| Rock burst is one of the main disasters that threaten the safe and efficient mining in Chinese coal mines.Statistical analysis shows that most of rock bursts occur in mining gateways.The existing research on rock burst has focused on the occurrence mechanism,forecast and controlling strategies,which can not completely prevent rock burst due to the complexity of occurrence mechanism,the diversity of influencing factor,and the uncertainty of bursting location.As the last barrier to prevent rock burst,anchorage bearing structure plays a crucial role in stability of roadway surrounding rock,which is composed of pre-tensioned bolt support system and its reinforced rock.The failure mechanism of bursting roadway is not the same as that of static loading roadway,as the anchorage bearing structure not only bears large static load but also dynamic load in bursting roadway.In this paper,based on the bursting roadway in Yima mining area with large mining depth,high mining-induced stress,dynamic load disturbance in roof and high rock burst tendency of coal,burst failure characteristics,failure mechanism and control technology of anchorage bearing structure under dynamic load disturbance have been studied by comprehensive methods of field measurement,laboratory test,theoretical analysis and numerical simulation.The main conclusions and innovative results are as follows:(1)The geomechanical field test of surrounding rock of bursting roadways in Yima mining area was carried out,and the bursting characteristics and reasons of anchored rock were statistically analyzed.The main bursting characteristics are:(1)Large transient deformation of surrounding rock;(2)serious floor heave;(3)The degree of deformation and failure positively related to the energy of source;(4)serious slow-deformation of surrounding rock before bursting failure;(5)Serious failure of bolt support system.The roadway after rock burst is characterized as the propagation and coalescence of discontinuities in surrounding rock,the reduction of strength and integrity of surrounding rock and the decrease of anchoring performance.The decrease of anchoring performance is manifested as the degradation of bond in anchoring interface,the plastic deformation of bolts,the drop of tension strength,extensibility and impact absorbing power of bolts,and the damage of microstructure in bolts which is characterized by that the grains in bolts are distorted and elongated with shearing and sliding,and the metallographic structure is disordered.The main reasons are: concentration of high static-loading stress,dynamic load disturbance in roof,structural characteristics of "external fragile and internal brittle" and unreasonable bolt supporting system.(2)The numerical model of gateway under combined effect of dynamic and static loads was established.And the dynamic-loading response and evolution process of bursting failure for anchorage bearing structure was numerically analyzed.The results are as follows: the failure of anchored rock by expansion deformation or anchoring system are easily resulted from repeated effect of compression and tension induced by dynamic load,finally causing bursting failure of anchorage bearing structure;The main failure factor of anchorage bearing structure is the stress difference at different depths of surrounding rock induced by stress wave for roof,while the main factor is the difference of dynamic response for rib.The evolution process of bursting failure of anchorage bearing structure is illustrated as: the dynamic-loading disturbance in roof? the damage in shallow surrounding rock ? the failure of anchoring system? the disabled control of anchorage bearing structure to deep surrounding rock? the sudden increase of the damage range of surrounding rock? the instantaneous release of energy in deep surrounding rock ? the bursting failure of anchorage bearing structure.(3)The criterion of bursting failure for anchorage bearing structure under dynamic load was put forward.The influences of single factor and interaction of multiple factors to stability of anchorage bearing structure were quantitatively analyzed using response surface method.The results are as follows: the bursting failure of anchorage bearing structure will happen when both satisfying the dual overload conditions of stress and energy.The stress overload is necessary condition,which means the superposition of the dynamic and static load in surrounding rock is greater than the bearing capacity of anchorage bearing structure.And the energy overload is sufficient condition,which means the superposition of residual energy from mine earthquake and elastic energy released from surrounding rock is larger than that needed for bursting failure of anchorage bearing structure,with the surplus energy converted to bursting kinetic energy.The amount of deformation for anchorage bearing structure is heavily influenced by the single factor such as the strength of surrounding rock,the source distance,the source intensity,in-situ stress,supporting strength and their interactions.The failure modes of anchorage bearing structure can be categorized as bolt breaking,anchorage debonding,rock dominating and the compositing mode under different combinations of influencing factors.(4)The control system of anchored rock for bursting roadway was put forward,including stability controlling principle of “reducing load-resisting burst-absorbing energy”,multilevel controlling technologies of “ releasing pressure for deep rock-strong support for shallow rock-protection for surface rock” and new anti-bursting bolts material.Releasing pressure for deep rock can reduce the degree and scope of stress concentration,reduce the magnitude of energy release during the burst failure of deep surrounding rock,increase the distance and attenuation coefficient of dynamic load propagation,slow down the disturbance of the rock vibration load on the anchorage bearing structure,and reduce the risk of burst failure of the anchorage bearing structure.For different failure modes,the ability of resisting burst and absorbing energy of anchorage bearing structure are effectively improved by means of lengthened or full-length anchorage with high prestress,the combined support of long and short bolts with super high strength and full-section support.The breaking load of newly developed connected bolt,which has a super high strength and high impact toughness,equals to that of SKP22-1/1770 type cable,with the average elongation(19.5%)and the impact absorbing power(128J)far larger than cable.(5)The field practice of stability control for anchorage bearing structure under dynamic load was carried out.The control effect was verified by mine earthquake simulated by artificial blasting.The monitoring result shows that under the dynamic load resulted from blasting,the monitoring area is failure by tension,with the shallowdamage greater than the deep in anchored rock.Due to the failure of anchored rock,the axial forces of most of bolts and cables drop suddenly.The loss rate of axial force decreases not only with the increase of pretensioned force,but also exponentially with the increase of distance to source.Under the effect of blasting load,rock burst has not occurred in the monitoring surrounding rock,with the deformation of roadway and the forces of bolts and cables both within the allowable range,which verifies the rationality of theoretical analysis and controlling strategies. |
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