Analysis Of Geostress Field And Rockburst Prediction In The Sang Zhuling Tunnel Of Lasa-Linzhi Railway Engineering

The Sang Zhuling Tunnel of Lasa-Linzhi railway engineering is a special railway tunnel in the Tibet Autonomous Region,and it is a whole-line control and difficult project.The tunnel is in the early stage of construction,the actual data is scarce,but the geologic datum in the region are accurate.The tunnel is in the early stage of construction,there are more obvious high ground stress characteristics,how to distribute the stress field in the tunnel through the area,and whether or not the rockburst activity have become major technical problem for the engineers.Due to the limitation of terrain and traffic conditions,it is difficult to carry out large scale geostress measurement.In view of this,the evolution history of regional tectonics and tectonic stress field is analyzed from the regional geological environment;the observation data of GPS deformation monitoring points around the Yarlung Zangbo fault zone are collected,the influence of the Yarlung Zangbo fault zone on the earth stress environment in the tunnel area is analyzed.Based on the field investigation,the geostress field characteristics of the tunnel area are analyzed synthetically by using the focal mechanism solution and the statistical analysis of the measured geostress data.Based on the measured data and the multiple linear regression principle,the three-dimensional geostress field inversion The results of the analysis and inversion are in good agreement with the comprehensive analysis results and the high ground stress phenomena identified in the field survey.The main results are as follows:(1)There are large-scale strike-slip and thrust-tectonic structures in the area,which are mainly along the east-west direction,and there are obvious signs of early and middle stage activities in the Quaternary.The distribution of shear or tensile fracture structure is mainly along the near SN or NNE into a strip of cloth,since the late Quaternary to a higher degree of activity.The tunnel line has Passed through the East Boundary fault and the Ba Yu fault in the tunnel area,and the two approaches the SN,tend to NW,which are small-scale faults.The distribution of the recent seismicity in the region is north and south,which occurs in the NNE strike-slip fault zone inside the block.The regional tectonic stress should correspond to the stress environment of the strike-slip fault.(2)Comprehensive analysis of the existing literature and research results,the regional tectonic activity is divided into 5 stages:(1)Shan pre-Sinian to Paleozoic stage-Guganwana Continental marginal basin development stage;(2)Shan Late Permian to early Jurassic stage-Tethys basin formation and development period;(3)the stage of the development and extinction of the Shan Jurassic to the Cretaceous stage-Tethys Basin;(4)the early tertiary to Miocene-the crustal shortening and thickening stage,(5)the Shan Pliocene to Quaternary-Himalaya mountain and the entire Qinghai-Xizang plateau greatly uplift stage.(3)There are more than 11 river terraces in the middle reaches of the Yarlung Zangbo river valley.The T6 follow is the stacking terrace,which shows that the tectonic activity is weaker in this period;the T6 above is the base terrace,the order width is generally less than the T6 below,which shows that the Yarlung Zangbo River cuts the earlier alluvial material to bedrock in a relatively short time,and the regional tectonic movement is strong in this period.Among them,the T6 Terrace has the largest width,which shows that during this period,there exists a transition period with relatively stable tectonic activity.(4)Based on collecting the GPS monitoring data in the vicinity of the Yarlung Zangbo fault,according to the regional movement and deformation characteristics of the crust,the characteristics of stress field and the controlling effect of the Yarlung Zangbo fault zone on the crustal stress distribution are analyzed.The results show that some deformation and tectonic stress are released in the Yarlung Zangbo fault zone,and the geostress in the tunnel area is reduced to some extent.(5)The direction of tectonic stress in the tunnel area is NE direction by using the stress environment database and the source mechanism solution.The deep hole geostress data of Sangri and Jiacha area are obtained by hydraulic fracturing method,the horizontal maximum princi Pal stress ?H is 9.41MPa~17.72 MPa and the horizontal minimum princiPal stress ?h is 5.61MPa~13.1MPa;The maximum horizontal princiPal stress dominance direction is N9oW~N7oE,which is consistent with the regional tectonic stress field.Based on previous research results,50 groups of measured geostress data were collected,and the princiPal stress direction of geostress field was analyzed by statistical method.The vertical stress ?v,the maximum horizontal princiPal stress ?H and the minimum horizontal princi Pal stress ?h are linearly fitted according to the buried depth,and the ground stress environment in the different depths of the regional shallow crust is quantitatively analyzed.(6)The stress field in the slope of the river valley can be divided into 3 zones,namely,stress reduction zone,stress increase area and original rock stress area.The range of stress reduction is from the surface of the slope to the horizontal depth of 80 m and the vertical vertical depth of 80 m.Under the depth of the stress reduction zone,the maximum princiPal stress of rock mass fluctuates,and the stress concentration in rock mass is significant,which divides the area into stress-increasing area.In the slope table,the hoizontal depth exceeds 240 m and the downward vertical depth exceeds the 160 m area,the main strength value tends to stabilize along the vertical and horizontal depth,which is divided into the original rock stress area.The range of the valley affected area is about 600 m from the vertical depth of the slope table,and the horizontal depth is about 800 m.(7)Maximum horizontal princi Pal stress(?H)on the Sang Zhuling tunnel axis obtains the maximum value near the mileage K178+600,the magnitude is about 23.0MPa,the minimum horizontal princi Pal stress(?h)is located near the mileage K177+000,the magnitude is approximately 13.5MPa,the maximum vertical stress(?v)is located near the K183+200,and the magnitude is up to 32.0MPa.The angle between the maximum horizontal princi Pal stress direction and the tunnel axis is 25.55 °~89.50 °,and the ground stress environment of tunnel construction is based on the stress field in the slope of the valley bank,(8)Based on the results of the ground stress analysis in the position of the tunnel axis,the possibility of rock burst is comprehensively judged by means of a variety of rockburst prediction methods from the angles of rock mass characteristics,high stress conditions and sectional shape,etc.The results showed that the length of the rock burst activity was about 2580 m,the proportion is about 15.68%,the length of the weak rock burst activity is about 4758 m,the proportion is about 28.92%,the length of the medium rock burst activity is approximately 6450 m,the ratio is about 39.2%,the length of the high rock burst activity is approximately 2667 m,and the ratio is about 16.21%.According to the mechanism of Rockburst,the prevention and control measures of rock burst are put forward.