The Research On The Growth Mechanics Of Crack In The Surrounding Rock Of Adjacent Tunnel Under The Blasting Load

For the drilling and blasting method with small spacing,the blasting load not only completes the necessary crush work,but also induces the damage of adjacent tunnel to a degree,which is specifically shown as the degradation of mechanical property and the lower of integrity in the surrounding rock.The current researches mainly involve the problem about the dynamic response of adjacent tunnel instead of the destruction induced by damage evolution of surrounding rock under the blasting load.Therefore it is necessary to carry out the research about the evolution rule and evolution mechanism of crack in the surrounding rock under the blasting load.Because of the complex of structure of surrounding rock and the difficulty of field monitoring,so the numerical analysis and the modeling test become the effective methods to solve above problem.At first,using numerical analysis software LS-DYNA,vibration velocity field of surrounding rock in the adjacent tunnel is studied under explosion loading to give the definition of dangerous spacing.Through analysizing the stress field and vibration field of the surrounding rock as well as the changing rule of stree and vibration velocity in the key location within the dangerous spacing,the most serious distribution region is acquired.In order to study the main damage area and the damage form of surrounding rock under the adjacent explosion load,the similar model test is carried out.When original crack exsits in the surrounding rock,the damage of tunnel becomes more serious,therefore,use digital laser dynamic caustics experimental system to simulate the original crack in the surrounding rock of adjacent propagation process under explosion loading and to analyse the change law of kinematics and dynamics parameters about crack propagation under various effection factors.Crack propagation path determines the damage form of surrounding rock,combined with the whole stress field,the influence factors of extension path are analysed.Through the experimental analysis,the crack growth path is researched under the comprehensive effect of various influencing factors.The destruction of the surrounding rock of tunnel tends to crack propagation and penetration,explosion gas and explosive stress wave are the power source of the crack propagation.Combined microscopic damage mechanics with macroscopic fracture mechanics,the growth law of microscopic crack and macroscopic crack are acquired.Meantime use the test method to analysis crack growth process.Based on above researches,the damage evolution mechanism of crack growth is researched.In the end,ABAQUS numerical analysis software based on XFEM is used to analyze the whole process of crack propagation,and research conclusions are verified.The detailed research findings are as follows.?1?The vibration rate of particle is an important indicator to measure the disturbation degree of the blasting load to the surrounding rock of adjacent tunnel.When the spacing between the blasting source and adjacent tunnel is less than the three diameter of tunnel,the vibration rate peek surpasses the specified safty value according to the standard and the tunnel is in a dangerous spacing.Through analysing the vibration field and stress field in surrounding rock of adjacent tunnel in a dangerous spacing,the face blasting side of tunnel is the most main distruction zone which is much lager than back blasting side and the distruction zones mainly focus on the zone between the straight wall and sprung arch.Among that the damage of the certain zone close to the straight wall of face blasting side is more serious owing to the superposition of reflection stress wave and the strong pressure wave.Because the changing of geometrical shape,the stress concentration in the base angle of straight wall in the back blasting side is also very obvious and new secondary wave source appears.Adopting similar model test to research the failure mode of surrounding rock of adjacent tunnel,the failure of tunnel shows that the radial cracks are induced and grow starting at the tunnel.And vertical cracks appear at tunnel roof and floor because of the gravity and unloading rebound effect.?2?The explosive charge,original crack length,original crack angle,the spacing between original crack and the tunnel and the sectional form of adjacent tunnel affect the crack growth in the surrounding rock.The explosive is the energy source inducing the growth of original crack.With the increasing of explosive charge,the tensile stress produced by the reflection wave from the freedom surface of tunnel are gradually strengthened,showing that the growth direction at the early stage changes from incline to vertical direction and the growth displacement along vertical direction gradually increases.The original crack length in the surrounding rock affect the damage degree.Under the action of stress wave,the longer of the original crack,the more concentrated of the stress,as result,the guidance to the main crack from the blasting source becomes stronger.When the length of original crack is equal to 5mm or larger,the main crack from the blasting source runs through the original crack.The angle of the original crack in the surrounding rock has little effect on the growth direction of crack,all the original crack in the surrounding rock of face blasting side show approximately vertical growth at the early stage,the growth parallel to main crack at the middle and penetrating to tunnel at the later stage.For the back blasting side,the growth direction show the horizontal direction at the early and middle stage and warp growth,and the growth displacement gradually decreases by Parabola form.The spacing between the original crack and the tunnel mainly affects the growth path of crack.With the increasing of the spacing,the growth path of crack changes from the tilt growth at the early stage,the horizontal growth at the later stage to the horizontal growth at the early stage,the tilt growth at the middle stage and the horizontal growth at the later stage.The sectional form affects the reflection and diffraction of stress wave,the reflection ability of plane is larger than cambered surface,which leads to the stress wave energy diffracts through the cambered surface easier than that of plane and takes more energy to the back blasting side,on the one hand,inducing the crack to grow towards the cambered surface,on the other hand,leading to that the growth displacement of original in the back blasting side of circle tunnel is larger than straight wall and arch tunnel and that of rectangle tunnel smallest.?3?The initiation direction of orginal crack in the surrounding rock of adjacent tunnel under the action of blasting load lies on the local stress field in the tip of crack and the growth path lies on the whole stress field under the blasting load.The blasting source,the location of original crack and tunnel freedom surface are the main factors to affect the growth path of crack,the crack angle has little effect on it.The stress wave from the blasting source causes the pressure action along radical direction and tensile action along tangential direction in the surrounding rock,the reflection wave from the tunnel freedom surface causes tensile action along the radical direction and the unloading wave generates tensile action along radical direction,among which the stress wave from the blasting source acts on the whole process.The growth path of crack in the surrounding rock of face blasting side at the early stage lies on the the tensile stress along the tangential direction under the stress wave from the blasting source and the tensile stress along the radical direction under the reflection wave from the freedom surface.The closer the spacing with the tunnel,the stronger the effect of reflection wave on the rock unit.When the original crack lies on the zone where the reflection wave plays a leading role,the crack grows along the approximate vertical direction.At the middle and later stage,the growing original crack gets rid of the effect scope of the reflection wave,and the effect of stress wave from the blasting source and freedom gradually weaken.As result,the quasi-static stress plays a leading role,the crack grows along the direction parallel to the main crack from the blasting source.The diffraction wave plays a leading role on the growth path of original crack in the back blasting side,the freedom surface does not change the quality of the preesure along radical direction and tensile action along tangential direction for the rock units.Under the action of diffraction wave,the crack grows along the radical directiong of blastbore.At the later stage,the crack appears to warp under the action of unloading wave.?4?The growth of original crack in the surrounding rock of the adjacent tunnel under the action of blasting load is the combined action stress wave from the blasting source and the explosion gas.The growth process of the original crack is divided into the formation of the weakest link near the crack tip and the macroscopic growth along the weakest link.Quasi static stress of explosion gas once again strengthened the original stress field,the peak belt of principal stress difference is formed,which driving the original crack continue to grow.The existing of freedom surface bring forward the crack initiation,but crack angle has little effection.The dynamic fracture toughness rangs among 0.39MN/m^3/2 to0.42MN/m^3/2,which shows that the freedom surface and crack angle has no effect on dynamic fracture toughness of crack.Under the action of blasting load,because of the stress concentration,the activated microcrack zone is generated in the spacing between the main crack from the blasting source and the original crack,which induces the main crack from blasting source to run through the original crack.The crack's penetration follows the transmission of energy,the original crack initiation and growth result from the superposition in the tip of stress wave energy which is taken by the main crack.After the crack penetrating,the energy release rate of original crack weaker than that of the main crack without penetration,but the crack growth rate increasing.The stress concentration zone of arch shield results from the explosion gas,in other words,the location of principal stress difference peak,which is related to the crack growth in the cack blasting side.The research based on this findings verifys that explsion gas plays a leading role on the crack growth in the back blasting side and deepens the mechanism research of blasting load on the adjacent tunnel.When the arch shield approaching to the growing crack tip,crack growth rate reaching to peak is about equal to the moving rate of arch shield.As result,the relative distance between the arch shield and growing crack.At this time,the crack growth rate oscillatory ranges in this peak up and down.Quasi-static stress mainly acts on this time where the arch shield runs after the growing crack.And at this moment,dynamic stress intensity factor reaches to the peak value and oscillatory changes.After the arch shield surpasses to the growing crack,the dynamic stress intensity factor gradually decreases and the crack arrest finally.?5?Under the action of blasting load,the growth of the original crack in the surrounding rock of adjacent tunnel shows that the left tip grows towards to the blastbore,whereafter,the right tip grows towards to the adjacent tunnel.For the crack right tip,the crack growth path of different tunnel is different.All of cracks at the early stage grows along the horizontal direction,but at the middle and later stage,the crack growth path is different.For the circle tunnel and straight wall and arch tunnel,the growth direction appears to deflection at the middle and later stage,and the deflection degree gradually becomes obvious with the growing crack approaching to adjacent tunnel.However,the crack in the rectangle tunnel still grows along the horizontal direction.The difference of growth path for original crack is related to the tunnel form.The reflection ability of plane is larger than arch surface,the crack grows towards to the direction which is easy to release energy.As result,crack deflects towards arch surface.The intensity of stress field changes with the different location and different time,so the crack growth rate is different at the different monment.The stress wave energy is mainly used to the increasing of crack length and width.When crack stops growing,the stress wave enery is mainly used to the increasing of crack width,vice versa.And the crack area increases linearly at last.Damage dissipation energy is mainly used to the increasing of crack width,not the crack growth.