Failure mechanism and coupled static-dynamic loading theory in deep hard rock mining: A review

Rock failure phenomena,such as rockburst,slabbing(or spalling) and zonal disintegration,related to deep underground excavation of hard rocks are frequently reported and pose a great threat to deep mining.Currently,the explanation for these failure phenomena using existing dynamic or static rock mechanics theory is not straightforward.In this study,new theory and testing method for deep underground rock mass under coupled static-dynamic loading are introduced.Two types of coupled loading modes,i.e.'critical static stress + slight disturbance' and 'elastic static stress + impact disturbance',are proposed,and associated test devices are developed.Rockburst phenomena of hard rocks under coupled static-dynamic loading are successfully reproduced in the laboratory,and the rockburst mechanism and related criteria are demonstrated.The results of true triaxial unloading compression tests on granite and red sandstone indicate that the unloading can induce slabbing when the confining pressure exceeds a certain threshold,and the slabbing failure strength is lower than the shear failure strength according to the conventional Mohr-Column criterion.Numerical results indicate that the rock unloading failure response under different in situ stresses and unloading rates can be characterized by an equivalent strain energy density.In addition,we present a new microseismic source location method without premeasuring the sound wave velocity in rock mass,which can efficiently and accurately locate the rock failure in hard rock mines.Also,a new idea for deep hard rock mining using a non-explosive continuous mining method is briefly introduced.

Dynamic tensile behaviour and crack propagation of coal under coupled static-dynamic loading

The fracture behaviour and crack propagation features of coal under coupled static-dynamic loading conditions are important when evaluating the dynamic failure of coal.In this study,coupled static-dynamic loading tests are conducted on Brazilian disc(BD)coal specimens using a modified split Hopkinson pressure bar(SHPB).The effects of the static axial pre-stress and loading rate on the dynamic tensile strength and crack propagation characteristics of BD coal specimens are studied.The average dynamic indirect tensile strength of coal specimens increases first and then decreases with the static axial pre-stress increasing.When no static axial pre-stress is applied,or the static axial pre-stress is 30%of the static tensile strength,the dynamic indirect tensile strength of coal specimens shows an increase trend as the loading rate increases.When the static axial pre-stress is 60%of the static tensile strength,the dynamic indirect tensile strength shows a fluctuant trend as the loading rate increases.According to the crack propagation process of coal specimens recorded by high-speed camera,the impact velocity influences the mode of crack propagation,while the static axial pre-stress influences the direction of crack propagation.The failure of coal specimens is a coupled tensile-shear failure under high impact velocity.When there is no static axial pre-stress,tensile cracks occur in the vertical loading direction.When the static axial pre-stress is applied,the number of cracks perpendicular to the loading direction decreases,and more cracks occur in the parallel loading direction.

Numerical analysis of deformation and failure characteristics of deep roadway surrounding rock under static-dynamic coupling stress

In actual production,deep coal mine roadways are often under typical static-dynamic coupling stress(SDCS)conditions with high ground stress and strong dynamic disturbances.With the increasing number of disasters and accidents induced by SDCS conditions,the safe and efficient production of coal mines is seriously threatened.Therefore,it is of great practical significance to study the deformation and failure characteristics of the roadway surrounding rock under SDCS.In this paper,the effects of different in-situ stress fields and dynamic load conditions on the surrounding rock are studied by numerical simulations,and the deformation and failure characteristics are obtained.According to the simulation results,the horizontal stress,vertical stress and dynamic disturbance have a positive correlation with the plastic failure of the surrounding rock.Among these factors,the influence of the dynamic disturbance is the most substantial.Under the same stress conditions,the extents of deformation and plastic failure of the roof and ribs are always greater than those of the floor.The effect of horizontal stresses on the roadway deformation is more notable than that of vertical stresses.The results indicate that for the roadway under high-stress conditions,the in-situ stress test must be strengthened first.After determining the magnitude of the in-situ stress,the location of the roadway should be reasonably arranged in the design to optimize the mining sequence.For roadways that are strongly disturbed by dynamic loads,rock supports(rebar/cable bolts,steel set etc.)that are capable of maintaining their effectiveness without failure after certain dynamic loads are required.The results of this study contribute to understanding the characteristics of the roadway deformation and failure under SDCS,and can be used to provide a basis for the support design and optimization under similar geological and geotechnical circumstances.

Mechanical properties of rock under coupled static-dynamic loads

Rock drilling machine,INSTRON testing system,and SHPB device are updated to investigate the characteristics of rocks at great depth,with high loads from overburden,tectonic stresses and dynamic impacts due to blasting and boring.It is verified that these testing systems can be used to study the mechanical properties of rock material under coupled static and dynamic loading condition and give useful guidance for the deep mining and underground cavern excavation.Various tests to determine the rock strength,fragmentation behavior,and energy absorption were conducted using the updated testing systems.It is shown that under coupled static-dynamic loads,if the axial prestress is lower than its elastic limit,the rock strength is higher than the individual static or dynamic strength.At the same axial prestress,rock strength under coupled loads rises with the increasing strain rates.Under coupled static and dynamic loads,rock is observed to fail with tensile mode.While shear failure may exist if axial prestress is high enough.In addition,it is shown that the percentage of small particles increases with the increasing axial prestress and impact load based on the analysis of the particle-size distribution of fragments.It is also suggested that the energy absorption ratio of a specimen varies with coupled loads,and the maximum energy absorption ratio for a rock can be obtained with an appropriate combination of static and dynamic loads.

Dynamic response and failure behavior of rock under static-dynamic loading

Dynamic response and failure behavior of rock under static-dynamic loading were studied. The effects of initial static load on the total energy dissipated during the failure process of specimen were analyzed. To simulate the engineering situation that in-situ rock experienced and obtain the dynamic loading with an intermediate strain rate, a low cycle fatigue load with the frequency from 0.5 to 5 Hz was adopted by servo-controlled Instron material testing system. The results show that the obtained strain rate increase with the increase of load frequency. The initial static load has great influence on both the energy and dynamic response of rock. Both the energy and the maximum failure load P_f decreases with the increase of initial static load. P_f under the static-dynamic loading is larger than that under only the static loading but less than that under only the dynamic loading. The load-displacement curves become nonlinear as the pre-added static load reaches the transition point which is about one third of static strength. With the increase of initial static load, Young’s modulus decreases and poisson ratio increases. It shows that rock has a lower strength and a tendency to soften under a higher initial static load. Rock may be broken more easily static-dynamic loading than under only the dynamic loading. The proposed method is useful in the investigation of constitutive relationship and failure behavior of rock under quasi-dynamic loading.

Crack propagation mechanism of compression-shear rock under static-dynamic loading and seepage water pressure

To reveal the water inrush mechanics of underground deep rock mass subjected to dynamic disturbance such as blasting, compression-shear rock crack initiation rule and the evolution of crack tip stress intensity factor are analyzed under static-dynamic loading and seepage water pressure on the basis of theoretical deduction and experimental research. It is shown that the major influence factors of the crack tip stress intensity factor are seepage pressure, dynamic load, static stress and crack angle. The existence of seepage water pressure aggravates propagation of branch cracks. With the seepage pressure increasing, the branch crack experiences unstable extension from stable propagation. The dynamic load in the direction of maximum main stress increases type I crack tip stress intensity factor and its influence on type II crack intensity factor is related with crack angle and material property. Crack initiation angle changes with the dynamic load. The initial crack initiation angle of type I dynamic crack fracture is 70.5°. The compression-shear crack initial strength is related to seepage pressure, confining pressure, and dynamic load. Experimental results verify that the initial crack strength increases with the confining pressure increasing, and decreases with the seepage pressure increasing.

Supporting characteristics analysis of constant resistance bolts under coupled static-dynamic loading

To study the tensile mechanical properties of constant resistance bolts, the RFPA(Rock Failure Process Analysis) statics software is used to perform a uniaxial tensile test on a constant resistance bolt. The numerical test results show that the plastic strain value is 12 times the magnitude of the elastic strain. During plastic deformation, the fluctuation in the stress magnitude is relatively stable, indicating that the bolt has good constant resistance characteristics. The numerical test results are in good agreement with the laboratory test results of M.C. He, and the accuracy and reliability of the numerical test method are verified. Therefore, the RFPA software with coupled static-dynamic loading is further adopted to study the supporting effects of traditional bolts and constant resistance bolts under coupled staticdynamic loading. The numerical comparison of the test results show that the constant resistance bolts can effectively control the deformation amount and rate of the laneway surrounding rock, reduce the total and rate of increase in the accumulated acoustic emissions,decrease the stress on the units in the model and protect the stability of the laneway. This paper verifies that a constant resistance bolt has better impact resistance mechanical properties than those of a traditional bolt and provides an effective way to control rock burst and soft rock that is prone to large deformation damage.

Global stability coefficient of large underground caverns under static loading and earthquake wave condition

Underground energy and resource development,deep underground energy storage and other projects involve the global stability of multiple interconnected cavern groups under internal and external dynamic disturbances.An evaluation method of the global stability coefficient of underground caverns based on static overload and dynamic overload was proposed.Firstly,the global failure criterion for caverns was defined based on its band connection of plastic-strain between multi-caverns.Then,overloading calculation of the boundary geostress and seismic intensity on the caverns model was carried out,and the critical unstable state of multi-caverns can be identified,if the plastic-strain band appeared between caverns during these overloading processes.Thus,the global stability coefficient for the multi-caverns under static loading and earthquake was obtained based on the corresponding overloading coefficient.Practical analysis for the Yingliangbao(YLB)hydraulic caverns indicated that this method can not only effectively obtain the global stability coefficient of caverns under static and dynamic earthquake conditions,but also identify the caverns’high-risk zone of local instability through localized plastic strain of surrounding rock.This study can provide some reference for the layout design and seismic optimization of underground cavern group.

Experimental study on the irreversible displacement evolution and energy dissipation characteristics of disturbance instability of regular joints

To investigate the disturbance-induced shear instability mechanism of structural catastrophe in the deep rock mass,MTS 815 material testing machine was used to carry out quasi-static loading tests and disturbance shear tests on symmetrical regular dentate joints of two materials at three undulation angles under specific initial static stress,disturbance frequency,and peak value.The test results indicate that:(i)the total ultimate instability displacement is only related to the intrinsic properties of the joints but not to the initial static stress and disturbance parameters;(ii)the cumulative irreversible displacement required for the disturbance instability conforms to the logistic inverse function relationship with the number of disturbances,displaying the variation trend of“rapid increase in the front,stable in the middle,and sudden increase in the rear”;(iii)the accumulation of plastic deformation energy is consistent with the evolution law of irreversible displacement of joints and the overall proportion of hysteretic energy is not large;(iv)the dissipated energy required for the instability of each group of joints is basically the same under various disturbance conditions,and this energy is mainly controlled by the initial shear stress and has no connection with the disturbance parameters.The stability of the total disturbance deformation and the disturbance energy law of the joints revealed in the tests provide data support for reasonably determining the disturbance instability criterion of joints.

Safety evaluation and the static-dynamic coupling analysis of counter-arched slab in plunge pool

The stability of plunge pool slab of drainage structure is crucial to energy dissipation,but the working conditions of counter-arched slab are very complex and its stress procedure is considerably nonlinear and coupling.Therefore,a nonlinear static and dynamic coupling method to analyze the counter-arched slab configuration is put forward,which can reflect the coupling of dynamic loads,slab,anchor bars,groundwork and abutment as well as the dynamic procedure of instability.Various nonlinear factors and static-dynamic coupling are taken into consideration,and the working conditions,stress mechanism,dynamic instability procedure and influential instability factors are revealed.The proposed method thus provides a comprehensive safety evaluation method of plunge pool,which further pro-vides an important theoretical basis for engineering design of counter-arched slab in plunge pool.

Catastrophe model and its experimental verification ofstatic loading rock system under impact load

According to the catastrophe model for impact buckling of static loading structures, a new catastrophe model for impact loading failure of a static loading rock system was established, and one dimension (1D) catastrophe model was analyzed. The analysis results indicate that the furcation collection where catastrophe may take place is not only decided by mechanical system itself but also relates to exterior loading, which is different from the results obtained under mono-static loading where the bifurcation collection is only determined by mechanics of the system itself and has nothing to do with exterior loading. In addition, the corresponding 1D coupled static-dynamic loading experiment is designed to verify the analysis results of catastrophe model. The test is done with Instron 1342 electro-servo controlled testing system, in which medium strain rate is caused by monotony rising dynamic load. The parameters are obtained combining theoretical model with experiment. The experimental and theoretical curves of critical dynamic load vs static load are rather coincided, thus the new model is proved to be correct.

Self-initiated static-dynamic state transition behavior and triggering mechanism of strain rockburst using three-dimensional discrete element method

The accurate understanding of rockburst mechanism poses a global challenge in the field of rock mechanics.Particularly for strain rockburst,achieving self-initiated static-dynamic state transition is a crucial step in the formation of catastrophic events.However,the state transition behavior and its impact on rockburst have not received sufficient attention,and are still poorly understood.Therefore,this study specifically focuses on the state transition behavior,aiming to investigate its abrupt transition process and formation mechanism,and triggering effects on rockburst.To facilitate the study,a novel burst rock-surrounding rock combined laboratory test model is proposed and its effectiveness is validated through experiment verification.Subsequently,corresponding numerical models are established using the three-dimensional(3D)discrete element method(DEM),enabling successful simulation of static brittle failure and rockbursts of varying intensities under quasi-static displacement loading conditions.Moreover,through secondary development,comprehensive recording of the mechanical and energy information pertaining to the combined specimen system and its subsystems is achieved.As a result of numerical investigation studies,the elastic rebound dynamic behavior of the surrounding rock was discovered and identified as the key factor triggering rockburst and controlling its intensity.The impact loading on the burst rock,induced by elastic rebound,directly initiates the dynamic processes of rockburst,serving as the direct cause.Additionally,the transient work and energy convergence towards the burst rock resulting from elastic rebound are recognized as the inherent cause of rockburst.Moreover,it has been observed that a larger extent of surrounding rock leads to a stronger elastic rebound,thereby directly contributing to a more intense rockburst.The findings can provide novel theoretical insights for the exploring of rockburst mechanism and the development of monitoring and prevention techniques.