Assessment of crack initiation and propagation in rock from explosion-induced stress waves and gas expansion by cross-hole seismometry and FEM–DEM method (original) (raw)
Related papers
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...
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
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.
International Journal of Engineering, Science and Technology, 2011
Damage to the surrounding rock during blasting either in underground or surface excavation, is a growing concern today. It has been found that the integrity of rockmass is reduced significantly from pre to post blast condition due to disregard paid to the surrounding rockmass. For exercising suitable engineering controls accurate measurement of the same is a prerequisite. To delineate the blast-induced rock damage zone seismic imaging technique can be used. Seismic imaging is a non-destructive method to determine the field seismic velocity of the rockmass for its characterisation. Further, it is considered that the blastinduced ground vibration is the root cause of rock damage due to structural dilation and should be controlled to localize its effect. Different researchers have related ground vibration to damage to arrive at the thresholds levels of damage. However, the accuracy in measurement/estimation of extent of damage is always a concern. This paper presents a case in which the seismic imaging technique was successfully used to delineate the extent of rock damage extent. The damage zone was also correlated with the vibration model developed to establish the same for its direct use in damage assessment.
Numerical Modelling of Blast Induced Fracture in Rock Masses
Proceedings of the Fifth International Conference on Engineering Computational Technology
Borehole blasting is a technique widely applied to form fracture networks in coal masses. There are many original cracks in coal masses, such as bedding planes and cleat planes. Coal masses are also subjected to high in-situ stress deep underground. However, the effects of pre-existing cracks and in-situ stress on blast-induced fractures are not well understood. In this paper, the isotropic and kinematic hardening plasticity model considering compression and tensile failure is introduced to numerical models, and the contact interface is used to simulate the effect of jointed planes on coal-mass blasting. The effects of jointed plane, constant in-situ stress and lateral pressure coefficient on blast-induced cracks are explored, and the relative peak displacement (Δu p) of jointed planes is considered as the basis for determining jointed plane failure. The results indicate that blast-induced cracks tend to expand along jointed planes in the coal mass; the in-situ stress enhances the compression effect and weakens the tension effect in the radial direction of the borehole; and the jointed plane failure zone in coal masses decreases with increasing in-situ stress. The lateral pressure coefficient also has a distinctive influence on blast-induced crack expansion.
Improved understanding of explosive-rock interactions using the hybrid stress blasting model
2012
Since 2001, the Hybrid Stress Blast Model (HSBM) project members have developed a software suite to model the complete blasting process from non-ideal detonation to muck pile formation. To preserve the physics and improve solution time, the breakage engine uses a combination of analytical models and 2D axisymmetric finite differences to model near-field crushing, coupled to 3D discrete lattice fracturing and distinct element numerical methods to model throw and muck pile development. The model has been validated by comparison with laboratory and field tests in kimberlite. Multiple blasthole simulations are used to demonstrate how changes to blasting parameters can influence downstream efficiencies. Case studies of wall control blasting show that the presplit design must balance the two opposing effects of increased damage with increased charge and decreasing attenuation of the seismic waves with decreasing charge. Modelling of decoupled explosives needs further development. Reducing...
Three dimensional numerical rock damage analysis under blasting load
Tunnelling and Underground Space Technology, 2013
In this study, the behaviour of rock mass subjected to blasting load is investigated using three dimensional finite difference numerical modelling. In the analyses, Mohr-Coulomb failure criterion has been used for the characterisation of the rock mass strength. Stresses acting on the borehole boundary have been simulated by an exponential function which reaches its maximum within a short time and then falls to zero value in a considerable period. The strain rate effect on the mechanical properties of rock material has also been taken into account in the analyses. Different explosive and site conditions have been studied to investigate the effects of loading rate and anisotropic high in situ stresses on blasting performance and blast induced damage zones. Results have shown that the most efficient explosive in rock blasting will be the one with low frequency content but with a sufficiently high borehole wall pressure. In addition, it has been verified that the directions and the magnitudes of major principle stresses affect the development of the crack zone around the borehole. Finally, it has been seen that proposed equation for the dynamic compressive strength of rock material fits very well to general suggestions.
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
Development of a continuum damage model for blasting analysis
International Journal of Rock Mechanics and Mining Sciences, 1997
This paper describes the development of a constitutive model for predicting rock damage due to explosive loading and the eonsequent fragment size distribution. The model is based on continuum mechanics and statistical fracture mechanics, assuming that the rock medium is an isotropic, continuous and homogeneous material with pre-existing micro cracks. Damage to the rock medium is defined as the probability of fracture at a given crack density which is obtained by integrating a crack density function over time. The minimum damage value at which the fragments may be formed is set by applying a micro crack coalescence criterion. Fragment size distribution is achieved by considering the equilibrium between kinetic energy and surface energy, with the changes in loading rate, material stiffness and damage taken into account. The simulation results are in accordance with the theory of explosive energy partitioning in rock medium and the damage zone, induced by the shock and stress waves, once established, remains stable. The model has been calibrated by field crater blasting and small scale bench blasting tests.