Study On Blasting Damage And Cumulative Effect Of Surrounding Rock In Excavation Of Great Wall Station

Great Wall Railway Station is the first underground high-speed railway station constructed by mining method in China.The station has the characteristics of large excavation section,large number of caverns and complex construction technology.Blasting excavation inevitably causes disturbance and damage of rock mass in the near area and vibration damage of rock mass in the far area.The repeated blasting loads make the cumulative damage effect of the rock mass especially significant,and seriously threaten the stability of the surrounding rock of the tunnel.Therefore,mastering the cumulative damage characteristics of rock mass under repeated blasting loads have become one of the key issues of this project.This paper takes the blasting excavation of the Great Wall Station tunnel as the research background,and combines the surrounding rock acoustic wave test and numerical calculation method to systematically and deeply study the surrounding rock damage and its cumulative effect under blasting.The main research contents and conclusions are listed as follows:(1)The cumulative damage growth law of tunnel surrounding rock under different excavation conditions is systematically studied by using acoustic testing technology.Based on the data measured in-situ,the cumulative damage degree and range of rock mass under blasting load are determined.The influence of tunnel working face distance and excavation sequence on cumulative damage of rock mass is analyzed.Through data analysis,it is concluded that the damage of tunnel surrounding rock includes the first impact damage caused by explosive stress wave and the cumulative damage caused by blasting vibration.(2)Based on the monitoring data of acoustic wave velocity and vibration velocity of rock mass in situ,the damage evolution law of rock mass in small spacing tunnel excavated by step method is analyzed.The evolution process of the blasting damage of the interlaid rock is:the blasting damage of the surrounding rock occurs when the later adjacent working face of the tunnel is flush with the monitoring section,and the damage increment is much larger than others.When the working face is gradually away from the monitoring section,the damage range of the interlaid rock remains unchanged,which indicates that the damage range of the intercalated rock is only related to the first overpass damage caused by explosive stress wave.During the excavation process,the disturbed rock mass of the excavated section continued to deteriorate,and the deformation damage caused by blasting vibration dominated the cumulative damage of surrounding rock.(3)Based on the damage weakening effect of blasting loading on rock mass,the Hoek-Brown strength criterion is revised.A new method is proposed to quantitatively determine the geological strength index(GSI)and disturbance parameter DB by applying the acoustic velocity of rock mass and the basic quality index(BQ)of rock mass,namely GSI=15vpm-7.5 and DB=2(1-10(BQd-BQud)/243.772).A link between rock and rock mechanics parameters based on acoustic velocity is established.The mechanical parameters of the surrounding rock of the Great Wall Station tunnel are estimated by using the modified Hoek-Brown criterion mentioned above.Based on the acoustic velocity and rock saturated compressive strength,a revised Hoek-Brown model of attaching rock and rock mass mechanics parameters is established.The mechanical parameters of the surrounding rock of the Badaling Great Wall Station tunnel were estimated by using the modified Hoek-Brown criterion.(4)Based on theoretical analysis and literature investigation,a set of experimental methods for obtaining parameters of RHT constitutive model are proposed.Based on the rock mass mechanics parameters derived from the modified Hoek-Brown criterion,the RHT constitutive model parameters of granite rock mass are determined to establish a rock mass blasting damage model,which provides a theoretical basis for further numerical analysis of blasting damage of tunnel surrounding rock.(5)According to the millisecond delay blasting of the large-section tunnel of Great Wall Station,the cumulative damage of surrounding rock under repeated dynamic loadings is calculated by using numerical simulation method.The cumulative step accumulation characteristics of the rock mass damage during the millisecond delay blasting are studied,and the impact and contribution of different blastholes to the final damage of the surrounding rock is obtained.The simulated damage range of surrounding rock has good consistency with the in-situ test results,which indicates that the proposed rock mass blasting damage model and parameter determination method are effective and feasible,which can provide some theoretical guidance for blasting excavations which are like this.(6)For the first time,rock mass damage model in the millisecond delayed blasting is established.Based on this model,the cumulative damage effect of the surrounding rock of the small spacing tunnels is numerically simulated.The law of the propagation and energy transfer of the explosive stress wave in the interlaid rock are studied.The occurrence mechanism and influencing factors of the damage are discussed,and the "energy absorption" effect of the damaged rock mass of the existing tunnel is analyzed.