In the process of tunnel blasting excavation, radial cracks could be induced. To investigate the effect of loading rates on crack propagation velocity and rock initiation toughness, a new cracked ...tunnel specimen was proposed in this paper, and drop weight impact experiments were conducted. The crack propagation velocity and crack initiation time were measured by using crack propagation gauges (CPGs), and the measuring results were applied in the determination of initiation toughness. In order to validate the effectiveness of the cracked tunnel specimens and to predict the test results, finite difference numerical models were established by using AUTODYN code. The simulation results of crack propagation speeds and delayed fracture time generally agree with the experimental results, and crack arrest phenomena occur during crack propagation. The dynamic stress intensity factors (SIFs) were calculated by using ABAQUS code, and the initiation toughness under different loading rates was obtained by using experimental-numerical method. The studying results show that crack propagation speeds and initiation toughness increase with loading rates, but as the loading rate is larger than a certain value, the crack speeds tend toward a stable value; Delayed fracture time decreases with the increase of loading rates, and as the loading rate is larger than a certain value, it tends toward a stable value.
Vertical stresses in the earth's upper crust may be evaluated by the depth times the average unit weight of the overlying rock mass; however, the horizontal stress is difficult to obtain. Rock ...usually contains joints or cracks, and its fracture toughness is limited. If the horizontal stress acting on a cracked rock body exceeds a certain range, the crack will propagate and lead to rock fracture; and if a cracked rock is stable, the horizontal stress must be within a certain range. Therefore, from the stability condition of cracks, the range of horizontal stress can be evaluated. In this paper, a collinear crack model is employed to establish the cracked rock stability condition, and our theoretical results generally agree with the in-situ measurement results. The theoretical results can well explain the phenomenon that the ratio of horizontal stress to vertical stress near crust surface is scattered in a wide range, but in deep zone, it is scattered in a narrow range.
► Using a collinear crack model to establish rock stability condition ► Experimental study to validate the stability condition ► Using rock stability condition to investigate the range of horizontal stress ► Explaining well the in-situ measurement results
Freeze-thaw has great deterioration effect on rock mechanical performance, which strongly affect the stability of rock masses engineering. Meanwhile, natural fractures in real rocks are ubiquitous ...with considerable variations in size, number, and orientation, which further accelerate the risk of geological disasters. In this study, the microstructure changes of specimens induced by cyclic freeze-thaw were measured using nuclear magnetic resonance (NMR) technique. Dynamic fracture tests were performed using single cleavage triangle (SCT) red sandstone specimens. Crack propagation gauges (CPGs) were applied to determine the crack velocity. Meanwhile, the microstructure of the fracture surface was obtained with the aid of scanning electron microscope (SEM). The results show that the cyclic freeze-thaw plays an important part in rock dynamic fracture behavior. The porosity and crack velocity increase with freeze-thaw cycles, whereas the crack initiation time and dynamic fracture toughness decrease with freeze-thaw cycles. Sandstone has three pore types and is susceptible to the freeze-thaw weathering. The fracture surface of specimen without freeze-thaw is smooth and the fracture mode is transgranular. With the rise of freeze-thaw cycles, the fracture surface becomes rougher, indicating that intergranular fracture plays a dominant role in sandstone failure.
•Impact tests were conducted using large-size single cleavage triangle specimen to study the fracture process of rock.•The crack initiation time and crack propagation velocity of red sandstone subjected to freeze-thaw cycles were detected.•NMR technique was used to study the microscopic damage evolution of sandstone subjected to freeze-thaw cycles.
Aiming at the disadvantage that the traditional creep model cannot describe the nonlinear creep acceleration stage (third-order creep stage) of rock. This paper explains the creep process of salt ...rock from a microscopic perspective based on the Riemann–Liouville type fractional-order calculus operator theory and acoustic emission (AE) theory, and describes the creep process of salt rock with the improved fractional-order derivatives. The results of uniaxial creep damage tests on rock salt specimens under quasi-static loading conditions are given, complete creep damage curves are obtained, and a creep model based on fractional-order derivatives for viscoelastic damage of salt rock is proposed, and finally, the best variable values are fitted to determine the optimum values. The AE characteristic parameter curves were compared with the creep strain curves, and it was found that the AE characteristic curves could predict the time point when the salt rock enters the accelerated creep stage in advance. According to this time point, the model is fitted in sections and compared with the experimental results. The predicted value of the model is in good agreement with the test results, and can better describe the nonlinear accelerated creep stage of salt rock. It is believed that the fractional-order model can simulate the whole process of rock creep well and has good practical application value.
Highlights
•Established a viscoelastic-plastic damage creep model of rock salt based on fractional derivative.
•Based on AE technique, the creep process of rock salt was explained from the microscopic perspective and the damage evolution was obtained.
•The AE characteristic curve can predict the time point when the salt rock enters the accelerated creep stage in advance, and the model can be fitted to the segment according to this time point, which can better describe the nonlinear accelerated creep stage of the salt rock.
•Segmental fitting can well simulate the whole process of salt rock creep, has good superiority and reliable, which can predict engineering disaster in advance.
In large scale rock blasting operations, such as in mining, quarrying and tunneling, there are usually a number of blast holes simultaneously, and the explosive charges are fired in different ...sequences. This indicates that the later-fired holes may have effect on the propagation of the cracks induced by the first fired blasting. This paper tried to investigate the effect of two empty holes on the behavior of the cracks propagating between them, and blasting experiments were conducted by using circular sandstone and PMMA specimens with a centralized blast hole, a pre-crack and two empty holes. The two-hole spacing was designed in the range from 10 mm to 45 mm, and the outer diameter of the circular specimens was 600 mm for sandstone and 400 mm for PMMA. The central blast hole was charged with a detonator and it was fired to provide dynamic loading which drives the pre-crack to propagate. Meanwhile, by using linear equation of state and the major principal stress failure criterion, numerical models were established by using AUTODYN code, and the effect mechanism of empty holes on crack propagation behavior was studied numerically. The crack propagation paths predicted by the numerical models agree well with the experimental results. The results show that: (a) Empty holes have the arrest function on outgoing cracks, and the arrest function depends on the two hole spacing; the shorter the two hole spacing is, the stronger the arrest function is; (b) As two hole spacing is less than a certain value, outgoing cracks can be completely arrested; (c) The stress perpendicular to the propagating crack plays a key role in arresting cracks.
•The RHT model was used to simulate the rock material and was calibrated.•The stress initialization was achieved in the first step of the simulation.•In-situ stresses can increase the compressive ...stress and reduce the tensile stress.•Both the crushed zone and the cracked zone are inhibited by in-situ stresses.
High in-situ stress can limit the generation of rock fractures induced by blasting, which usually shows different states of rock fragmentation with those under low-stress conditions. In this paper, the stress distribution around the blasthole under coupled in-situ stress and blasting load was theoretically analyzed. Then, the single-blasthole blasting process, which is calibrated by field blasting tests, was numerically investigated using the Riedel-Hiermaier-Thoma (RHT) model, and the effects of in-situ stress magnitudes and lateral pressure coefficients on the crushed zone and the crack propagation were investigated. After that, influences of lateral pressure coefficients, buried depths, and blasthole layouts on the behavior of double-blasthole blasting were studied. It is concluded that in-situ stresses can increase the compressive stress and reduce the tensile stress caused by blasting load. The area of the crushed zone decreases with increasing in-situ stresses. The crushed zone is elliptical in shape in anisotropic pressure conditions. The gap between the long axis and the short axis of the crushed zone widens as the difference between the stress in the horizontal and vertical directions increases. Cracks preferentially propagate in the higher stress direction. At a buried depth of 1000 m, connecting cracks can be formed at lateral pressure coefficients ranging from 0.25 to 3.0 when blastholes are drilled along the horizontal direction. The rise in buried depths and the angle between the centerline of adjacent blastholes and the higher stress direction can limit the formation of connecting cracks. The research results can provide guidance for analyzing the behavior of rock blasting in deep underground.
Zonal disintegration is a curious phenomenon and has received considerable attention recently. Using traditional static mechanics cannot well explain the phenomenon of the alternately distributed ...cracked zones. In this paper, the process of a slowly unloading P-wave reflecting from a free surface has been investigated and the result shows that as a slowly unloading P-wave encounters a free surface, it could cause the phenomenon of zonal disintegration. In order to confirm this result, two numerical models of defected rock around an excavation under the action of a slowly unloading P-wave are established. The slowly unloading P-wave is induced by a steel plate impacting the target rock with a proper speed, and their interaction lasts for a long time. The simulation results show that if the strength of slowly unloading P-waves is in a certain range, the phenomenon of zonal disintegration could be observed, and if the incident wave is not a slowly unloading P-wave, such as a rectangle wave, the phenomenon of zonal disintegration will not occur.
•Slowly unloading P-waves can induce material discrete failures.•Numerical simulation results show that zonal disintegration could happen.•As P-wave amplitude is in a certain range, it could cause zonal disintegration.•Quickly unloading P-waves cannot induce zonal disintegration.
Under compression, cracks extend, branch and coalesce. These fracturing processes have received much attention recently. In this paper, an attempt is made to find the analytical solution of stress ...intensity factors for the special case of cracks situated along a straight line, and to set up a fracture criterion. Under compression, cracks close and the crack surface friction can resist crack surface sliding. Considering crack surface friction, a set of complex stress functions is proposed for the special case of cracks situated along a straight line. The analytical solution is formulated, and for the case of only two collinear cracks inside an infinite plate, the exact analytical solution of stress intensity factor is presented. Finally, an alternative form of crack propagation criterion for two collinear cracks under compression is developed, which is expressed in terms of principal stresses. For the case of materials without pre-existing macrocracks, this new propagation criterion becomes the well known Coulomb—Mohr criterion.
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.
