Coupling Effect Of Blasting And Transient Release Of In-situ Stress During Deep Rock Mass Excavation

Excavation of deep-buried rock masses is now becoming an increasingly common construction work in the projects of hydropower, mining and radioactive waste disposal. Deep masses of various natures are characterized by high in-situ stress. When the high-stress rock masses are excavated with the drill and blast method, in-situ stress on excavation boundaries is suddenly released for an instant of rock fragmentation by blasting, which is a dynamic mechanical process. During this process, some new deformation and failure phenomenons, suah as rock burst, large abrupt-deformation and microseismic events have been observed. However, the conventional theory system based on the assumption of quasi-static unloading is unable to satisfactorily explain these new phenomenons. Therefore, investigating the mechanism of the transient release of in-situ stress and its dynamic effects will be a great help to a better understanding of initiation mechanism and propagation process of the excavation disturbed zone (EDZ). Furthermore, it also finds its engineering applications in optimizing design of deep rock mass blasting excavation and controlling engineering disasters.The deep rock mass excavation by blasting involves two major mechanical processes, blast loading and the transient release of in-situ stress on excavation faces. This dissertation focus on the coupling mechanism and effects of the two mechanical processes and a series of related studies are conducted with methods of theoretical analysis, numerical calculations and engineering example verification. Some main research results are concluded as follows.According to the basic theories of explosion mechanics, fracture mechanics, fluid mechanics, etc, explosive detonation propagation, blasthole expansion induced by explosion pressure, crack propagation driven by detonation gases, stemming movement and detonation gas venting, the whole phases of explosion for a cylindrical charge are theoretically analyzed. Under this scenario, a new blast loading pressure variation with time, fully describing the complicated physical and mechanical action of blast, is derived to approximate the blast loading as a direct load input in numerical modelling.During deep rock mass excavation by blasting, the transient release of in-situ stress on excavation faces occurs along with the process of rock fragmentation by blasting and formation of new free surfaces. Its mechanical process can be described with parameters including a magnitude of the redistributed in-situ stress corresponding to sequences of millisecond-delay blasting, starting time, duration of time and a stress release path. When blast-induced cracks between adjacent blastholes are interconnected and the blast loading decays to a level equaling the magnitude of the stress on the excavation faces, the in-situ stress begins to be entirely released along the newly-formed excavation faces, and when the blast loading declines to the same level of atmosphere pressure, the process of the in-situ stress release simultaneously ends. For full-face excavation by shallow-hole blasts adopted in most tunneling projects in China, if the in-situ stress reaches a magnitude of20～50MPa, the duration of the stress release on excavation faces during blasting ranges from2to5ms, and the resulting strain rate in the immediate vicinity of the excavation faces lies in the range from10-1to101s-1. It is generally accepted that when the strain rate caused by loading or unloading exceeds the critical value of10-1s-1, the inertial force should not be ignored and the process is a dynamic one.A mechanical model for the coupling action of blast loading and transient release of in-situ stress is proposed and developed in this dissertation. On this basis, space-time variation of the stress field under the blast loading, the transient release of in-situ stress and the coupling action of both are calculated by using the dynamic finite element method. The results indicates that, compared to redistributed secondary in-situ stress, the transient release of in-situ stress on excavation faces produces additional dynamic stress in surrounding rock masses, amplifying the effects of the radial unloading and circumferential loading. Magnitude of the additional dynamic stress is closely related to in-situ stress level, dimension of excavation faces and release rate of stress. During deep rock mass excavation by blasting, surrounding rock masses are firstly subjected to disturbance of the blast loading, resulting in radial and circumferential stresses first increase then decrease, and then disturbed by the additional dynamic stress due to the transient release of in-situ stress, which leads to the radial and circumferential stresses first decrease then increase, and finally tending to the redistributed secondary stress state. In comparison, the blast loading plays a major role in the coupling effects of dynamic loads on the stress field in surrounding rock masses.By developing a damage model of rock masses under the coupling action of dynamic loads and static loads, this dissertation presents numerical simulation of rock damage mechanisms and evolution for a deep-buried tunnel blasting excavation, with a special emphasis on the influence of repeated dynamic disturbance corresponding to sequences of millisecond-delay blasting. When highly stressed rock masses are excavated by the method of drill and blast, behavior of surrounding rock damage is attributed to the combined effects of stress redistribution, blast loading and additional dynamic stress due to the transient release of in-situ stress. Pattern of damage for rock masses subjected to blast loading is mainly represented by tensile damage. For deep rock masses, the presence of in-situ stress suppresses the tension effect of blast and equivalently improves the tensile strength of rock masses. If the magnitude of the in-situ stress reaches up to a high level, the stress release (unloading) due to excavation induces compression-shear damage in a wide zone of surrounding rock masses, while the blast-induced damage is limited in a small zone near the tunnel surface, and the excavation-induced stress release is mainly responsible for the rock damage. Compared to redistributed secondary in-situ stress, the transient release of in-situ stress on excavation faces produces greater extent of damage in surrounding rock masses, furthermore, the extent increases with increase in in-situ stress level, dimension of excavation faces and release rate of stress.Attenuation of peak particle velocity (PPV) and characteristics of vibrational frequency induced by different dynamic loads are also calculated by the numerical method. The results show that blasting excavation-induced vibration in deep rock masses are attributed to the combined action of blast loading and transient release of in-situ stress. Generally, near-field vibration is mainly caused by the blast loading. However, because the frequency of the blast loading-induced vibration is higher than that of the transient release of in-situ stress and high-frequency vibration always attenuates with distance faster than low-frequency vibration, in middle and far field, the transient release of in-situ stress will exceed the blast loading and become the main factor causing vibration in surrounding rock masses.With the methods of time-energy density analysis, amplitude spectrum analysis and numerical simulation of whole time-history blasting vibration, vibration induced by transient release of in-situ stress is identified and separated from vibrational waves measured during blasts of actual deep rock mass projects. Aamplitude spectrums of the measured vibrational waves have two dominant frequency bands and the low-frequency component results mainly from the transient release of in-situ stress, while the high-frequency component originates primarily from the blast loading. According to this identification, vibrational waves induced by the transient release of in-situ stress can be approximately obtained by separating the low-frequency component from the measured signals with a low-pass filter, where the cutting frequency is determined as the inflection point between two peaks in amplitude-frequency curves. Analysis results of the actual projects demonstrate the occurrence of the transient release of in-situ stress and its dynamic effects, and also confirm the dependability of the theoretial analysis and numerical calculations conducted in this dissertation.