Research On Coal Bump Prevention Using Energy-absorption Bolts And Its Application

There are lots of fracture or damage of bolts and cables in bump-prone roadway,which has become a potential seriously restricts the safety mining.In order to solve the problem issued above,the failure characteristic and mechanism of rockbolt subjected to impact loads were analyzed through methods combined with field investigation,theoretical analysis,numerical simulation,laboratory test and field tests.And the method of coal bump prevention using energy-absorption bolt was also proposed.Furtherly,the influence of energy-absorption bolts on forming process of coal bump and the dynamic responses of rock mass anchored with energy-absorption bolts under the action of impact loads were conducted systematically.(i)Failure characteristic and mechanism of rockbolts under the action of impact loads.On the basis of a large number of damage investigations on bump-prone roadways supported by bolts,it is found that there are four typical failure forms for rockbolts under the action of impact loads:(1)tensile break of bolt shank near plate face,which leads to the loss of bearing plate;(2)debonding between bolts and rock,which results in the bolts sliding from borehole;(3)failure of shallow surrounding rock,which will contribute to the loss of anchored rock mass;(4)overall failure of rock and bolts.The failure mechanism of bolted rock structure under the action of impact loads was studied by theoretical analysis and numerical simulation.Main factors which influence the dynamic failure of bolted rock structure were also analyzed.The results showed that under the action of impact loads,a phenomena of non-synchronous vibration appeared in bolted rock for bolt and rock,here the phenomena is defined as “time difference effect”.Due to this effect,the dynamic interaction between bolts and rock occurred,which would trigger the debonding along bolt and rock interface or the break of bolt shank.Time difference effect was dramatically affected by grouted encapsulated length of bolts.Along with the increasing of bond length,the contribution of time difference effect to the failure model of bolted rock decreased obviously,which resulted in the synchronous vibration between bolts and rock,and eventually avoided the debonding between bolts and rock.With the increase of velocity of impact loads,the contribution of this effect also decreased,and the failure mode of bolted rock transformed from the type of debonding or shank break into the type of overall failure of bolts and rock.The resonance phenomenon presented as the excitation frequency approaching natural frequency of bolted rock structure,the bolts or its surrounding rock would be damaged even under the action of impact load with a relative small peak value.For a given bolted rock structure,an excitation load with a critical frequency would cause a half vibration period between bolt and rock,which would result in greater relative vibration velocity between bolt and rock,and finally yield the debonding or shank break.(ii)The principle of bolts supporting in bump-prone roadways was put forward,and the distributed energy-absorption bolt was invented.It was pointed out that two aspects should be considered for the support of bump-prone roadways.On one hand,some methods should be made to enhance the strength and improve anti-shock capability of surrounding rock.On the other hand,support structure with the energy-absorption components should be designed,which can quickly absorb impact energy and reduce the impact strength on support structure and surrounding rock.From this perspective,the distributed energy-absorption bolt was invented,which consists of energy absorption sleeve with built-in hex pipe,whorl–steel bolt with friction roller on the head,prestressed strand and energy absorption devices at bolt tail.Then mechanical analysis on the hex pipe extrusion deformation is made by using plastic bending theory and numerical simulation method,during which it was explored that the influence law of geometric parameters and assembly form of hex pipe and friction roller on the working resistance of energy absorption bolt.Ulteriorly,structure parameters of energy-absorption bolt were determined based on the mechanical properties of common bolt under the condition of static stretching,tensile impact and prestressed impact tension.And the static tensile test and impact tensile test on energy absorption bolt were conducted by using home-grown static and dynamic combination loading system,the test result shows that energy absorption bolt has good self-protection and adaptive of impact compared with common bolt.(iii)The influence law of energy-absorption bolt on forming process of coal bump was studiedBased on instability theory,coal bump in circular tunnel supported by bolts was analyzed with analytical solution by simplifying bolt support action as compressive stress no surrounding rock in plastic softening region.And effects of bolt intervals on forming process of coal bump were discussed.The results show that,when the burst tendency of coal is moderate,with the increase of bolt density,the depth of critical resistance zone,the critical load and release energy become greater,which cause the stronger damage of coal bump,and the phenomenon is more obvious as burst tendency decrease.However,it has unconspicuous effects when the burst tendency of coal is very strong.Increase of bolt density makes coal bump more difficult,but it also improve the destructiveness of the coal bump.Fortunately,because of energy absorption device,the bolt begin to play a role of energy absorption avoiding the critical load continues to increase when the critical loads increases to the energy-absorption resistance.When coal bump occurs energy-absorption device can absorb part of elastic energy,which alleviates the impact on bolted rock mass.(iv)Dynamic response characteristics of rock mass supported by energy-absorption bolt and was studied and its preventment mechanism was revealed.Three-dimensional axis-symmetry model of energy-absorption bolt and surrounding rock affected by impact loads was established by simplifying the energy-absorption sleeve as spring and damper situated on end of bolt parallelly.Analytical solution of displacement impedance function were obtained by analyzing the dynamic response of the bolt top under the coupling vibration of energy absorption bolt and surrounding rock with viscous damping.Then the parameter sensitivity analysis of three-dimensional axis-symmetric mechanical model was carried on.The results show that,with the greater parameters such as viscous damping and strength of surrounding rock,anchored length and damping of energy-absorption bolt,the greater complex stiffness and resistance ability for deformation of supporting system can be achieved.Consequently,the stability of the system under the action of impact load are improved significantly.(v)The field test results show that the distributed energy-absorption bolt can satisfy the supporting requirements of coal bump roadway,which have great significance on safety and efficiency of coal mining.