Study On The Influence Of Blasting On The Radius Of Broken Rock Zone And Its Correction Calculation

The thickness of the broken rock zone is the basis for the design of the roadway support,which can be obtained by theoretical calculation,numerical simulation,field measurement and intelligent prediction.However,due to the complexity and diversity of the roadway site conditions,there are many factors affecting the formation of the broken rock zone,including initial ground stress,surrounding rock strength,excavation method,roadway section shape and size,etc.For various reasons,it is difficult to measure the thickness of the loosening circle in the field,and the discrete type of the measured data is large,which still needs to be combined with the experience of the tester to make a comprehensive judgment.Compared with the field measurement method,the theoretical calculation of the loosening circle of the surrounding rock is more convenient and faster.At present,the theoretical calculation of the perimeter rock loosening circle is mainly based on the strength criterion method.Through the study of the theoretical calculation of the loosening circle radius formula based on Mohr-Coulomb strength criterion,Hoek-Brown strength criterion and unified strength,it is found that the thickness of the perimeter rock loosening circle obtained by the theoretical calculation method is smaller than the size of the field actual measurement in most cases.The reason for this is that the theoretical calculation method is generally based on the "static" excavation and does not consider the effect of the "dynamic" blasting.Based on the consideration of the influence of blasting excavation on the surrounding rock damage,the theoretical formula of Mohr-Coulomb strength criterion method to calculate the loosening circle of surrounding rock is proposed to be modified by using the blasting damage factor η.Based on the methods of geological radar and borehole photography,we measured the broken rock zone of two roadways under the same conditions in Chengjiao coal mine,and found that the thickness of the loosening circle of the surrounding rock in blasting excavation is generally greater than that in "static" excavation by roadheading machine.Compared with the "static" excavation,the thickness of the broken rock zone of the tunnel under blasting excavation increased by 2.4%～7.3%.The numerical simulation software ANASYS/LS-DYNA was used to construct a numerical calculation model for dynamic excavation of the roadway and to analyze and study the stresses in the broken rock zone.The numerical simulation takes into account the ground stress,rock properties,and factors such as light blasting and normal blasting,etc.By comparing and analyzing the loosening circle in the gang area of the roadway,the reference value of blasting damage coefficient η for the correction calculation of the loosening circle of the surrounding rock is 1.02～1.10,in which the radius of the loosening circle of the surrounding rock will be increased by 2%～5% by normal blasting compared with light blasting.The value of blasting damage factor η decreases with the increase of rock strength and decreases with the increase of ground stress.Based on the above three aspects of research and analysis,it is considered necessary to make blast damage corrections to the Mohr-Coulomb strength criterion for calculating the radius of the perimeter loosening circle formula,and the correction coefficients can be referred to the range of values studied in this thesis.