Experimental Study On Zonal Disintegration In Deep Rock Mass Under Blasting Load

With continuous increase in exploitation depth of underground engineering, deep rock mass is in a complex and special engineering environment of high geo-stress, high geothermal temperature, high seepage pressure and blasting disturbance, then a series of unique and new geological hazards show up, such as zonal disintegration, large deformation, and rockburst. Aiming at zonal disintegration phenomenon in high axial geo-stress condition, the influence mechanism of blasting load on zonal disintegration and deformation and failure rule of deep surrounding rock under blasting excavation have been investigated by theoretical analysis, three dimensional geomechanical model experiment, particle flow code numerical simulation and in-situ monitoring.Based on Hoek-Brown criterion, elastic stress fields around deep tunnel in both high axial geo-stress state and hydrostatic pressure state have been analyzed theoretically and the fracture zones around deep tunnel are obtained. Therefore, the influence range of blasting load on fracture zone around deep tunnel is deduced.Three dimensional geomechanical model experiments for zonal disintegration in deep rock mass have been carried out under three kinds of excavation methods, manual digging, transient pulling out embedded part and blasting excavation. During the model experiment, the experiment procedure is first loading, second excavation, then overloading. The results show that both stress distribution and strain distribution around model tunnel under three kinds of excavation methods present a non-monotonic change with distance to model tunnel increasing after excavation, interval distribution of peaks and troughs, which indicates that zonal disintegration is a unique failure mode in high axial geo-stress state. During overloading, ring fractures appear around model tunnel, and strain measuring points in the fracture zone increase rapidly while strain measuring points around the fracture zone decrease sharply. After overloading, strain distribution around model tunnel presents a non-monotonic change with distance to model tunnel increasing, interval distribution of peaks and troughs, which corresponds to a fracture zone and a relatively intact zone. Radial stress distribution around model tunnel shows a similar change rule. The increase of loads at the vault and side wall confines the fracture zone around model tunnel. Blasting load during blasting excavation induces many micro-cracks in the surrounding rock around model tunnel. It not only deteriorate the mechanical properties of surrounding rock, but also reduce its integrity. In high axial geo-stress state, these micro-cracks propagate and connect to form a macro fracture zone. Hence, the fracture level of surrounding rock is intensified and the fracture zone is also extended. The radius of next fracture zone is about 1.28 times bigger than that of former fracture zone.After setting up on three dimensional particle models for deep rock mass under blasting excavation, numerical simulations have been carried out under various inclinations of layered joints. Then the influences of action time for blasting load and inclinations of layered joints on zonal disintegration in deep rock mass of underground engineering have been investigated. With the increase of joint inclination, the fracture zone around tunnel, the number of tensile crack and shear crack, and the strain energy increment all present a change of first increase then decrease. When the inclination of layered joints is 60°, the fracture zone of surrounding rock is the biggest of all.Model test of anchorage rock mass in high axial geo-stress state has been carried out by blasting excavation. There is no zonal disintegration in positions supported by combination of anchor bolt and anchor cable, while an evident zonal disintegration presents in positions without support. The combination of anchor bolt and anchor cable can transfer and redistribute the stress of surrounding rock. Hence, the restraining mechanism of anchorage support for zonal disintegration in deep rock mass has been revealed.In-situ blast-induced damage monitoring indicates that the blast-induced damage zone is within 6 m for left sidewall of roadway excavated by drilling and blasting method left sidewall, and it is within 7 m for right sidewall. Moreover, the surrounding rock within 3 m presents a much bigger blast-induced damage effect.