Mechanism Of Rock Burst Induced By Excavation Unloading Under High Tri-axial Stress

Due to the double influence of tectonic stress and geostatic stress,the stress of rock under deep high geostress presents complexity,the mechanical response produced by excavation is obviously different from that of shallow rock.The related research results of deep underground space lag behind the requirements of engineering practice.Therefore,it is of great significance to study the mechanical mechanism of excavation unloading under high ground stress,which can provide theoretical basis for rock burst prediction,and provide scientific basis and analytical means for disaster prevention and control in the development and utilization of deep underground space and energy in China.In this paper,based on the non-uniformity characteristics of rock,using RFPA3D numerical analysis code,the stress transformation path caused by rock excavation is simulated by constructing true triaxial single-sided unloading and reloading test,so as to study the pregnant process and failure mechanism of rock failure induced by excavation unloading effect and loading effect of stress concentration.The main achievements of this paper are as follows:Numerical experiments on rockburst induced by unloading along different principal stress directions under true triaxial conditions were carried out,and the effects of unloading in different principal stress directions on rockburst failure modes,failure ranges,and failure mechanisms were analyzed.When unloading along the third principal stress direction,it is easy to produce local tension shear composite failure close to the free surface,and finally form shear dislocation rock burst failure;when unloading along the second principal stress direction,it is easy to produce integral layered plate crack failure,and finally form tension buckling rock burst failure.Under true triaxial condition,the failure mode of rock burst after unloading is also closely related to the stress state of rock after unloading.It is found that the failure mode changes from the single tension mode to the composite mode along the direction of the third tensile stress However,the higher the intermediate principal stress before unloading,the more the number of cleavage surfaces and the smaller the thickness of the splitting plate.The failure position of combined rock under true triaxial loading and unloading is related to hard and soft rock:the failure of rock firstly occurs and concentrates on the interface of soft rock and soft rock,and extends to the inside of hard rock.In the soft rock,the failure crack is closer to the free surface,and the hard rock crack is closer to the inner part of the rock.The failure mode of composite rock is related to the height of soft rock and hard rock:with the increase of the ratio of soft rock to hard rock,the failure mode of composite rock changes from shear type to tensile type.The failure mode of the composite rock is related to the dip angle of the soft and hard rock combination:under the combination of 0-15° the tensile failure of the rock through the soft and hard rock is formed.The V-shaped rockburst pit is formed along the interface of soft rock and hard rock.The sliding failure of rock with 60-75° combined dip angle is obvious,and the tensile failure is mainly caused by 90° combined dip angle.Under the condition of true triaxial double-sided unloading of principal stress in different directions,obvious acoustic emission agglomeration occurs near multiple free surfaces of rock,and microcracks gather near two free surfaces,forming two V-shaped rock burst pits with symmetrical failure.The results of the re loading test under true triaxial two-sided unloading are consistent with that of single-sided unloading,that is,the tensile failure cracks near the two free faces are easy to be formed when the intermediate principal stress is removed on both sides,and the shear mode failure with symmetry is easy to be formed by the minimum principal stress of double-sided unloading.At the same time,the failure mode of rock after unloading is related to the principal stress state before unloading.Compared with the unanchored rock,the localized failure characteristics are significant,and the location of the failure is related to the number and spacing of bolt support.The damage of the rock is first concentrated near the open surface of the unanchored area and spreads to the inside of the rock,forming shear-type through cracks.Increasing a certain number of supporting bolts and reasonable bolt arrangement spacing can restrain rockburst damage to a certain extent,improve the bearing capacity of the rock after unloading,and effectively resist the lateral deformation of the rock.