Study On Mechanical Response And Damage Mechanism Of Surrounding Rock During Roadway Excavation Unloading

The excavation unloading effect of roadway/tunnel surrounding rock is directly related to the stability of surrounding rock,the design of support and the selection of construction technology.With the continuous in-depth development of geotechnical engineering,the environment of rock mass is more complex,and the problems of roadways/tunnels stability and support are more and more difficult,the challenges and dangers are increasing.During or after the excavation of underground rock engineering in deep complex environment,unloading and creep often occur in the rock mass near the entrance of the cave,leading to large deformation and rock burst,etc.,and resulting in a series of engineering disasters such as casualties and economic damage.Therefore,it is necessary to study the deformation law,strength characteristics,stress distribution characteristics and damage evolution mechanism of surrounding rock under the condition of roadway/tunnel excavation and unloading.In order to study the excavation unloading mechanical response and the mechanism of damage and fracture of surrounding rock,small cement mortar thick-walled cylinder specimens（290 mm in height,200 mm in outer diameter and60～100 mm in inner diameter）were taken as the research object to carry out the following work.The research was performed as follows:the method research about model testing to simulate tunnel excavation unloading,deformation law and failure characteristics of surrounding rock during roadway excavation unloading,the study of control effect of excavation face spatial effect on radial deformation of surrounding rock using the improved sealing device,acoustic emission（AE）characteristics and damage estimation of surrounding rock under excavation unloading,PFC^3D numerical simulation study on damage evolution and fracture mechanism of surrounding rock under tunnel excavation unloading,yield criteria and yield mechanism about thick-walled cylinder model.The three-directions deformation law and macroscopic failure characteristics of surrounding rock specimens under unloading were studied,and the macroscopic damage evolution of surrounding rock under unloading was further comprehensively studied from the microscopic point of view through AE laboratory tests and PFC^3D numerical simulation.Details of the research are provided below:（1）A set of methods for burying strain gauges in small cement mortar thick-walled cylinder specimens and a testing method for the deformation inside the specimens were proposed.Acoustic emission testing technology was integrated into the testing system that simulates tunnel excavation and unloading.Finally,a testing method using small cement mortar thick-walled cylindrical specimens to simulate roadway excavation unloading was established,which can obtain the axial,radial and tangential deformation inside and on the surface of the specimen,the failure,and the AE characteristics.（2）Based on the established testing method,unloading tests with different unloading rates and different stress levels were carried out to systematically analyze the deformation law and macro failure characteristics of surrounding rock under different influencing factors.The radial,tangential and axial strain variation rules of the tunnel wall and surrounding rock at different locations,and the failure characteristics of surrounding rock specimens were obtained.The effect of unloading speed and stress levels on the deformation law and failure modes of surrounding rock were revealed.The failure characteristics of the specimens were analyzed from the macroscopic point of view,and it was found that the unloading effect would continue to affect the surrounding rock after the end of unloading.（3）The laboratory unloading testing study based on the spatial effect of excavation face need to consider the supporting effect of the face.Therefore,the testing system was improved,and the partially hollow thick-walled cylinder specimens（290 mm in height,200 mm in outer diameter,60 mm in inner diameter,and 60 mm high solid rock mass reserved at the bottom）with one end open and the other solid were used to carry out the testing of the excavation face spatial effect.The control law of the excavation face spatial effect on the radial deformation and the influence of the stress level on the excavation face spatial effect was obtained.（4）In order to study the micro-fracture inside the surrounding rock specimen,the AE characteristic tests under different unloading rates and different stress levels were carried out based on the established testing method,and a good corresponding relationship between the AE characteristics and unloading deformation was obtained during excavation unloading.Based on the analysis of AE parameter response in time-domain and frequency-domain and b value characteristics,the damage estimation of the specimen was carried out.The relative damage of surrounding rock during and after unloading was quantitatively analyzed by the damage variableD_SB.（5）In order to further study the microscopic crack propagation law and mechanical characteristic response inside the specimen under unloading,a thick-walled cylinder model was established by using PFC^3D to reveal the crack propagation law and damage evolution process of surrounding rock during the unloading process.The influence of stress levels on the mechanical properties of surrounding rock was analyzed.According to the process of accumulation-release of energy stored in surrounding rock,the unloading failure mechanism of surrounding rock was revealed.（6）Based on the mechanical model of thick-walled cylinder specimens,the strength criterion was established.It is found that the stress circle of the thick-walled cylinder specimens will reach the elastic limit faster during the unloading process,compared with the stress circle of loading and unloading of core specimens（?50×100mm）.Therefore,it was easier to reach the elastic limit state of the thick-walled cylinder specimens during unloading.