Numerical Analysis For The Excavation Failure Zone Of Deep Underground Opening In Hard Rock With High Geostress

Along with the construction of deep underground opening, the formation ofexcavation failure zone and its prediction have become a focus of rock mechanics in deepunderground excavation both in China and abroad. Based on the analysis of mechanicalproperties of hard rock during its failure process, the conventional constitutive models forcontinuous media and the Cohesion Weakening-Friction Strengthening（CWFS）modelwere compared. The post-peak mechanical characteristics of various constitutive modelsand their rationality on the simulation of brittle failure and the excavation failure zone insurrounding rock of deep opening in hard rock with high in-situ stress were alsodiscussed.A parametric sensitivity study was carried out to study the relative importanceof different strength parameters and their significance on the simulation of excavationfailure zone by using different numerical models in virtue of the numerical analysissoftware FLAC which has been suitable for geotechnical engineering. Then, a circularMine-by Experiment test tunnel at the URL in Canada was simulated by using FLAC withelastic-perfectly plastic, elastic brittle, strain softening and CWFS constitutive modelsrespectively. The mechanical parameters used in those models were chosen by consideringthe post-peak mechanical characteristics of various constitutive models and the previousparametric study. In virtue of comparison of displacement, stress distribution, principalstress values at key points and plastic region among different models, the results show thatCWFS model can simulate the stress transfer and stress concentration to a deep area,which is the real situation when stress-induced failure occurs; the stress distributioncalculated from CWFS model is much closer to the one when the excavation failure zoneis deleted. Compared to other models, the extent and depth of excavation failure zonecalculated by CWFS are larger and they are in agreement with the field measurement.Baced on the parametric study and the study of CWFS model in predicting the excavationfailure zone, a simplified method of parameter determination with certain accuracy wasfurther proposed. Finally, this proposed method combined with CWFS model were used toinvestigate the excavation failure zone in two cases, the Kobbskaret road tunnel inNorway and the Garpenberg vertical raise in one of the Boliden mines in Sweden. Thedepth and extent of the excavation failure zone were predicted rather well by comparingthe numerical results with field measurement, showing the rationality on the simulation ofthe depth and extend of excavation failure zone by using this proposed method, and it again proves the rationality of using CWFS model in predicting the excavation failurezone of hard rock in deep tunnel.This simplified method for parameter determination of CWFS model not only reflectsthe post-peak mechanical characteristics of CWFS model，but also shows the relativeimportance of different strength parameters and their significance on the simulation ofexcavation failure．Even on the condition that design and survey data are1imited，thismethod can still provide higher precision in predicting the excavation failure zone of hardrock in deep tunnel. The research has an important role in guiding people to learn theexcavation failure process of surrounding rock, judge stability and predict the excavationfailure zone of tunnel.