Study On Mechanism Of Directional Fracture And Surrounding Rock Damage In Tunnel Smooth Blasting

At present,the drilling and blasting method in tunnel driving still plays an irreplaceable position due to the advantages of flexible construction and low cost.After blasting,smoothness of the excavation contour directly determines safety,efficiency and economy of construction.Smooth blasting technology can make the excavation contour smooth,reduce the stress concentration degree of the rock wall surface greatly,and control overbreak,underbreak and damage of surrounding rock effectively,so it is widely used in rock engineering.However,from the field blasting effect,poor effect of directional fracture and large damage of surrounding rock still exist,which results in the serious overbreak and underbreak,and thus the expected smooth blasting effect is far from achieved.The main reason is that the understanding of the mechanism of directional fracture and surrounding rock damage in smooth blasting is not clear enough.Therefore,the mechanism of smooth blasting is studied by the combined methods of the field test,the theoretical analysis,the numerical simulation and the model experiment.And the relationship between smooth blasting effect and smooth blasting parameters is further determined.Field tests of directional fracture and surrounding rock damage are respectively conducted under the non-smooth blasting and the smooth blasting scheme.Half-hole ratio,amount of overbreak and underbreak and excess spraying amount of concrete are made a statistic to reflect the effect of directional fracture,and meanwhile the P-wave velocity of surrounding rock is tested by the cross-hole acoustic wave method to determine the damage range of surrounding rock.The test results show that the excavation contour is uneven under the non-smooth blasting scheme,while smooth under the smooth blasting scheme.Meanwhile,the average linear overbreak amount under the non-smooth blasting scheme is about 3 times as much as that under the smooth blasting scheme,and the damage range of surrounding rock under the conventional blasting scheme is about 3～4 times as much as that under the smooth blasting scheme.It is evident that the smooth blasting scheme realizes directional fracture and meanwhile controls surrounding rock damage effectively,and then the importance and necessity of applying smooth blasting technology to perimeter holes in the field construction are determined.The respective contribution of explosion stress wave,quasi-static gas and the combined action of the two to directional fracture is explored in theory and it is preliminarily pointed out that the action of quasi-static gas is the active force of directional fracture.Meanwhile,based on the finite difference software,FLAC 3D,the numerical calculation program of rock mass fracture is secondarily developed by using the principal strain to determine the element force state,by damage equivalent strain containing triaxial strain ratio and by critical fracture strain considering the influence of stress state.Under the action of explosion stress wave,quasi-static gas and the combined action of the two,the principal stress,the principal strain and the fracture characteristics of rock mass are respectively simulated to further determine that the action of quasi-static gas is the active force of directional fracture,while the action of explosion stress wave has no positive effect on directional fracture on the premise that radial cracks are produced around the boreholes.According to the results of putting together the blasting blocks in the smooth blasting layer and observing directional cracks by high speed photography in the model experiments,it is determined that the initial position of directional cracks is on the borehole wall near the charge segment and the longitudinal cracks growing from the borehole wall to the free surface and the transverse cracks near the charge segment are produced in the smooth blasting layer.Meanwhile,directional cracks grow toward the direction of the connecting line of the borehole center in the two-dimensional approximation and the growing characteristics are greatly affected by the free surface,and then the growing physical model of directional cracks in smooth blasting is built.The damage evolution equation built through the theory of damage thermodynamics is improved by determining the element force state through the principal strain,by damage equivalent strain and by critical fracture strain.And the improved equation is embedded into the finite difference program to further simulate the evolution laws and distribution characteristics of damage in smooth blasting.The simulation results show that damage around the boreholes is evenly and symmetrically distributed under the action of explosion stress wave,while asymmetrically distributed under the action of quasi-static gas due to the influence of the free surface.Surrounding rock damage under the action of quasi-static gas presents a continuous saddle-shaped distribution along the connecting line of the borehole centers.Damage around the boreholes under the combined action of the two is the superposition result of the two and is increased.Meanwhile,damage characteristics under non-smooth blasting and smooth blasting are simulated to further demonstrate the necessity of using the smooth blasting driving from the perspective of damage.In addition,the radial distribution characteristic formula of the damage factors under the combined action of explosion stress wave and quasi-static gas is derived by the attenuation law of explosion stress wave and the theory of the thick wall cylinder.Moreover,based on the attenuation law of PPV near the blasting source,the distribution characteristic formula of the damage range of surrounding rock along the borehole is derived by combining the load distribution characteristics of the smooth blasting hole.The results show that the radial distribution laws of the damage factors attenuate in a power exponential function,and when the decoupling coefficient is less than the critical value,the damage factors tend to increase first and then decrease,conversely,they tend to decrease.The damage range of surrounding rock during the expansion process of detonation gas presents a continuous saddle-shaped distribution along the borehole and the damage range near the charge segment is the largest,but it is evenly distributed only during the process of quasi-static gas and is corrected by using the results in the expansion process to predict the damage range of surrounding rock.The formula of the stress intensity factors at the tip of directional cracks,considering the influence of multiple collinear linear cracks and the free surface,is derived theoretically.By putting forward the concept of the quasi-plastic zone,the theoretical relationship between the damage factors of surrounding rock in the semiinfinite body and the minimum burden is studied,and the mutual relationship between directional fracture and surrounding rock damage is analyzed from the perspective of energy.By the theoretical calculation,it is found that the smaller the minimum burden is,the bigger the stress intensity factor at the tip of directional cracks is,and the closer the two directional cracks grow towards each other,the faster the increasing velocity of the stress intensity factor at the tip is.Meanwhile,stress and damage factors in surrounding rock increase with the scaled distance and attenuate in a power exponential function,and increase with the increase of the minimum burden until the change is not obvious.When the linear charge density is constant,the energy consumed by directional fracture,surrounding rock damage and the smooth blasting layer crushing,and so on,presents a reciprocal relationship.In addition,the influence laws of the minimum burden,the borehole space,the radial decoupling coefficient and the linear charge density on damage distribution characteristics of surrounding rock are respectively calculated by using the numerical program,and the relationships between the maximum damage range of surrounding rock and smooth blasting parameters are fitted.Through simulation,it is found that the maximum damage range of surrounding rock increases nonlinearly with the increase of the minimum burden and the linear charge density,and decreases nonlinearly with the increase of the borehole space and the radial decoupling coefficient.Finally,it is pointed out by the test data of the dynamic strain that the dynamic strain is not the leading factor of directional fracture in smooth blasting.From the perspective of realizing directional fracture and controlling surrounding rock damage,the theoretical formulas of the value range of charge parameters and borehole net parameters in smooth blasting are established,and the suggested values of smooth blasting parameters of different surrounding rock are given by combining with experiments.According to the geological conditions of the field tunnel,the smooth blasting parameters of the corresponding tunnel are optimized through theoretical calculation and numerical analysis,which are applied to many tunnels.A good smooth blasting effect is achieved,which ensures the integrity and stability of surrounding rock.The rapid driving of hanging arch under the continuous blasting is also realized,which brings great economic and social benefits.Only in Jiang Shuiquan tunnel,the direct economic benefit of nearly 60 million and the enormous social benefit of being opened to traffic 8 months ahead of schedule are achieved.