Characteristic Of The Dynamic Loading And Unloading Responses And Dynamic Mechanism Of Rocks Under High Initial Stress

Abstract:Deep underground rocks are naturally stressed and stored energy by their mass and tectonic movement. Before excavation, a quasi-static equilibrium exists between the rock ahead of the tunnel face and the surrounding ground. When rock is excavated, the pre-existing equilibrium is disturbed, leading to the redistribution of the primary in-situ stress field and energy release, which will induce the accumulation and development of the initial crack of rocks, and then produce macroscopic aging fracture. Mining-induced stress change, release and related rock mass relaxation can drastically influence the stability of underground opening. One of the consequences is that the excavation may induce rock failure or even rock burst around of underground cavities, thus, the deep underground rock failure responses are particular complex. Meanwhile, in the underground excavation processes spalling and other progressive failure modes are often induced. Therefore, mining-induced stress change not only influences the excavation safety, it also seriously influences and changes the demand on the rock support and support capacity。Currently, the research methodologies of deep rock mechanics are main based on separate static, quasi-static and dynamic views, however, the combined effect of the initial stress of rock mass and the outside dynamic disturbance are commonly ignored. Therefore, at the present thesis focus on the influence of the initial stress for elastic stress wave propagation, spalling induced by shock loads, and the theoretical, experimental and numerical simulation methods are applied to explore the failure mechanism of high initial stress rock under the loading and unloading processes.The main contents are as follows:(1) The definite problem of loading and unloading process of rocks is solved by using the mathematical physics method. The initial stress and initial stress gradient on the wave equation, wave velocity and so on are analyzed. An experimental of stress wave propagate in the pre-stressed granite specimen is designed and conducted; the experimental results indicate that compressional and shear wave velocity increase with the magnitudes of initial stress.(2) For the different failure forms of spalling under shock load processes, analysis of the spalling characteristic of one-dimensional (1D) bar, surface spalling induced by underground explosion and spalling in underground structures. Using a long strip of granite specimen conducted the1D spalling fracture process in laboratory; the experimental results indicate that the obvious damage exist in the spalling process.In addition, considering the special condition of the underground rock mass in pre-stressed state, the granite axial spalling strength under confining pressure are achieved by the way of a sphere stress turns to plan stress.Furthermore, the initial stress and stress gradient are introduced to analysis the surface spalling induced by the underground explosion, and a new spalling model is generated。(3) Based on the features of underground rock mass are naturally energy storage body and in a quasi-static equilibrium state, the initialization-dynamic unloading processes of high initial stress rock mass are conducted by using the hybrid implicit-explicit element method. Numerical results indicate that:in the high stress state, when the initial stress was quickly unloaded, rock failure is induced in the initial stress release processes. Meanwhile, for different unloading processes, the concept and calculation method of "equivalent initial stress release rate" and "equivalent strain energy density rate" are proposed, by the numerical simulation method, which are proved can be well characterized the different unloading processes in different initial stress states.(4) Numerical simulation the different loading processes of deep underground tunneling excavation.By setting different initial stresses, different loading paths, and so on, numerical modelling generated a second fracture zone in the far-field and an elastic non-fracture zone between the two fields, i.e., fracture and non-fracture zones occurred alternately around a deep cavity. In addition, the further research demonstrates that the dynamic load conduct on a material with increasing static stress gradient, in addition to the loading location, it will case another stress peak zone at a certain distance away for the loading location, which is the main reason why the fracture and non-fracture zones occurred alternately around a deep cavity, i.e., because of the superposition effect of the static increasing initial stress and decreasing dynamic loading.