Study On The Interaction Between Blast Wave And The Rock Mass With Structural Plane And The Induced Crack Propagation

With the rapid development of geotechnical engineering in China,higher requirements have been put forward on the rock blasting technology.On the one hand,make the energy released along the pre-blasting direction as much as possible to maximize the efficient energy release rate of rock breaking;On the other hand,control the direction of blast induced crack propagation effectively to reduce the damnification of the remaining rock mass.Based on the above purpose,grooving borehole,explosive cartridge with energy grooves and the other direction fracture blasting technologies have been proposed.However,the rock fragmentation has been greatly affected,due to the presence of the structural planes in natural rock mass,such as joints,fractures,holes and so on,which has a significant influence on both the spread of blast wave and the propagation of induced cracks.Especially in the well developed jointed rock masses,a well contour blasting is very hard to form.Thus,to solve this problem,the interaction mechanism between blast waves and the rock masses with structural plane,as well as the induced crack propagation behavior,should be studied firstly.In this paper,theoretical analysis,experimental study and numerical modeling are carried out to study the mechanism of blast wave transformation in the rock masses with structural plane,the interaction mechanism between induced crack and the structural plane in rock masses,and the propagation behavior of induced crack.The main conclusions are as follows:Based on the stress wave theory in continuous medium,the analytical equations for the reflect coefficient of reflected waves were derived when the P waves/S waves encountered the dynamic crack with different angle.When the propagation direction of the explosive P-wave is perpendicular to the propagation direction of the dynamic crack,the superposition of both reflect tensile P-wave and the incident P-wave generated at the crack surface,the stress field continusly change from compression to tensile,making the dynamic crack presents a "wave-like" fracture pattern during tcrack propagation process.While when the P wave obliquely enter the dynamic crack,the stress field near the crack tip changed from the pure tensile stress field to the combined stress field,making the crack propagates toward the origin of the P waves.When the propagation direction of the P wave is opposite to the propagation direction of the moving crack,only the reflect P wave generated along the crack surface,and it is equivalent to applying a constant stress parallel to the crack surface in the stress field of the crack tip,which has a great influence on the crack propagation angle and the fracture toughness.It is the transformation between the P waves and the S waves when the wave encountering structural plane that change the stress distribution character in rock masses,which determined both the position and the propagation angle of the running crack.Numerical simulate the evolution process of the P-wave / S-wave interact with cracks at three different angles of 0 °,45 ° and 90 °,respectively,which visualized the process of wave transformation for the interaction between P-wave / S-wave and the crack.Both the law of particle vibration near the crack tip,the character of stress distribution and the fracture mode of induced crack was obtained.Laboratory experiments was carried out to analyze the fracture mechanism of the blast-induced cracks under both the grooving borehole blasting and the ordinary borehole blasting.When the crack vertically propagates to the non-filled joint,the crack stoped abruptly at the joints,and wing cracks generated from the two ends of the crack,which propagates along the same direction as the main crack.Combined with the numerical results,it was found that the open crack obstacled the propagation of blast waves,which producing a diffraction wave at both ends of open-crack,and forming a stress concentration at the crack ends,and,in turn,leding a wing crack propagates from both ends of the non-filled joint.The stress behind the non-filled joint is introduced by the diffraction waves originating from the interaction between the blast wave and the non-filled joint.Compared to the ordinary borehole blasting,in grooving borehole blasting,the blast-induced crack initiates from the grooving direction is 10 ?s earlier than the non-grooving direction,leding the blast energy preferentially released along the grooving direction;the initiation of wing crack is mainly mode-I failure mode,while the initiation of wing crack is mainly type II failure mode;Both the fracture velocity and the stress intensity factor KI d of the crack presents a decrease trend,which resulting in a 22.7% and 17.8% increase in the crack growth duration and the crack length,respectively,compared to ordinary borehole blasting.The dynamic fracture character of the crack during the interaction between the mode I running crack and the hole is analyzed experimentally.Both the velocity and the dynamic stress intensity factor of the running crack decreased with the crack propagating toward the adjacent hole.The bigger the hole is,the more the suppression effect is.After the crack-hole coalescence,the running crack arrest abruptly and the crack tip become blunt,the initiation toughness of the blunt crack is increased to 9.58%~13.87%,the crack velocity and dynamic stress intensity factor shows an obvious jump after crack initiates from the hole.Fracture surface analysis indicates that the fracture face is smooth and propagate path is straight before the coalescence of the crack and the hole;after the running crack initiates at the top of the round hole,the fracture face is relatively bumpy and the propagate path is relatively twist.There exists a critical diameter Dc,which would store maximum strain energy,leding a remarkable blunt effect.Experimentally studied the interaction mechanism between the filled structure plane and the dynamic crack.The results show that,due to the difference of the loading rate and the angle of the structural plane,there are three possible fracture mode when the crack endures the weak plane,that is,propagates through the weak plane directly,diverts to fracturing along the weak plane,and propagates through the weak plane after the crack fracturing along the weak plane for a short distance.It is the combination between evolvation of the fracture behavior in time and the superposition of the induced cracks in space during the changing of the stress loading rate and the angle of structural planesa that formed the intensive blast crack nets in the jointed rock mass.Combined with the Split Hopkinson Pressure Bar(SHPB)and the Digital Image Correlation(DIC)method,the influence of the stress intensity,the angle of the weak plane and the strength of weak plane on the fracture behavior of dynamic crack was analyzed quantitatively.The evolution of displacement field and strain field during the interaction between the dynamic crack and the weak plane was obtained.With the increase of stress loading rate,the crack velocity increases and more crack propagates through the weak plane,while the divert distance decreases.The offset value of the crack in the direction of the structure plane decreased nonlinearly with the increase of the joint angle.When the angle of structure plane ranging from 30 ° to 60 °,the offset distance of crack is almost constant,while it linearly decreased when the angle of structure plane is beyond 60 ° or less than 30 °.When the loading force along the structural plane is lower than the tensile strength of the structural plane,the offset distance along the structural plane is controlled by the energy release rate along the crack propagation direction.When the loading force along the structural plane is higher than the tensile strength of the structural plane,the offset distance is determined by the magnitude of the loading force along the structrual plane.The fracture behavior and its interaction mechanism of two cracks propagating in opposite direction under tensile stress loading was studied.The caustic experimental results show that the two cracks first propagate opposite along the vertical distance before they encountered each other in space,and then propagate to the other crack surface after they encountered in space.Combined the results of dynamic photoelastic experiment and the numerical results,it is found that the constant stress(T stress)along the direction of the crack surface is significantly increased during the interaction of two opposing propagating cracks,which significantly influence the fractured angle of crack,the crack velocity and the fracture toughness.When the T stress is negative,the fractured angle of the crack decreases,as well as the vertical distance of the crack.While,the fractured angle of crack increased for a positive T stress,and the vertical distance increased correspondingly.Therefore,three parameters(KI,KII and T stress)should be adopted to determine the crack fracture character,such as the fractured angle,the fracture length,during the interaction of multiple cracks.In actual blasting engineering,by changing the stress state in rock masses,increasing the negative T stress along the crack growth direction,which helps to control the crack direction and realizes fine blasting construction.On the contrary,increasing the positive T stress can attribute to extending the range of rock fragmentation by blasting and improving the effect of loose blasting.Numerically studied the influence of the ratio of principle stress,the vertical spacing between cracks and the mechanics properties of material on the fracture behavior of two opposing fractured cracks.When the distance between two crack tips is far away from each other,the Tresca stress of crack tip changed from butterfly shape to flat as the ratio of principle stress increasing,which expanding the interaction distance between two cracks.When the distance between two crack tips is relatively small,altering the ratio of principal stress would have few influence on the change of stress field parameter of crack tip(KI,KII and T stress),as well as the path of crack propagation.The maximum vertical deviation distance from its initial distance between two crack tips decreased non-linearly with the increasing of the initial vertical distance between two cracks.There exists a critical vertical distance S_c,when the initial vertical distance of two cracks S is less than the critical vertical distance S_c,the maximum offset distance Wmax along the vertical distance is basically unchangeable.When S is greater than the S_c,the Wmax decreased with the increasing S.With the increase of elastic modulus and the reduce of poisson's ratio,the critical vertical distance S_c decreased.For the marble,granite and polymethyl methacrylate(PMMA)materials,the significance level of two opposing cracks under the same initial vertical distance can written as: marble < granite < polymethyl methacrylate(PMMA).Applying T stress theory into the study of the interaction mechanism between the blast waves and blast-induced cracks.When the propagation direction of blast-induced crack is opposite to the propagating direction of P wave,the effect of P wave on blast-induced crack is equivalent to applying a negative T stress at the crack tip,which increasing the fracture toughness,blocking the propagation of crack,and decreasing the crack velocity.On the contrary,when the propagation direction of blast-induced crack is accordance with the propagating direction of P wave,the fracture toughness decreased,while the crack velocity increased.From the experimental point of view,the crack velocity decreased when the the dynamic crack encountered S wave,which significantly changing the propagation direction of crack.