Experimental Study On Rockburst Process And Several Key Influencing Factors And Support Strategies Of Deep Buried Tunnel

As China’s underground engineering is getting deeper,rockbursts have become frequent geological disasters during the construction of deep underground engineering,which pose a huge threat to tunnels,transportation tunnels,mining,nuclear waste storage and underground rock laboratories(URLs)and have become an urgent problem in the"deep ground strategy".Up to now,the rockburst process,mechanism,and support are far from meeting the needs of the project,and rockbursts still occur frequently.Therefore,it is of great scientific and engineering significance to carry out studies on rockburst process,mechanism and support strategy in order to promote China’s"deep ground strategy".Based on in-situ rockbursts survey,loading path combined with true-triaxial rockburst testing systems(true-triaxial testing machine,high-speed cameras,acoustic emission(AE),etc.)are used to investigate the rockburst in deep buried tunnels.In addition,the influences of axial stress,mineral grain size and bolt support on rockburst are investigated.Then,based on the shortcomings of traditional rockburst support,preconditions for rockburst is summarized,and an“active support strategy”is therefore proposed and verified.Finally,the strainburst and rockburst support are discussed in detail,and a“rockburst integrated support system”is established.The research results provide scientific reference for understanding and preventing rockburst.The main conclusions are as follows:(1)The formation process of a strong rockburst in a tunnel with a rockburst band is a gradual process of multiple local rockbursts to overall rockburst,with obvious spatial distribution characteristics and structural response.Before overall rockburst,it is accompanied by the AE"quiet period".In addition,the occurrence time and position of overall rockburst are closely related to the AE event density,and AE event density can be used as a potential indicator for time and location prediction of rockburst.In the whole rockburst process,extensile and dilatants cracks,generated in the form of intra-and trans-granular cracking,accounted for97.2%,and only when the tangential stress reached 67%of the slabbing strength,shear cracks began to occur.After that,micro-cracks continuously interacted and coalesced to form fractures,followed by slabbing,buckling and V-shaped rockburst failure area.The whole development process of rockburst is dominated by tensile splitting and local rockbursts can be composed of multiple bursts and can last for a long time,similar with the in-situ slight rockburst.(2)Rockburst is significantly influenced by the axial stress.As the axial stress increases,the vertical stress increment and duration of macroscopic failure from particle ejection to rockburst tend to decrease first and then increase.However,observations of cross-polarized light image of a cross-section show that the fracturing mechanisms(trans-granular and extensile micro-cracking)of the V-shaped failure zone are not affected by the axial stress.Characteristic stresses(z-direction)corresponding to crack initiation,particle ejection,crack coalescence,slabbing failure and rockburst,as well as cumulative AE energy are a function of the axial stress and their variations with increasing axial stress are all characterized by an increase first,followed by a plateau and then a reduction.Therefore,the rockburst possibility will decrease first and then increase with increasing axial stress.In addition,the double Poisson effect caused by tangential stressσθand axial stressσ_x promoted the axial slabbing,because the propagation of cracks in the radial direction was suppressed but the extensile cracks in the axial direction are encouraged to propagate.As a result,cracks will develop more in the preferred axial direction,resulting in more axial fracturing in rockburst.During the test,it is found that the rockburst stress is higher(about 13.4%)than the slabbing failure stress and dependent on the axial stress.Therefore,the in-situ rockburst failure envelope should be established as a 3D model.(3)Rockburst process and failure mode are affected by grain feature size:1)The experiment found that fine to medium-grained granite suffers from slabbing mainly caused by static brittle failure,while medium-and coarse-grained granite experiences rockburst mainly caused by violent ejection;2)Under the condition that the stress concentration is high enough and the storage capacity of the elastic strain energy before peak strength of the rock is basically the same,rocks with coarser grain size have a stronger rockburst proneness;3)A large number of small and large rupture events before the final instability are found for fine to medium-grained granite.These ruptures are relatively continuous and concentrated in the spatial and temporal distribution,which results in slabbing with lighter intensity in surrounding rocks.However,the small and large rupture events occurred randomly at various stages of loading for medium to coarse-grained granite.These ruptures are relatively scattered in the spatial and temporal distribution,which easily results in the ejection of particles and fragments of the surrounding rocks,and even rockbursts.In addition,the geometric structure and heterogeneity of the mineral grains also have an important impact on the tensile stress field and rockburst intensity during the rockburst,and should be paid attention to.(4)Rockburst is significantly affected by bolt support.It is found that rockburst process under bolt support is mainly composed of four processes:particle ejection,splitting into slabs,buckling,and fragment ejection,but it is significantly affected by the bolt support.When the bolting space is large,rockbursts always occur in the weak areas after the support,and are presented in the form of localized damage.When the bolting space is reduced to a certain extent,the weak areas after the support will no longer be obvious,and rockburst will occur in the area with reinforcement and even through the entire free surface of the specimen.In addition,rockbursts under bolt support are more serious,but the intensity decreases as the bolting space decreases.Therefore,rockbursts under bolt support should be given sufficient attention.With the decrease of the bolt spacing,crack initiation stress,crack damage stress,and peak strength continue to increase,the cracking activity inner the rock is delayed.Using bolt,tensile splitting of the free surface is weaken and continues to be weaken with the decreasing bolting spacing.However,with the decreasing bolting space,the number of shear micro-cracks increases.In addition,bolt density can be appropriately increased,local reinforcement of"critical parts"can be paid attention to,and the use of surface element can be combined.(5)An active rockburst support strategy of“suppressing buckling and therefore rockburst by increasing the tensile strength of the rock surface”is proposed,and a“rockburst integrated support system”is established,which provide some references for rockburst prevention.The results show that increasing the surface tensile strength(using material with high tensile strength,such as basalt fabric)can effectively increase the allowable buckling deformation of rock,prolong the buckling process,and increase the rock ductility and deformation.This results in that the internal cracking activity before the final failure is more intense and the law of energy release is changed,i.e.,more energy is consumed and dissipated in a more gradual manner.As a result,failure mode changed from rockburst to brittle fracturing and rockburst was actively and effectively suppressed.In addition,a“rockburst integrated support system”that considers bolts,active support strategies,and surface elements is established,and"Barrel effect"should be paid attention to.The first layer of the system is bolt,including rigid bolts,such as mortar bolts,and energy-absorbing bolts,such as MCB super bolts,to achieve the reinforcement and hold of broken rocks.In addition,local strengthening of"critical parts"should be considered.The second layer enhances the tangential tensile strength of the surrounding rock surface to achieve active suppression of rockbursts.The third layer adopts a rockburst passive support strategy,i.e.,combined with meshes,straps,etc.to achieve the function of remaining the broken surrounding rocks and connecting the support element to comprehensively and effectively prevent rockbursts.