Propagation Characteristics Of Blasting Vibration In Slopes Subjected To Open-Pit To Underground Mining And Safety Criterion

With the continuous exploitation of near-surface mineral resources in China,the open-pit mining slopes continue to become higher and steeper,whose stability is declining.Because of the depletion of near-surface mineral resources and increasing mining,underground mining will be adopted in more and more mines in the condition of high and steep slopes.As the necessary means in mining,rock excavation and other constructions,blasting brings huge benefits,on the other hand,blasting-induced landslides,collapses and other geological disasters are nothing new across the world.Compared with the open-pit mining slope,the open pit to underground mining slope has experienced both disturbances of open-pit mining and underground mining,and its stability is more deteriorated.In open pit to underground mining,the sublevel caving has two major characteristics of space location concentration of explosion source and large scale,which will have a significant impact on the blasting dynamic stability of mining slopes.It is the fundamental work to study the blasting dynamic stability that the propagation characteristics of blasting vibrations in slopes subjected to open pit to underground mining are studied and also the multidisciplinary frontier and hot topics of engineering geology,explosive mechanics,rock dynamics and so on.It has both important theoretical significance and practical application value.In this paper,the combined method of field monitoring,numerical simulation and theoretical analysis will be adopted to study the characteristics of medium-long hole blasting loads in underground mining,the propagation characteristics of blasting vibrations in intact slopes,slopes with a out-dip structural plane and slopes with a vertical crack in the face and the safety criterion.The main contents and achievements are as follows:(1)Study on the characteristics of medium-long hole blasting loads in underground mining.Based on the actual situation of underground mining in the east of Daye Iron Mine,the numerical model for simulating medium-long hole blasting in underground mining is established by using the dynamic finite element software LS-DYNA.The characteristics of blasting loads outside the cracked area are analyzed,and a method for appling blasting loads is proposed.Based on the numerical modeling results and field measurement data by my research team,its reliability has been tested and verified.(2)Study on the propagation characteristics of blasting vibrations in intact slopes under the influence of different mining depths.Based on the engineering background of 12m-high blocks for sublevel caving method in Daye Iron Mine and the current slope design situation of the open pit mines in China,the slope angle is chosen as 50 o.The simplified numerical models of mine slopes are established.The verified blasting loads appling method is adopted to study the propagation law of blasting vibrations on and in the slope under the influence of different mining depths(i.e.36 m,48m,60 m and 72m).The results show that:(a)The propagation law of the peak particle velocity(PPV)on the slope face is mainly attenuated,and PPVs decrease with an increasing mining depth.(b)Within a certain distance from the slope face,the slope face has a magnifying effect on PPVs and the slope face effect is obvious,but the range of slope face effect is not the same when the mining depth is different.On the whole,the range of slope face effect for horizontal-component PPVs decreases with an increasing mining depth,but that for vertical-component PPVs is basically the same.(3)Study on the prediction model of PPVs close to the slope face under the influence of different mining depths.Based on the principle of dimensional analysis,the traditional Sadowski formula is modified by considering both the influence of mining depth and elevation effect.The modified prediction model is used to predict PPVs close to the slope face under the influence of different mining depths and the results are compared with that of the traditional Sadowski formula.It is found that the accuracy of the modified PPV prediction model is greatly improved.(4)Study on the influence of out-dip structural planes on the propagation characteristics of blasting vibrations under the influence of different mining depths.In actual mine slopes,the slope rock mass usually contains a variety of structural plane of different dimensions,which will have an important impact on the propagation characteristics of blasting seismic waves.First of all,the slope with the slope angle of 50 o and the dip angle of 30 o of the out-dip structural plane is taken as an example to study the propagation law of blasting vibrations on and in the slope under the influence of different mining depths.Then the influence of normal stiffness,the shear stiffness and the dip angle of the structural plane is analyzed.The results show that:(a)The structural plane has a significant effect on the propagation of the nearby blasting vibration on the slope face,the decay rate of PPV is obviously reduced before passing through the structural plane,and then suddenly increases behind the structural plane,but the mining depth has a little influence on the entire distribution law.(b)The distribution of PPV on the both sides of the structural surface is not consistent along the plane,and the mining depth has an important effect on the distribution;compared with the lower side,PPVs on the upper side are greater on the whole.(c)The structural plane makes the distribution of PPV on the two sides of the structural plane in the vertical direction quite different,which is different from the intact slope,and the mining depth will also have an important influence.(d)The increase of normal stiffness of the structural plane can significantly improve the transmission of blasting seismic waves within a certain range;(e)The increase of shear stiffness of the structural plane can significantly make the horizontal-component PPV increase but the vertical-component PPV decrease.(f)The influence of dip angle of the structural plane on the PPV on the slope face and in the slope is different.For the monitoring point S6,horizontal-and vertical-component PPVs increase first and then decrease with the increasing dip angle of the structural plane.For the monitoring point D2,horizontal-and vertical-component PPVs increase with the increasing dip angle of the structural plane.(5)Study on the influence of vertical fracture in the slope face on the propagation characteristics of blasting vibrations under the influence of different mining depths.In open pit to underground mining,due to the long-term geological movement,weathering and artificial excavation and other factors,it is often encountered that vertical cracks are in the slope face,which will have an important impact on the propagation characteristics of blasting seismic waves.First of all,the slope with the slope angle of 50 o and the vertical crack with the depth of 20 m is taken as an example to study the propagation law of blasting vibrations on and in the slope under the influence of different mining depths.Then the influence of the depth of the crack is studied.The results show that:(a)The crack will have an important effect on the propagation law of blasting vibrations on the slope face.Within a certain range in the left side of the crack,PPV on the slope face will increase,but its range decreases with the increasing mining depth.(b)There is a significant slope effect on the PPVs on the both sides of the crack but the range of the slope effect is not consistent.On the whole,the range of slope face effect for horizontalcomponent PPVs decreases with an increasing mining depth,but that for verticalcomponent PPVs increases.(c)The decay rate of PPVs across the crack increases with the increase of the elevation,but decreases with the increasing mining depth.(d)The influence of mining depth on the PPVs on the slope face at different locations is not the same.At the right side,the mining depth has little effect on the distribution of PPV along the slope face,but it has some influence on its value.At the left side,the PPV-increasing range increase with the increasing depth of the crack.(e)The depth of the crack has a large influence on the distribution of PPV in the vertical direction,especially the range of slope face effect.For horizontal-component PPV,the range decreases with the increasing depth of the crack;but for the vertical-component PPV,the range increases with the increasing depth of the crack.(f)The decay rate of PPVs across the crack increases with the depth of the crack.(6)Study on the safety criterion of blasting vibration of defect slopes subjected to open pit to underground mining.(a)For the slope with an out-dip structural plane,the formulas for the transmission and reflection coefficient is derived according to the principle of displacement discontinuous method when the blasting seismic wave is incident on the structural plane.The blasting vibration safety criterion is determined based on the safety factor.The slope with the slope angle of 50 o and the dip angle of 30° of the structural plane taken as an example,the blasting vibration safety criterion is 3.12 cm / s when the frequency of the incident wave is 100 Hz.(b)For the slope with a verticle crack in the slope face,according to the theory of fracture dynamics,the expression of the stress intensity factor at the tip of the crack is derived when the blasting seismic wave is incident on the crack,and the blasting vibration safety criterion is determined based on the fracture toughness.The slope with the slope angle of 50 o and the crack depth of 20 m taken as an example,the blasting vibration safety criterion of blasting seismic wave is 100.614 cm / s when the frequency of the incident wave is 100 Hz.(c)With the increase of the frequency of blasting seismic waves,the blasting vibration safety criterion of the above two types of slopes increases,which indicates that the low-frequency blasting seismic wave is more likely to cause the damage of the defect slope.In the actual projects,we should avoid the low-frequency blasting seismic waves.