Study On The Mechanical Mechanism Of Coal Burst Induced By Fault Instability

In recent years,as coal resources enter deep mining in China,the intensity and frequency of coal burst are very high.According to statistics,coal burst accidents near fault structures are particularly complex and serious.Taking Qianqiu coal mine of Yima mining area as engineering background,physical simulation test with the existence of a fault under mining conditions is carried out.In this thesis,a mechanical model with the existence of a fault considering mining disturbance is established and a point-by-point integral method for calculating the relative displacement between the hanging wall and footwall of fault is derived.Meanwhile,the concept of fault potential energy is proposed to characterize the evolution of energy field and the temperature order increasing function model of fault instability related to friction is presented.The mechanism of coal burst induced by fault instability are revealed.The results show that the dynamic evolution characteristics of following displacement field,stress field,energy field and temperature field are simultaneously triggered when the fault is unstable under mining disturbance.1.Precursory characteristics of abrupt change in the displacement fieldBased on the physical model,the dynamic fault slip under the mining disturbance and the movement of overburden strata in the fault-influenced are simulated.The slip vector distribution on the fault surface is analyzed by using the point-by-point integral method for calculating the relative displacement between the hanging wall and footwall of fault.The results show that the fault instability is characterized by the abrupt evolution from slow slip to sharp slip.During the process of fault slip,strain release zone accelerates expansion and connection on the fault.The relative slip has evolved from isolated local static slip to whole fault slip.It can be seen that the maximum fault displacement increases from 5 mm to 15 mm and the position of the maximum fault displacement dynamically moves from the top to the middle region where a fault intersects with extra-thick rock strata.2.Precursory characteristics of differentiation in the stress fieldThe evolution characteristics of stress field on the fault during the mining activities are studied.The correlation between stress distribution and friction is analyzed by using mechanical model considering mining-induced stress.The results show that the normal and shear stress evolve from synchronous change to differentiated development.It is characterized by the increase of shear stress and the decrease of normal stress or the decrease of shear stress and the increase of normal stress.The position of maximum shear stress on the fault surface is critical region where the friction effect transforms from static friction to dynamic friction.Meanwhile,it is also the triggering position of fault slip.3.Precursory characteristics of lagging surges in the energy fieldAcoustic emission(AE)is used to monitor evolution of AE events during fault instability.The spatial distribution characteristics of fault potential energy before and after the mining face passes through the fault are analyzed.The impact failure caused by violent fault slip is explained from the point of view of fault potential energy.The results show that although AE events can be sporadically detected before fault instability,the number of AE events at the critical moment of fault instability is almost zero.The sudden increase of AE events can only be monitored when the fault severely slips.In addition,the surge of AE events lags behind the time of abrupt change of displacement field.Therefore,the stage of a relatively long suspension before the sharp increase of AE events can be regarded as the precursor characteristics of fault slip.In the process of coal seam mining,the position of peak fault potential energy coincides with the position of AE event surge.Fault potential energy changes from almost zero to sudden increase when the working face is mined through the fault.It can be revealed that the large amount of strain energy released from the violent fault slip will cause serious impact on the mining face.4.Precursory characteristics of order increasing in the temperature fieldThe influence of high stress environment and mining activities on the temperature field of fault is studied.The correlation between the dynamic change of mining stress and the temperature field response on fault plane is analyzed.The temperature evolution on the fault under the mining disturbance is explained by using the order increasing function model.The results show that the high stress environment leads to the interphase distribution of heating zone,cooling zone and heating zone.There are two regions with significant temperature and these two regions take the lead in cooling with the advancement of mining face.During the coal seam mining,temperature field of fault experiences temperature decrease caused by strain release,local temperature rise caused by strain accumulation and whole temperature rise caused by friction.Severe fault slip occurs when the temperature field of fault changes from small changes caused by strain release to sharp changes caused by strain accumulation.Therefore,the order increasing in the temperature field can be regarded as the Precursory characteristics of fault slip.5.Mechanism of coal burst induced by fault instabilityA formula for risk of fault relative slip is further proposed and the formation mechanism of reverse fault under the high stress environment is studied.The disaster mechanism caused by extreme pressure from coupling effect of fault and extra-thick rock strata is illustrated.Displacement mutation characteristics in the superposition area of double fault is researched and the mechanism of coal burst with the existence of fault is also revealed.The results show that the increase of fault dip angle causes the abrupt change of displacement field near the fault and it increases the risk of fault slip.The high stress environment aggravates the phenomenon that the normal and shear stress evolve from synchronous change to differentiated development and promotes the instability of reverse fault structure.The compacted area in the extra-thick rock strata with the existence of fault can mitigate the disturbance of mining activities and roof collapse to the fault.This leads to the lagging surges in the energy field.The process of strain rate varying from relatively low to sharp changes in the overlying strata with the existence of double fault is easy to cause friction between the fault hanging wall and footwall.This can induce order increasing effect in the temperature field.In conclusion,the mechanical-temperature field characteristics of fault instability indicate the occurrence of coal burst.