Study On Stress Path Variation Of Surrounding Rock And Mechanism Of Rockburst In Coal Roadway Excavation

The rockburst is a typical coal-rock dynamic disaster.The occurrence of rockburst during roadway excavation is frequent,which poses a serious threat to the workers.It is of great significance to study the mechanism of rock burst in coal roadway excavation and its control technology.The rockburst of roadway excavation can be divided into rockburst of substance roadway excavation and driving roadway along gob.The rockburst of substance coal roadway excavation is less affected by the movement of overburden rock,and the mechanism of rockburst is lack of systematic research.For this reason,the paper comprehensively uses the methods of field investigation and data statistics,theoretical analysis,laboratory tests,computer numerical simulation and engineering practice to carry out the research on the impact mechanism and counteracting countermeasures of substance coal roadway excavation.The characteristics and influencing factors of rockburst in roadway excavation are analyzed statistically.It was found that more than 90% of the rockburst occurred in the 100 m behind the head-on which was driving disturbance area;the geological anomaly area was an important influence factor for the excavation rockburst;with the increase of coal thickness and the driving speed,the frequency of the rock burst increased.The true triaxial loading and unloading experiment device and numerical simulation methods were used to study the stress path and failure characteristics of the surrounding rock during the roadway driving.It was found that during the coal roadway excavation,the surrounding rock has the characteristics of stress-strain path("loading-unloading" stress path)with "vertical loading-inclination unloading-going strain constant".The elasto-plastic boundary stress equation of the sample,the stress equation at the unloading side during failure,and the stress increment equation at the unloading side during failure are established according to the “loading-unloading” stress path experiment results.The experiment found that when the coal stress reached 60 MPa or more and the unloading rate reached 2.7 mm/min or more,the coal sample showed a shock.When the shock appears,the stress on the loading side of the coal sample instantaneously decreases,and the stress on the unloading side increases instantaneously.After the impact damage,the coal body is loose;the stress of the coal body without shock changes slowly and presents the characteristics of layered fracture,and the fracture surface is parallel to the unloading.surface.The rock burst mechanism of coal roadway excavation,which was that rock burst was caused the combination of "loading-unloading" stress path transformation and high initial stress in surrounding rock of coal roadway,was proposed.From the test results of the “loading-unloading” stress path experiment and the stress path characteristics of surrounding rock in roadway excavation,the failure characteristics of roadway surrounding rock are obtained,the stress equation of roadway surrounding rock is established,and a discriminant model of rock risk index is established.Taking into account of the differences in the surrounding rock properties of coal roadways,the stress equations of beam surrounding rock and the stress equations of arch surrounding rock based on the "loading-unloading" stress path are established.Two kinds of rockburst mechanisms are proposed for coal roadway excavation: a)The effect of expansion pressure: the volume expansion of the coal body at the moment,the instantaneous stress of the inclinatical direction increased,which caused the displacement of the coal to the free surface,and this type of failure occurred mostly in the soft coal.b)Reflection and stretched of stress wave: The stress wave is generated by the shock,and it is reflected as a tensile stress wave at the free surface of the roadway,throwing the free-faced coal body into the roadway.This type of failure occurs mostly in hard coal.Based on the rockburst damage form,a model for identifying the rockburst risk index of roadway surrounding rock was established.Simulation experiments on roadway excavation in geological anomalies,revealing the effect of geological anomalies on rock burst,have been carried out.Under the effect of disturbance,the weak surface of the geological anomaly zone expands and immediately damages,which release large energy shock waves.When the shock waves transfer to the roadway surface the deformation rate of roadway surrounding rock instantaneously increase.At the same time,the local high stress formed in the geological anomaly zone makes it easier for the stress of surrounding rock to reach the critical stress.High strain rate and high stress together cause geological anomalies to easily form the rockburst.Through the numerical simulation analysis,the influencing factors of the source energy in roadway driving are obtained.The increase of the original rock stress,lateral pressure coefficient,thickness of the coal seam,thickness of the roof,and excavation speed play a reinforcing role for the source energy.The reduction scheme is proposed.The research results have been carried out in the Zhangshuanglou Coal Mine,and the effect is remarkable.