ANALYSIS OF INSITU GEOSTRESS AND ROCKBURST |AT NO.2 VENTILATION SHAFT TUNNEL OF QINLING HIGHWAY TUNNEL

The No.2 ventilation shaft of Qinling highway tunnel is mainly located in the mixed gneiss stratum. The main technical problems encountered in the shaft excavation are high insitu geostress and rockburst. The paper examined the engineering geology of the shaft and the geostress measurement results of nearby engineering by hydraulic fracturing method. Then, the geostress field in the shaft area is calculated and regressed with 3D finite element method. It is shown that the geostress in the shaft area has a high horizontal geological structure stress and is ly distributed in zones. The maximum horizontal geostress is 28.7 MPa. The vertical geostress is controlled by weight of rockmass. Based on the geostress field, the rockburst prediction in the shaft is made according to the Tao's rockburst criteria. The depth of rockburst failure area is estimated with the constantdeviatoricstresscriterion of brittle rock mass. The results show that there are high possibilities of occurring weak and moderate rockbursts. The maximum depth of failure is 0.6 m in the surrounding rocks of the shaft. The results from the geostress and rockburst analysis are well compared with the actual situations of rockbursts after shaft excavation.