Rock damage characteristics of tunnels under impact splitting and the mechanism of in-situ fracture expansion by blasting

Stabilizing the surrounding rock provides an important environmental safeguard for tunnel blasting. To investigate the damage characteristics and rupture mechanism of the surrounding rock in long-distance tunnels under cyclic explosive loading, it takes the in-situ surrounding rock and rock samples of diversion tunnel underneath the highway as the study object. The impact splitting test was carried out with the SHPB test system. It revealed the mechanical behaviors and energy evolution characteristics of the rock body. It is also introduced the 4D ultra-high-definition imaging technology of the mine roadway to further analyze the in-situ fissure expansion mechanism of the tunnel peripheral rock under the cyclic explosive loading. The results show that: ① The splitting stress-strain curves of granite and tuff under impact splitting are different from the “stress yield” characteristic of impact compression. But they have the “residual stress” characteristics. The peak stress and dynamic growth factor DIF show a positive linear growth trend; The strain time curve does not exhibit the characteristic of “later reduction” deformation during impact compression. It exhibits the characteristic of “later stability” deformation as an elastic brittle material. ② The rock mass impacting energy - time curve undergoes an evolutionary stage of “initial slow growth - fast linear growth - later stable”. The energy conversion ratio of the rock mass under the same impact energy level satisfies the following conditions: reflected energy ratio > dissipated energy ratio > transmitted energy ratio; The overall damage and failure of the rock mass exhibits a state characteristic of “central splitting + loading end crushing → central splitting + loading end crushing + semi circular fracture → overall crushing”. ③ A peephole imaging test combining “less domains and more tests” and “more domains and less tests” is proposed. The crack propagation characteristics of the in-situ surrounding rock hole wall are mainly manifested in following evolutionary stage: “microcrack development → crack propagation → slow propagation → stop extension”. There is no significant phenomenon of fragmentation detachment and further inward extension. ④ The observation results of the D1 hole of the top pipe tunnel at the foot of the mountain and the D2 hole of the Shancha maintenance tunnel have verified the good boundary control blasting effect; The impact splitting results of rock samples taken from the original site of the tunnel provide an effective theoretical basis and reference for the design of pre blasting drilling and blasting schemes.