Study On Rockburst Precursors And Dynamic Failure Mechanism Of Deep-buried Tunnels

With the development of underground caverns in the fields of hydraulic engineering,mining,railway and highway,the frequency and intensity of rockburst and dynamic instability have gradually increased,which has become a bottleneck restricting the safety of deep-buried caverns.In order to ensure the safety and stability of the deep-buried tunnel under high stress and violent dynamic disturbance,this paper studies the rockburst precursors and dynamic response characteristics of deep-buried TBM-constructed tunnels.This paper systematically studies the above problems by establishing rockburst prediction models,revealing microseismic precursors of rockburst and conducting tunnel tests under dynamic disturbances.The main results are as follows:(1)Based on machine learning algorithms and 325 sets of rockburst cases,nine rockburst comprehensive prediction models that considering multiple factors were established.In the process of model establishment,multiple data preprocessing techniques were used to clean,normalize and reduce the data,and the grid crossvalidation technique was used to obtain the optimal parameters of the models.The results of the model performance evaluation show that the accuracy of the model built in this paper is much higher than the classification performance of the widely used theoretical criteria.Based on the theoretical criteria and the model established in this paper,the rockburst prediction was carried out for the Duoxiongla tunnel.The results are in good agreement with the field situation.(2)Based on the microseismic monitoring results of the Duoxiongla tunnel in Tibet,the characteristics of microseismic activities during the instability of surrounding rock and quantitative precursors of rockburst were revealed.The method for evaluating the stability of surrounding rocks based on spatiotemporal anomalies of microseismic activities and b-value(the slope of the linear section of the magnitude-frequency curve)was established.The relationship between microseismic activities and TBM excavation was built.The excavation damage area was identified based on the double difference tomography method.To reveal the precursors and rules of rockburst,the spatiotemporal distribution,frequency-magnitude distribution,and signals of microseismic events were analyzed.The evolution characteristics of the frequency-spectrum and magnitude-frequency relationship of microseismic activities prior to rockburst were obtained.Based on the critical point theory,the degree of acceleration of energy release was revealed.The results show that prior to rockburst,microseismic activities become more concentrated in a certain area,accompanied by a clear acceleration of energy release,as well as the decrease of the main frequency and b-value.(3)Based on digital image correlation(DIC)technology,the deformation and failure characteristics of a deep-buried tunnel under coupled biaxial static stress and high strain rate dynamic disturbance were experimentally investigated.The effects of lateral pressure coefficient,overbreak and underbreak,fault,and dynamic disturbance direction on the tunnel failure were considered.The crack density was defined to measure the degree of damage of the specimen.The results indicate that the contribution of the dynamic disturbance on the failure of the tunnel is closely related to the static in-situ stress.Sufficient static stress is essential for tunnel failure under dynamic disturbance.The effect of the lateral pressure coefficient is not monotonic but depends on its relationship with 1.For specimens with prefabricated defects,the cracks always initiate from the protruding defects with rough boundary due to the large stress concentration.The effect of the fault on tunnel stability depends on both the distance between the fault and the tunnel and the dip of the fault.Most devastating failure occurs when the dynamic disturbance is applied in the direction of the higher static in-situ stress.