| Tunnel excavation and blasting operation will cause repeated disturbances to the surrounding rock of the tunnel,and the failure and instability of the surrounding rock caused by it is a hot and difficult point in the study of rock impact dynamics.In order to deeply understand the dynamic mechanical response characteristics of mountain tunnel rock under different stress states,based on the sub-topic"Experimental Study on Static and Dynamic Mechanical Properties of Surrounding Rock"of the transverse subject "Research on Key Technology and Engineering Demonstration of New Type of Energy-accumulating Hydraulic Blasting Construction for Highway Tunnels in Plateau Mountainous Areas",taking the three-level surrounding rock of Mengsheng Super-long Tunnel as the research object,the one-dimensional and three-dimensional dynamic and static SHPB loading tests of tunnel surrounding rock are systematically carried out,and the dynamic mechanical response and energy transfer law of tunnel surrounding rock are systematically studied.The main research contents and conclusions are as follows:(1)The impact tests of limestone specimens under different dynamic loads(σA=0MPa、14.40MPa、21.60MPa、28.80MPa和 36.00MPa)were carried out by using one-dimensional dynamic and static SHPB system.The deformation characteristics of specimens at strain rate of 80s-1 and the strength characteristics of specimens at strain rate of 80s-1～130s-1 were analyzed respectively.It was found that when the strain rate was constant,the dynamic compressive strength showed a relationship of "y=ea+bσA+cσA2 "with the increase of axial pressure.The transmission ratio exhibits a" ηT=ea+bWI(t)+cWI(t)2 relationship with the increase of incident energy.The failure process of limestone recorded by high-speed camera is used to verify its macroscopic failure mode,which shows the same regularity:when the axial pressure increases from 0 to 36MPa,the failure mode changes from transverse tensile failure to tensile-shear failure and compression-shear failure one by one.(2)Limestone specimens under different axial pressures(σA=12、24 and 36MPa)and different confining pressures(Cp=2.5、5、7.5、10 and 15MPa)were subjected to different amplitudes of cyclic impact with 0.2MPa as the initial impact pressure(gradient of 0.1MPa)by using a three-dimensional dynamic-static combined SHPB system.It was found that the mechanical mechanism of the first compaction and then damage of the specimen was reflected in the whole cyclic impact process.There was a critical threshold for the anti-deformation ability of the limestone specimen affected by the axial confining pressure,and the impact resistance of the limestone specimen exceeding this critical threshold would be weakened.With the increase of impact times,the reflection of the specimen increases first and then decreases,the transmittance decreases first and then increases,and the higher axial confining pressure has a deterioration effect on the mechanical properties of the rock.When σA/Cp=7.2,the size of the axial pressure is the key to affecting the energy absorption rate.When σA/Cp=2.4,the size of the confining pressure is the key to affecting the energy absorption rate.(3)Combined with SEM and EDS energy spectrum analysis,the macro-meso failure and damage law of limestone under cyclic impact of three-dimensional dynamic and static combination(σA=24 MPa、Cp=5、7.5、10、15 MPa)was studied.It was found that under the condition of cyclic impact of lower axial confining pressure stress,the plastic deformation of limestone accumulated more rapidly,the dimple of ductile fracture first appeared,and finally plastic deformation occurred.The damage variable is defined based on the longitudinal wave velocity method.Under the condition of low confining pressure,the damage accumulation rate of limestone is the fastest,and the critical value of cumulative damage before mutation is the largest.The variable D of limestone shows an exponential growth trend with the increase of cumulative specific energy absorption value v:"D=ea+bv+cv2". |
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