Influence On Damage Characteristics Of Surrounding Rock Induced By In-situ Stress Transient Unloading

Large-scale and high-intensity excavation of wide-span underground caverns and extra-long tunnels is required in a large number of large-scaled and extra-large-scaled hydropower projects. And excavation of deep rock mass is also involved in nuclear waste deposit, communication and division tunnels, where blasting excavation is mainly adopted, there is an excavation disturbing problem in surrounding rock of deep-buried engineering.According to the two major factors which induce the damage of urrounding rock:blast load and in-situ stress transient unloading, problems are discussed on the action mechanism of dynamic load, dynamic response of surrounding rock, etc. It is of significance theoretical meaning and engineering application value on improving excavation methods for deep rock mass, evaluating safety level and reducing engineering cost.The dynamic stress field excited by blast load, in-situ stress transient unloading and the coupling effect of both them is mainly studied. And based on that, the damage area distribution characteristics is further determined.Based on relevant theories such as explosion mechanics, fracture mechanics, fluid mechanics, etc, adopting finite element method (FEM), the peak value, duration and process of blast load of column explosive in the borehole is calculated. According to the characteristics of the blast load, and the mechanical boundary of excavation surface in the blasting excavation process, the starting and ending moments, duration and effect process of in-situ stress transient unloading are determined.The strain rate distribution of the rock mass around the borehole induced by the blast load is calculated with theoretical methods. It is revealed by results that, the strain rate is high in the rock mass close to the borehole, and is rapidly decreased with the increase of the distance to the blasting source. And the decreasing rate is lower when the distance to the blasting source is longer. Rock mass dynamic problems and static problems are distinguished by strain rate. The zone division of different strength selection is determined for dynamic and static numerical simulation of surrounding rock. The potential stressed state of surrounding rock under dynamic load is comprehensively analyzed. Corresponding safety criteria is suggested for different damage pattern.Calculation models are put forward and built for dynamic stress field excited by different dynamic load in the surrounding rock. Adopting theoretical calculation method, the dynamic stress field excited by the in-situ stress transient unloading in surrounding rock during the excavation process of round-section tunnel in the hydrostatic stress field is studied, the influence of unloading duration, rock mass medium features, etc, is analyzed. The results show that the transient unloading of in-situ stress has an obvious dynamic effect. The radial stress rapid unloading shows extra-loosening phenomenon, while shows extra-concentrating phenomenon in circumferential stress.Adopting Dynamic FEM, the variation curve of dynamic stress with time is calculated for the surrounding rock of caverns in different section shapes under non-uniform stress field. It is revealed by the result that, the dynamic effect of in-situ stress transient unloading is more obvious in the direction of greater stress, where greater disturbance is applied on the surrounding rock. In the same time, the cavern structure is also influencing the unloading stress field distribution. Radial tensile stress is generated on the side walls of city-gate section channel. When the corresponding excavation load on the unloading boundary is higher, the induced tensile stress in the surrounding rock would be higher.As for the dynamic stress field excited by the coupling of static and dynamic load, due to the high peak value and fast attenuation of blast stress wave, the coupled stress field is mainly blast stress wave in the nearby area, and unloading stress wave in the far area.Base on the dynamic stress distribution in the surrounding rock during the blasting excavation of deep cavern, adopting damage mechanism and relevant safety criteria for rock mass in different stressed state, the damage zone distribution effected by different dynamic load is studied. Under different in-situ stress conditions, the blast load induced damage in the surrounding rock is limited within the surface layer, which is quite shallow and is obviously influenced by the circumferential stress of the secondary stress field. Under high in-situ stress condition, the damage zone formed by in-situ stress transient unloading is larger than that of quasi-static condition, and is similar in shapes, which are determined by the secondary stress field in the surrounding rock. The final damage scale can be regarded as the coupling of the blast load induced and in-situ stress induced damage zones in the surrounding rock. And the in-situ stress transient unloading is the major cause to large-scale damage zone.Based on the actual monitored vibration signals in surrounding rock around the diversion tunnel of Jingping Second Cascade Hydropower Station during the excavation, adopting modulus maximum method and time-energy density method in wavelet analysis, the existence of dynamic effect of in-situ stress transient unloading in actual engineering is proved. Through tests and numerical calculation on damage zone induced by blasting excavation in auxiliary tunnel at Jingping Second Cascade Hydropower Station, the surrounding rock damage induced by in-situ stress transient unloading and in-situ stress redistribution (in-situ stress quasi-statically unloaded) are compared, and the formation mechanism and distribution of different damage zones are analyzed. In order to control and reduce the disturbance of in-situ stress transient unloading in surrounding rock, specific optimizing measures are suggested on both excavation procedure and blasting network in the excavation of deep-buried and large-scale caverns.