## 5 Conclusions  
Rockburst hazards induced by high geostress are particularly prominent in underground engineering construction. Prevention and control of rockburst is still a global challenge in the field of geotechnical engineering, which is of great significance. Based on the tunnels of the Jinping II hydropower station in China, this paper analyzed the mechanical principle of support in the construction process, and discussed the active release and passive support by numerical simulation and field application in detail. The following conclusions were drawn:  
The surrounding rock of auxiliary tunnel is mainly marble with poor water absorption, the effect of common measures like surface water spraying and borehole water injection is very little. Boring holes in tunnel wall can release the energy of the microseismic source and transfer the high stress to the deeper surrounding rock, and the surface rock wall with relatively low stress acts as a protective barrier. The stress release rate of surrounding rock increases linearly with the diameter of borehole. The smaller the borehole spacing, the greater the stress release rate. However, increasing the boreholes length has little effect on rockburst prevention. In this project, with holes diameter of 150 mm and length of 2 m and spacing of 2 m, the stress release rate was about 12%.  
Advanced stress relief blasting can also release the energy and transfer stress to the deeper surrounding rock and form a protective rock wall. Comparing to the stress release hole, advanced stress relief blasting reaches is more useful for stronger rockburst conditions, which reaches about 33% stress release rate. It can also make the surface rock wall with relatively low stress act as a protective barrier. Considering the limit of tunnel small section and longtime needed for boring holes, a new 14-hole advanced stress relief blasting scheme was proposed, which was composed of main shallow holes on the tunnel face and supplementary deep holes on the periphery.  
Different from active measures that stress release hole and advanced stress relief blasting releasing and transferring the stress of surrounding rock, passive measures are also an effective way to prevent and control the rockburst under high geostress. Based on the simulation of tunnel with different thickness of shotcrete, it could be found that high-strength rapid support could reduce the damage zone of shotcrete itself, effectively control the occurrence of rockburst. In order to achieve that better, this paper introduced two measures: Water swelling anchor and nano-admixture shotcrete. The installation speed of water swelling anchors is five times higher than normal ones, but because of low corrosion resistance it needs to cooperate with mortar bolts. The nano-admixture shotcrete has high early strength, high one-time shotcrete thickness and other excellent characteristics. The combined support composed of swelling anchor, nano-admixture shotcrete, steel mesh and steel arch ribs were proposed, which achieves good effect in practice.  
[PERSON] analyzed the calculation results and wrote the article; [PERSON] carried out the calculations; [PERSON] provided the information of the parameters; [PERSON] processed the data; [PERSON] offered useful suggestions for the preparation and writing of the paper.  
The study was supported by Sichuan Science and Technology Program (No. 2019 YFG0460) and the National Natural Science Foundation of China (NSFC) under Grant No. 51508037.  
We also highly appreciate the contribution of data collection from 2 nd Engineering Pte Ltd. of the Chinese Railway 2 nd Bureau, Ertan Hydropower Development Company, Ltd., and East China Investigation and Design Institution. Finally, the authors would like to thank reviewers for useful comments and editors for improving the manuscript.  
The authors declare no conflict of interest.