Study On Influence Factor And Control Technology Of Overbreak And Underbreak In Tunnel Smooth Blasting Construction

At present,the smooth blasting construction is the most important method during tunnel construction in our country.When using smooth blasting technology,the phenomenon of overbreak and underbreak is inevitable.Overbreak and underbreak will significantly increase the cost of tunnel construction and affect the construction safety seriously,so it is very important to study the control measures of overbreak and underbreak.The causes of overbreak and underbreak are complex,these factors include the influence of surrounding rock geological conditions,the influence of drilling equipment and initiating explosive products,the limitation of smooth blasting theory and construction technology,and the unreasonable blasting design scheme.To a certain extent,these factors will affect the shaping effect of smooth blastingBased on the blasting engineering of Badaling tunnel / Great Wall Station of the new Beijing-Zhangjiakou Railway,and referring to many scholars' field test research on the factors affecting the overbreak and underbreak,combined with theoretical analysis and numerical simulation.The following aspects are studied and the following conclusions are obtained:(1)Based on the blasting design scheme in the blasting engineering of Badaling tunnel/Great Wall Station of the new Beijing-Zhangjiakou Railway,the numerical simulation of smooth blasting of the complete rock mass is carried out.Combined with the actual measured overbreak and underbreak data in many practical projects,it can be concluded that: in the actual construction,the opening contour line of peripheral holes should deviate from the design contour line by an appropriate distance,which can make the tunnel contour more consistent with the design contour after blasting.(2)Through the numerical simulation of smooth blasting for the surrounding rock with joints,combined with the theoretical analysis and the field tests of some scholars,it is concluded that when there is a joint between two blastholes,the angle between the joint and the blast hole connection,the strength and width of the joint will also affect the overbreak and underbreak of the tunnel after blasting.(3)Due to the different structure of the pneumatic rock drill and rock drilling jumbo,the scope of application is also different.The construction of drilling jumbo is highly mechanized and the driving speed is fast,this greatly releases the labor force.It has gradually replaced the hand-held pneumatic drill in many occasions.However,due to the large volume of the drilling jumbo,the required operating space is much larger than pneumatic rock drill.In some cases,the application of the air drill will cause the phenomenon of overbreak and under break.At this time,the advantage of using pneumatic rock dril is greater.(4)Based on the theoretical analysis and the field test of some scholars,the numerical simulation of the influence of the distance between the peripheral holes,the arrangement of the empty holes and the axial uncoupling coefficient of the charge on the overbreak and underbreak is carried out.It is concluded that choosing the appropriate distance between the peripheral holes,the arrangement of the empty holes at the proper position and the reasonable axial uncoupling charging coefficient in the blasting design will improve the blasting effect and reduce overbreak and underbreak after blasting.