rock mass and soil deposits. Propagation of stress waves from the blast hole is simulated by a time domain 3-D finite element analysis, which is able to provide the time history of all the relevant quantities describing the motion at any given distance. The numerical results can be post processed in order to derive attenuation laws for the most relevant quantities to which the codes of practice usually refer to, i.e., peak particle velocity and principal frequency of the vibration. The model is energy-conserving, thus the energy supplied by the explosive is correctly partitioned into fracture energy of the rock mass close to the blast hole, elastic energy providing the stress wave propagation and kinetic energy of the fragmented rock blocks. Numerical simulations of two literature case-histories are presented, and the numerical results are compared to the available experimental data. Experimental peak particle velocity could be captured remarkably. Principal frequencies for the rock mass could be reproduced as well. In layered sites, the ratio between the stiffness of the different media where stress waves propagate seems to play a key role in the determination of principal frequencies, while less influence is observed on peak particle velocities.

Effects of Rock Damage on Seismic Waves Generated by Explosions

Pure and Applied Geophysics, 2001

In studying the physical processes involved in the generation of seismic waves by explosions, it is important to understand what happens in the region of high stresses immediately surrounding the explosion. This paper examines one of the processes that takes place in this region, the growth of pre-existing cracks, which is described quantitatively as an increase in rock damage. An equivalent elastic method is used to approximate the stress ®eld surrounding the explosion and a micromechanical model of damage is used to calculate the increase in damage. Simulations for a 1 kt explosion reveal that the region of increased damage can be quite large, up to ten times the cavity radius. The damage is initiated on a damage front that propagates outward behind the explosive stress wave with a velocity intermediate between that of P and S waves. Calculations suggest that the amount of increased damage is controlled primarily by the initial damage and the extent of the region of increased damage is controlled primarily by the initial crack radius. The motions that occur on individual cracks when damage increases are converted to seismic moment tensors which are then used to calculate secondary elastic waves which radiate into the far ®eld. It is found that, while the contribution from an individual crack is small, the combined eect of many cracks in a large region of increased damage can generate secondary waves that are comparable in amplitude to the primary waves generated by the explosion. Provided that there is asymmetry in the damage pattern, this process is quite eective in generating S waves, thus providing a quantitative explanation of how S waves can be generated by an explosion. Two types of asymmetry are investigated, a shear pre-stress and a preferred orientation of cracks, and it is found that both produce similar eects.

Blast Induced Crack Propagation and Damage Accumulation in Rock Mass Containing Initial Damage

Shock and Vibration, 2018

Blast induced rock mass damage and crack propagation play important roles in structure safety and stability in mining, quarrying, and civil constructions. This paper focuses on the effect of small blasthole diameter blast on crack propagation and damage accumulation in water-bearing rock mass containing initial damage composed of inherent geological discontinuities and previous multiblast induced damage. To elucidate this effect, theoretical analysis of calculation method for several important blast influencing factors is firstly presented. Secondly, definition of a practical damage variable using ratio of longitudinal wave velocity in rock mass before blast occurrence to that after blast occurrence and derivation of a damage accumulation calculation equation accounting for initial damage and blasting effect are described. Lastly, a detailed description of the conducted in situ blast tests and plan layout of the sonic wave monitoring holes is reported. The results indicate that blas...

Assessment of crack initiation and propagation in rock from explosion-induced stress waves and gas expansion by cross-hole seismometry and FEM–DEM method

International Journal of Rock Mechanics and Mining Sciences, 2015

Single-hole blast-induced damage in a granitic outcrop has been assessed through both controlled experiments and numerical simulations with a combined finite-discrete element method (FEM-DEM). Damage tomographies from decoupled short and long explosive charges in flooded boreholes were obtained through a high-resolution cross-hole system, by measuring pre-and post-blast seismic velocities around the blastholes. Damage was assessed through crack density, which was calculated by inversion of P-wave velocity measurements through an Effective Medium Theory (EMT) method. The resulting damage was found to be highly asymmetrical around each blast hole, both along the vertical and horizontal planes, despite the apparent isotropy and homogeneity of the granitic rock mass. Experimental results combined with numerical simulations carried out to assess damage from stress waves alone, showed that most damage from experiments was caused by the expansion of gases, while its magnitude and extension were strongly dependent on confining conditions along the blasthole. The difficulty of quantifying the relative contribution of stress waves and gas expansion and the problems inherent in prediction of blast-induced damage are described.

Numerical investigation of blasting-induced crack initiation and propagation in rocks

International Journal of Rock Mechanics and Mining Sciences, 2007

To investigate the dynamic fracture mechanism related to blast-induced borehole breakdown and crack propagation, circular rock models containing a single centrally located source of explosive were numerically blasted using the AUTODYN 2D code. According to the material properties and loading conditions, four kinds of equations of state, linear, shock, compaction and ideal gas, are used. A modified principal stress failure criterion is applied to determining material status. The dynamic stresses at the selected target points in a rock sample are computed as a function of time following application of explosive load. It is shown that shear stress (resulting from intense compressive stress) causes a crushed zone near the borehole, the major tensile principal stress causes radial cracks, and the reflected stress wave from free boundary causes circumferential cracks some distance away from the free boundary. The influences of the factors of boundary condition, coupling medium, borehole diameter, decoupling and joint on rock dynamic fracture are discussed. r

Seismic waveforms from explosive sources located in boreholes and initiated in different directions

Journal of Applied Geophysics, 2012

Peak particle acceleration and velocity Frequency spectra Linear explosive charges Direct and reverse initiation of explosive A study of the effect of explosive source orientation in a borehole on the nature of emanating seismic waves was made. High frequency triaxial accelerometers mounted on the surface and in boreholes in underground mines were the diagnostic sensors. A variety of explosive sources initiated with the detonation reaction propagating towards the detector and away from the detector were studied in highly competent rock. The results show significant differences in both the amplitude and the frequency spectra of the signal for the two modes. The respective azimuthal distribution of P and S wave amplitudes from propagating linear sources are also found to be at variance with those predicted by existing theoretical approaches.

Numerical investigation of blasting-induced damage in cylindrical rocks

International Journal of Rock Mechanics and Mining Sciences, 2008

In order to investigate rock fracture and fragmentation mechanisms under dynamic loading, a cylindrical rock model with a centralized borehole is developed through the use of AUTODYN code. According to the material properties and loading conditions, four kinds of equation of state (EOS), linear, shock, compaction and ideal gas, are applied to the four kinds of materials employed in this numerical model. A modified principal stress failure criterion is applied to determining material status, and a well-behaved explosive, PETN, and a relatively homogeneous igneous rock, diorite, are used in this rock model. A single centrally located line source of explosive is fired numerically to produce the dynamic loadings operating on the surrounding rocks. This numerical model is applied to actual blasting conditions. The rock failure mechanism under dynamic loading is first analyzed, and then the influences of the following factors on rock fracturing are discussed: (a) coupling medium, (b) confinement, (c) boundary condition, (d) initiation location in an explosive column, and (e) air ducking. The results show that all these factors have significant effects on rock fracturing under dynamic loading. r

New theory for the rock mass destruction by blasting

To develop a new theory for the rocks destruction by blasting using a description of the formation processes of zones with various mass state around the charging cavity. Methods. The new theory for the rock mass destruction by blasting has been developed based on the use of the well-known elasticity theory laws and the main provisions of the quasi-static-wave hypothesis about the mechanism of a solid medium destruction under the blasting action. The models of zones of crumpling, intensive fragmentation and fracturing that arise around the charging cavity in the rock mass during its blasting destruction, depending on the physical and mechanical properties of the rock mass, the energy characteristics of explosives and the rock pressure impact, have been developed using the technique of mathematical modeling. Findings. Based on the mathematical modeling results of the blasting action in a solid medium, the mathematical models have been developed of the zones of crumpling, intensive fragmentation and fracturing, which are formed around the charging cavity in a monolithic or fractured rock mass. Originality. The rock mass destruction by blasting is realized according to the stepwise patterns of forming the zones of crumpling, intensive fragmentation and fracturing, which takes into account the physical and mechanical properties of the medium, the energy characteristics of explosives and the rock pressure impact. Practical implications. When using the calculation results in the mathematical modeling the radii of the zones of crumpling, intensive fragmentation and fracturing in the rock mass around the charging cavity, it is possible to determine the rational distance between the blasthole charges in the blasting chart, as well as to calculate the line of least resistance for designing huge blasts.

Geotechnical Investigation of Fracture Patterns in a Rock Mass during Excavation by Blasting

Overcharging in rocks (wall faces) during blasting and excavation usually causes damage to rock mass in most mining and quarry industries. This creates blast-induced fractures which can relates with pre-existing fracture pattern thereby increasing sliding and rockfall from the crest and body of an excavated wall. The spacing and orientation of pre-existing fractures are predominant at a small-scale mining (galamsey) site at ‘Atta ne Atta’, a town near Beposo, in the Western Region of Ghana. Geotechnical field studies were carried out to investigate the possibility of any instability within the area to eradicate the occurrence of an unexpected future wall failure (rockfall). The geotechnical mapping conducted was focused on fracture distribution and spacing. Mean spacing (Sm) of existing fractures was calculated and corrections were made to obtain calculated spacing (Sc). The scanlines of wall face 001 and wall face 002 intersect with their corresponding strike and dip at 78° and 80° respectively creating a slightly favourable fracture pattern and rock wall stability. The fracture pattern created at Wall Face 003 and Wall Face 004 were unfavourable for rock stability with their corresponding scanlines having a strike and sip of 67° and 73° respectively. The instability of these wall faces (003 and 004) is as a result of parallel orientation of the induced fractures to the strike of the pre-existing fractures. Observations made from the stereographic projections and rose diagram indicate a cluster of fracture patterns with a general strike of NNE-SSW. Hence, the fracture patterns in the study area are composed of favourable (stable) rock mass at some walls and unfavourable (unstable) rock mass at other wall faces due to overcharging of blast holes.

Modeling of dynamic fracture mechanism in rock masses due to wave propagation

International Journal of Engineering & Technology

Finding a new oil well is a stimulating experience at all levels, however, it’s only an important milestone on the road towards exploiting oil and gas. When it comes to well drilling, the condition of the ground that surrounds the oil plays a major role. While there are many factors that dictate the success of exploring and drilling wells, porosity and permeability of the surrounding stone are some of the most important components.This paper focuses on the effective way to increase the porosity and the permeability of the rock using explosives without damaging the rock. In order to reach our aim, a numerical simulation was conducted. In fact, a 2D distinct element code was used, and 4 models were constructed; in each model the number of explosives increase while the blast load per explosive decreases.The dynamic stresses, and velocity vectors of the wave propagation were analyzed to evaluate the behavior of rock masses in each model. Moreover, a grid of history points was studied in...