ROCKBURST FAILURE CHARACTERISTICS AND FORMATION MECHA-NISM OF DEEP GRANITE TUNNEL WITH EXTREMELY HIGH GEO-STRESS
During the construction of a deep granite tunnel with extremely high geo-stress,rockburst disasters are extremely serious.The rock mass flakes off,forms slab-shaped fractures,and violently ejects rocks due to rock-bursts.To reveal the formation mechanism of rockbursts in the granite tunnel,the formation,characteristics,and influencing factors of tunnel high geostress and rockbursts were discussed and analyzed.This includes engineering geological survey,monitoring and measurement,rockburst record statistics,indoor rock mechanics tests,micro-scopic analysis,numerical simulation,et al.According to statistical analysis of rockburst records,rockburst disas-ters are prominent in the construction of the granite tunnel,with complex and diverse damage characteristics and concentrated occurrence times.When affected by horizontal tectonic stress,the intensity of rockbursts near the side of the Yarlung Zangbo valley increases significantly,and when controlled by vertical stress,rockbursts are mainly distributed on the face and vault.Based on measured in-situ stress data of boreholes and inversion analysis of the in-itial geostress field,the granite tunnel is dominated by horizontal tectonic stress and vertical stress.82.0％of the area along the tunnel is in a state of high to extremely high geo-stress,providing internal dynamic geological condi-tions for the occurrence of rock bursts.Laboratory rock mechanics tests show that both monzogranite and granodiorite have a moderate rockburst tendency,and the rock mass with thick layers,high strength,high stiffness,and integri-ty provide sufficient provenance conditions for rockbursts.Controlled by the special tectonic structure of the Yarlung Zangbo suture zone and the influence of Cenozoic magmatic activities,the granite tunnel presents high geothermal characteristics with a geothermal gradient of 5.5 ℃/100m,and the highest measured ground temperature reached 89.6 ℃.The high-temperature thermo-mechanical coupling accelerates the occurrence of rockburst and increases the rockburst risk level.The quality of tunnel construction technology will change the stress redistribution of sur-rounding rock,affect the stress concentration of surrounding rock,cause a certain size effect,and affect the intensi-ty level,time,and spatial distribution of rockburst.
Granite tunnelExtremely high geo-stressRockburst failure characteristicsFormation mechanismHigh temperatureRockburst tendency
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