Study On The Technology And Mechanism Of Active Control Of Rock Damage Under Explosion Load

With emergence and development of green mining aimed at mineral resources in our country,how to dig blasting-rock efficiently to minimize damage of retained rock mass has become a hot issue.Energy in rock mass release randomly after explosion in blasting engineering,and it's bound to damage surrounding rock.Therefore,how to prevent the spread of the explosion energy to surrounding rock,and guide the explosion energy to be propagated to blasting-rock become urgent problems.Based on the above,pre-splitting blasting and the derogative mechanism of protecting borehole wall blasting were studied,and double-system detraction technique was put forward.The results of the study are as follows:1.In order to investigate the effect of pre-splitting crack on the dynamic fracture behavior of blasting cracks and original cracks,a series of studies were carried out based on the digital laser dynamic caustic experiment.(1)The pre-splitting crack can stop blasting-cracks from expanding to the retained rock mass and increase the damage of the blast rock mass.Under the action of blast load,wing cracks generated at both ends of the pre-crack extend to the retained rock mass.With the increase of the pre-crack length,the crack propagation length of the wing increases.With the increase of pre-crack width,the crack propagation length of the wing decreases.Changes in the length and width of the pre-crack have some influence on the original crack propagation in the retained rock mass.The shorter pre-splitting crack(L=20~80 mm)can promote the propagation of the original crack B in the retained rock mass.Only when the pre-splitting crack length increases to a certain degree(L=100mm),it can inhibit the expansion of the original crack in the retained rock mass.The extension length of primary crack B in the retained rock mass decreases first and then increases with the increase of pre-crack width.The pre-crack has an optimal width,which makes the original B crack propagation length in the retained rock mass the shortest.(2)The B_Z fracture toughness is less than the B_Y fracture toughness.When the pre-splitting crack exists,the fracture toughness of B_Z and B_Y is less than that of the non-pre-crack,and the fracture toughness of B_Z decreases with the increase of the pre-splitting crack length and width.The peak values of the stress intensity factors of the wing cracks B_Z and B_Y decrease with the increase of the length of the pre-crack,and the mean value of the stress intensity factor of the B_Z and B_Y decreases first and then increases with the increase of the pre-crack width.(3)The holes can not only guide the directional cracks generated by the pre-splitting crack,but also inhibit the new wing cracks in the pre-splitting crack tip.With enlarging of the hole radius,the air hole orientation is more and more obvious,the principal stress difference is smaller.(4)The pre-splitting hole was arranged between the main blast area and the surrounding hole,and the starting order was the pre-splitting hole,the main hole and the surrounding hole.Design can increase the broken of main blasting area,block blasting crack expand to the retained rock mass,weaken the stress around the retained rock mass on blasting area,inhibit retains the original crack crack in the rock mass.There is a"safe distance"from the reserved rock mass,and the damage and crack in the vicinity of the hole will not be extended to the retained rock.After the pre-splitting hole and primary detonation area,the surrounding holes are filled with an empty surface.The empty surface will guide the explosion energy to be released in the direction of the blasted rock mass,reduce the release to the reserved rock,and protect the retained rock mass.Under the blast loading,the interpenetrating crack between the pre-split hole and the peripheral hole will initiate crack growth.The wing crack easily initiated from the branched crack with the smaller stress wave incident angle and longer length,and propagated along the vertical direction of maximum tensile stress.The branched crack with smaller stress wave incident angle and longer length may inhibited the propagation and the stress intensity factor of branched crack with bigger stress wave incident angle and smaller length.The mean of dynamic stress intensity factor decreased with the increasing of stress wave incident angle.The velocity of branched wing crack propagation presents oscillating change law.After obtaining the law of interpenetrating crack propagation,the hollow hole can be set on the path of the wing crack propagation,and the extension of the wing crack to the retained rock mass can be suppressed.2.The effect of pre-splitting crack on explosion stress,the mechanism of pre-splitting crack and stress wave and the evolutionary process of pre-fracture detraction were studied by numerical simulation software to build the computational model containing pre-splitting crack.(1)The compressive stress wave propagated to pre-splitting crack after explosive detonation will produce a reflection stretching wave,and the tensile stress wave increases the damage degree of the explosive rock mass between blasthole and pre-splitting crack.So the existence of pre-splitting crack prevents the damage from extending to the retained rock mass.(2)Under explosion load,crack tip produces stress concentration.With continuous loading of explosive load,concentrated stresses generated from ends of pre-splitting crack and surrounding of blasthole connect and form a v-shaped stress concentration zone finally,and blast damage guided by interlinked stress concentration zone extend to the two ends of the pre-splitting crack.The stress concentration of crack tip and the stress peak is increasing with the increase of pre-splitting crack length.(3)Under explosion load,two crack surfaces of pre-splitting crack have closed trend,and the closed crack surface is extruded under the expansive action of explosion gas,resulting in compressive stress.With the increase of pre-splitting crack length,the compressive stress produced by the compression of the closed crack surface increases first and then decreases.With the increase of pre-splitting crack width,the closed time of the two crack surfaces is more later,and the peak of pressure stress is smaller when pre-splitting crack length is certain.3.In order to analyze the macroscopic fracture,microdamage and vibration signal of the cement mortar model with pre-fracture,three dimensional cement mortar model,RSM-SY5(T)and TB4850 were used.(1)Macro-fracture analysis:Linear pre-splitting crack and circular pre-splitting crack can block the blast-induced crack to retain rock mass expansion.The closed circular pre-splitting crack can increase the damage of the explosion rock mass more than the linear pre-splitting crack.(2)Microscopic damage analysis:With the further distance from the source,there is an approximate linear decline in the unprefracture damage degree.The pre-splitting crack model presents a"cliff-type"drop,and the damage degree of the test points on both sides of the pre-splitting crack is obvious.The peak value of the straight-line pre-splitting crack damage is 1.55 times that of the unpreformed crack,and the linear pre-splitting crack can increase the damage of the explosion rock mass.The pre-splitting crack can reduce the damage degree of retained rock mass,and the closed circular pre-splitting crack is better than the linear pre-splitting crack.The pre-splitting crack depth increases from 150mm to 180mm,and the peak and mean value of the retained rock mass decreases by 7.4%and 10.8%respectively.The increase of pre-splitting crack depth can reduce the loss of retained rock mass.(3)The pre-splitting crack has the effect of decreasing the vibration velocity.The deeper the pre-splitting crack,the more obvious the peak vibration velocity.The peak value of single circular pre-splitting crack vibration is 81.4%,and the peak of double circular pre-splitting crack is 68.6%,which is better than linear pre-splitting crack vibration reduction.Linear pre-splitting crack can make the blasting vibration energy transfer from"low frequency"to"high frequency",and the depth of pre-splitting crack more deeper,the vibration energy produced by blasting is more evenly distributed on the frequency band,and the peak is smaller;The circular pre-splitting crack model is lower than the energy peak of the linear pre-splitting crack model,and the energy is moved to the high frequency band,and the energy distribution is more uniform.4.In order to explore the mechanism of wall-protecting explosion on blast-induced cracks,primary cracks and directional cracks,a series of studies based on digital laser dynamic caustic experiments,model experiments and numerical simulations were conducted.(1)Dynamic caustic line experiment shows that wall-protecting explosion can suppress blast-induced and primary cracks propagation on the side of the retaining wall;Dynamic stress intensity factors of primary cracks on the side of the retaining wall are53%and 77%of those on the side of non-retaining wall,and the wall-protecting pipe plays a significant role in weakening the loading of blast waves and gas.(2)Cement mortar model experiment shows that wall-protecting blasting can produce directional cracks at the end of the wall-protecting tube.The degree of damage of each model in the direction of the protective wall is less than the degree of damage in the direction of the corresponding non-protected wall.The shorter the length of the blockage,the weaker the quasi-static effect of the explosion gas,the smaller the degree of rupture of the specimen,the lower the degree of damage at each measurement point,and the damage gap between the protection point and the non-protection point diminishes gradually.The wall-protecting pipe plays a significant role in weakening the loading of blasting gas.(3)The numerical simulation results of free field show that:Due to the reflection of the wall-protecting pipe,there are two stress peaks appearing in the direction of the non-protective wall and the end of the wall.There is a significant diffraction phenomenon in the detonation wave near the end of the wall-protecting pipe,and there is a stress concentration phenomenon.During the whole detonation process,the end of the wall-protecting pipe has been in the state of compressive stress due to the continuous impact of the detonation product.Under the action of detonation products,the wall-protecting pipe is in an expansive state,and the lateral expansion rate is obviously larger than the longitudinal expansion rate.(4)The numerical simulation results of the constraint field show that:After explosion,the jet is formed at the end of the wall-protecting pipe,and then acts on the borehole wall and causes initial damage.Subsequently,the jet continues to impact the rock mass near the end of the wall-protecting pipe and forms“jet effect”,which promotes the damage of primary cracks and eventually produces directional cracks.The wall-protecting explosion can not only protect the surrounding rock,but also produce directional fracturing effects,which has a wide application prospect in engineering practice.(5)Based on the research results of 2~5 chapters,this paper proposes a dual system to reduce the damage technology,and realize the combined use of the damage reduction system and the damage reduction system of the wall blasting.The pre-splitting hole is arranged between the peripheral hole and the main detonation zone,and the pre-hole and peripheral hole spacing are larger than the two adjacent gun holes.The pre-hole is used to produce the pre-crack,and the peripheral hole is used to protect the surrounding rock and form a regular contour.The detonation sequence is the pre-hole first,then the main detonation area of the hole detonation,and finally the peripheral hole detonation.5.In order to study the attenuation rule of the stress wave and the protective effect on the rock of PVC,PVC and silicon rubber,SHPB experiment system was used.(1)Impact experiments under different speeds,hard PVC specimens were not fracture,stress peak value is smaller than the incident stress wave transmission,protecting wall material on the transmission of stress wave has"clipping"role.With the increase of the impact velocity,the reduction degree of protecting wall material to the stress wave is maintained at about 40%,and the energy dissipation of the hard PVC specimens shows exponential growth.(2)B~E plans can protect the surrounding rock well.The rock specimens under the four protection schemes have no obvious macroscopic fracture.(3)Four different values of the transmission of stress wave under combination scheme are 61.8 MPa,53.6 MPa,7.2 MPa,53.6 MPa,less than the transmission of stress wave of unprotected value of 78.6 MPa.It shows the four kinds of solutions and protecting all can weaken the stress wave propagation,have a favourable protection effect on surrounding rock,the plan D has the best protection effect owe to minimum value transmission of stress wave.(4)The transmission energy and damage degree of the plans B~E are less than the transmission energy of the non-protective material plan A.Compared with the plan B and plan C for PVC,the multi-layer plan C has less energy of transmitted stress wave,lower damage degree and better protective effect.Compared with the plan B~E,we find that plan D and E whose breast wall materials mixed with soft rubber with less impedance have minimum transmitted stress wave energy and damage,best protection effect.