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Editor Work2021 Vol. 29, No. 2
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2021, 29(2): 275-288.
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The Sichuan-Tibet railway corridor has high crustal stress, temperature and water pressure. The soft rock is a difficult problem that cannot be ignored. However, the distribution characteristics and engineering effects of the soft rock have not been deeply studied. This paper is based on the geological map of railway corridor, rock mass structure investigation and rebound test as well as microanalysis. It finds that suture zones had obvious control effect on the formation of soft rocks. The paper has the following findings. (1)There are Jinshajiang, Lancangjiang, Nujiang and Yarlung Zangbo suture zones along Sichuan-Tibet railway corridor. Under the influence of the special tectonic activity pattern, temperature and pressure conditions in the suture zone, the soft rocks such as schist, slate, mylonite, cataclasite and damaged rock zones are mainly distributed in the suture zone and on both sides of the boundary faults. (2)The tectonic damage, deformation and metamorphism of rock mass were caused by the plate splitting and converging movement, which led to the deterioration of rock mechanical properties and form soft rocks in the area. (3)The large deformation risk of railway tunnel engineering is higher in the soft rock section controlled by suture zone. Water and mud inrush will be easy to occur when the tunnel engineering passes through tectonic soft rocks near the active bound faults of suture zone. Besides, mountain hazards in suture zones mainly occur in soft rock area with intense river erosion. These findings can provide scientific basis for the prediction and prevention of engineering geological problems of soft rocks along Sichuan-Tibet railway corridor, and also lay foundation for the research of deformation mechanism and engineering prevention technology of soft rock under plate collision and compression.
The Sichuan-Tibet railway corridor has high crustal stress, temperature and water pressure. The soft rock is a difficult problem that cannot be ignored. However, the distribution characteristics and engineering effects of the soft rock have not been deeply studied. This paper is based on the geological map of railway corridor, rock mass structure investigation and rebound test as well as microanalysis. It finds that suture zones had obvious control effect on the formation of soft rocks. The paper has the following findings. (1)There are Jinshajiang, Lancangjiang, Nujiang and Yarlung Zangbo suture zones along Sichuan-Tibet railway corridor. Under the influence of the special tectonic activity pattern, temperature and pressure conditions in the suture zone, the soft rocks such as schist, slate, mylonite, cataclasite and damaged rock zones are mainly distributed in the suture zone and on both sides of the boundary faults. (2)The tectonic damage, deformation and metamorphism of rock mass were caused by the plate splitting and converging movement, which led to the deterioration of rock mechanical properties and form soft rocks in the area. (3)The large deformation risk of railway tunnel engineering is higher in the soft rock section controlled by suture zone. Water and mud inrush will be easy to occur when the tunnel engineering passes through tectonic soft rocks near the active bound faults of suture zone. Besides, mountain hazards in suture zones mainly occur in soft rock area with intense river erosion. These findings can provide scientific basis for the prediction and prevention of engineering geological problems of soft rocks along Sichuan-Tibet railway corridor, and also lay foundation for the research of deformation mechanism and engineering prevention technology of soft rock under plate collision and compression.
2021, 29(2): 289-306.
The strong and complex movement of active fault zones inevitably causes different degrees of damage to the crustal rock mass. These damages can significantly affect the processes of earthquake rupture, geomorphic evolution, geological disaster, and engineering rock mass. However, the tectonic damage of crustal rock mass around large active faults is still not well understood. The concept of the tectonic damage of crustal rock mass is firstly proposed. The tectonic damage is irreversible, accumulative, heterogeneous, and healable. We measure tectonic discontinuities of the outcrops in the six major fault zones on the eastern margin of the Tibetan Plateau. They are the Xianshuihe fault zone, the Longmenshan fault zone, the Anninghe fault zone, the Daliangshan fault zone, the Zemuhe fault zone, and the Xiaojiang fault zone. The crustal rock mass outside fault core can be divided into a damage zone and wall rock according to the intensity of tectonic discontinuities. This intensity is a quantitative index of tectonic damage. Then we analyze the spatial pattern, the characteristics of damage, and the influence factors of the tectonic damage of crustal rock mass. The main conclusions are as follows: (1)Damage degree shows a decrease with perpendicular distance from the main fault trace. High damage areas alternate with low damage areas in the damage zone. The widest damage zone is 3100 m; (2)The rock mass in damage zone and wall rock is damaged in brittle regime at high strain rate and low strain rate, respectively. And the orientations of tectonic discontinuities in damage zone show a strong correlation with recent crustal stress field; (4)The cumulative displacement that characterizes the development stage of faults determines the scale of the damage zone, and the local variations are mainly controlled by fault geometry and lithology; (5)A new conceptual model based on cumulative displacement, fault geometry, lithology, and depth is proposed to explain the pattern of the tectonic damage of crustal rock mass around the active fault zone. This study can provide evidences for constraining the structure of crustal rock masses in seismic dynamics, tectonic geomorphology, geological disaster, and engineering, and help to deepen the understanding of the crustal rock mass of active fault zones.
The strong and complex movement of active fault zones inevitably causes different degrees of damage to the crustal rock mass. These damages can significantly affect the processes of earthquake rupture, geomorphic evolution, geological disaster, and engineering rock mass. However, the tectonic damage of crustal rock mass around large active faults is still not well understood. The concept of the tectonic damage of crustal rock mass is firstly proposed. The tectonic damage is irreversible, accumulative, heterogeneous, and healable. We measure tectonic discontinuities of the outcrops in the six major fault zones on the eastern margin of the Tibetan Plateau. They are the Xianshuihe fault zone, the Longmenshan fault zone, the Anninghe fault zone, the Daliangshan fault zone, the Zemuhe fault zone, and the Xiaojiang fault zone. The crustal rock mass outside fault core can be divided into a damage zone and wall rock according to the intensity of tectonic discontinuities. This intensity is a quantitative index of tectonic damage. Then we analyze the spatial pattern, the characteristics of damage, and the influence factors of the tectonic damage of crustal rock mass. The main conclusions are as follows: (1)Damage degree shows a decrease with perpendicular distance from the main fault trace. High damage areas alternate with low damage areas in the damage zone. The widest damage zone is 3100 m; (2)The rock mass in damage zone and wall rock is damaged in brittle regime at high strain rate and low strain rate, respectively. And the orientations of tectonic discontinuities in damage zone show a strong correlation with recent crustal stress field; (4)The cumulative displacement that characterizes the development stage of faults determines the scale of the damage zone, and the local variations are mainly controlled by fault geometry and lithology; (5)A new conceptual model based on cumulative displacement, fault geometry, lithology, and depth is proposed to explain the pattern of the tectonic damage of crustal rock mass around the active fault zone. This study can provide evidences for constraining the structure of crustal rock masses in seismic dynamics, tectonic geomorphology, geological disaster, and engineering, and help to deepen the understanding of the crustal rock mass of active fault zones.
2021, 29(2): 326-341.
The Sichuan-Tibet Railway is the most challenging railway project ever in the world. The project region is characterized by various adverse geological conditions including active tectonic movement, intensive mega faults, abrupt topographic relief and frequent geological disasters. Therefore, the construction of this project is challenged by complex geological disaster risks from both ground surface and underground. This study comprehensively and quantitatively analyzes the ground surface and underground geological risks along the Sichuan-Tibet Railway, and further evaluates their consequential influences on the project so as to support its feasibility study. The risk analysis utilizes abundant spatial-temporal data, and various approaches including three-dimensional structure modeling, numerical modeling, statistical modeling, dynamic modeling, time series modeling and spatial distribution modeling. The results show that there are three areas concentrated with high ground surface geological hazards risk along the Sichuan-Tibet railway, namely the Xianshuihe fault zone, the Jinshajiang fault zone and the eastern Himalayan syntaxis area. However, ground surface engineering risks of this project are significantly reduced, because its underground tunnels are over 80% of the total railway line length. The universal quantitative risk assessment models are developed for major underground engineering geological risks, including fault activities, rock bursts and large deformations. For several major tunnels such as the Yigong tunnel, risks of typical individual underground geological hazards(e.g. rock burst and large deformation) as well as the integrated risk, are quantitatively evaluated. The results show that both surface and subsurface engineering risks are significant and can threaten the safety of project. This study provides a scientific and technical support for the feasibility study of the Sichuan-Tibet Railway, and is also expected to be a reference for major engineering geological risk control for similar line engineering projects around the world.
The Sichuan-Tibet Railway is the most challenging railway project ever in the world. The project region is characterized by various adverse geological conditions including active tectonic movement, intensive mega faults, abrupt topographic relief and frequent geological disasters. Therefore, the construction of this project is challenged by complex geological disaster risks from both ground surface and underground. This study comprehensively and quantitatively analyzes the ground surface and underground geological risks along the Sichuan-Tibet Railway, and further evaluates their consequential influences on the project so as to support its feasibility study. The risk analysis utilizes abundant spatial-temporal data, and various approaches including three-dimensional structure modeling, numerical modeling, statistical modeling, dynamic modeling, time series modeling and spatial distribution modeling. The results show that there are three areas concentrated with high ground surface geological hazards risk along the Sichuan-Tibet railway, namely the Xianshuihe fault zone, the Jinshajiang fault zone and the eastern Himalayan syntaxis area. However, ground surface engineering risks of this project are significantly reduced, because its underground tunnels are over 80% of the total railway line length. The universal quantitative risk assessment models are developed for major underground engineering geological risks, including fault activities, rock bursts and large deformations. For several major tunnels such as the Yigong tunnel, risks of typical individual underground geological hazards(e.g. rock burst and large deformation) as well as the integrated risk, are quantitatively evaluated. The results show that both surface and subsurface engineering risks are significant and can threaten the safety of project. This study provides a scientific and technical support for the feasibility study of the Sichuan-Tibet Railway, and is also expected to be a reference for major engineering geological risk control for similar line engineering projects around the world.
2021, 29(2): 342-352.
Climate warming and engineering actions impact and accelerate permafrost degradation, resulting in significant changes of engineering stability in permafrost regions. This article mainly discusses the relationship between climate warming and engineering stability from the view of permafrost changes and engineering hazards under the impacts of climate warming and engineering. We summarize the changes in active layer thickness and permafrost temperature under climate warming and in permafrost table and temperature under the embankment of Qinghai-Tibet Highway and Railway and their subgrade deformation. Especially, we summarize the engineering measures of preventing permafrost thaw under the embankment in Qinghai-Tibet Plateau and discuss the role of these measures of cooling underlying permafrost under the future climate warming and its impact on the engineering serviceability. The results show that the Plateau permafrost has undergone remarkable degradation during the past decades and engineering thermal impacts accelerated permafrost degradation under the embankment, resulting in engineering instability. Qinghai-Tibet Railway used some measures of cooling the underlying permafrost, which can adapt to climate warming of 1.0 ℃. Under the future climate warming of 1.5 ℃, reinforcement measures of Qinghai-Tibet Railway is required and has to be planned as soon as possible.
Climate warming and engineering actions impact and accelerate permafrost degradation, resulting in significant changes of engineering stability in permafrost regions. This article mainly discusses the relationship between climate warming and engineering stability from the view of permafrost changes and engineering hazards under the impacts of climate warming and engineering. We summarize the changes in active layer thickness and permafrost temperature under climate warming and in permafrost table and temperature under the embankment of Qinghai-Tibet Highway and Railway and their subgrade deformation. Especially, we summarize the engineering measures of preventing permafrost thaw under the embankment in Qinghai-Tibet Plateau and discuss the role of these measures of cooling underlying permafrost under the future climate warming and its impact on the engineering serviceability. The results show that the Plateau permafrost has undergone remarkable degradation during the past decades and engineering thermal impacts accelerated permafrost degradation under the embankment, resulting in engineering instability. Qinghai-Tibet Railway used some measures of cooling the underlying permafrost, which can adapt to climate warming of 1.0 ℃. Under the future climate warming of 1.5 ℃, reinforcement measures of Qinghai-Tibet Railway is required and has to be planned as soon as possible.
2021, 29(2): 365-374.
Under the comprehensive influences of tectonics, weathering, and acidification of acid rain, the rock mass along the Sichuan-Tibet Railway has extensive deep shattered zones. When the rock masses are excavated due to constructions, a series of deep and cataclastic rock slopes can be formed. These slopes are highly susceptible to collapse as well as overall instability, which is hard to be reinforced by the traditional safety netting system(SNS). This study proposes a new composite structure for slope protection, anchored flexible system, replacing bolts of SNS with prestressed anchor cables to constrain the surface displacement of the cataclastic rock slope while simultaneously stabilizing the slope body. Here, we conduct 1 ︰ 10 large-scale indoor model tests to explore the stress-strain characteristics, the coordinated loading mechanism, and the design method of the anchored flexible systems for cataclastic rock slope protection under heavy rainfall conditions. Model test reveals the coordinated loading between both prestressed anchor cables and nets in anchored flexible systems when the cataclastic rock slope deformation happens. The stress of the flexible nets has significant impacts on the working state of the prestressed anchor cable, and it can form an obvious re-tension, which should be fully taken into account in the design.
Under the comprehensive influences of tectonics, weathering, and acidification of acid rain, the rock mass along the Sichuan-Tibet Railway has extensive deep shattered zones. When the rock masses are excavated due to constructions, a series of deep and cataclastic rock slopes can be formed. These slopes are highly susceptible to collapse as well as overall instability, which is hard to be reinforced by the traditional safety netting system(SNS). This study proposes a new composite structure for slope protection, anchored flexible system, replacing bolts of SNS with prestressed anchor cables to constrain the surface displacement of the cataclastic rock slope while simultaneously stabilizing the slope body. Here, we conduct 1 ︰ 10 large-scale indoor model tests to explore the stress-strain characteristics, the coordinated loading mechanism, and the design method of the anchored flexible systems for cataclastic rock slope protection under heavy rainfall conditions. Model test reveals the coordinated loading between both prestressed anchor cables and nets in anchored flexible systems when the cataclastic rock slope deformation happens. The stress of the flexible nets has significant impacts on the working state of the prestressed anchor cable, and it can form an obvious re-tension, which should be fully taken into account in the design.
2021, 29(2): 375-382.
With the extension of China's railway to the western mountainous areas such as the Qinghai-Tibet Plateau, there are more and more deep buried tunnels. High in-situ stress has become a kind of complex geological condition which controls the route selection scheme. If it is possible to quantitatively assess the in-situ stress state of the tunnel site before lifting, then the risk control is the most active in the stage of route selection. We have measured a lot of in-situ stress data along the Lalin section of Sichuan-Tibet railway. The structure of in-situ stress in the study area is mainly reverse stress regime and strike-slip stress regime. The direction of in-situ stress is distributed between N19°W~N30°E. The average transition depth of in-situ stress state is about 500 m. Less than that depth, the maximum horizontal in-situ stress has disordered change with buried depth. But the maximum value does not exceed the maximum stress level. More than that depth, the maximum horizontal stress can be estimated approximately by Helm formula. This is the most important rule of in-situ stress with depth. Based on this: If the buried depth of the tunnel is less than the transition depth, the supporting structure and construction method should be dynamically adjusted during the construction period. Because this is conducive to control the risk of high in-situ stress. If the buried depth of the tunnel is more than the transition depth, the in-situ stress of tunnel site can be estimated by Helm formula. According to the understanding, we can use the method of route selection to choose the better scheme of economy and technology. Finally, the route selection program of high in-situ stress area is established. The paper gives useful ideas for solving the problem of lack of quantitative evaluation method of in-situ stress in railway scheme selection stage.
With the extension of China's railway to the western mountainous areas such as the Qinghai-Tibet Plateau, there are more and more deep buried tunnels. High in-situ stress has become a kind of complex geological condition which controls the route selection scheme. If it is possible to quantitatively assess the in-situ stress state of the tunnel site before lifting, then the risk control is the most active in the stage of route selection. We have measured a lot of in-situ stress data along the Lalin section of Sichuan-Tibet railway. The structure of in-situ stress in the study area is mainly reverse stress regime and strike-slip stress regime. The direction of in-situ stress is distributed between N19°W~N30°E. The average transition depth of in-situ stress state is about 500 m. Less than that depth, the maximum horizontal in-situ stress has disordered change with buried depth. But the maximum value does not exceed the maximum stress level. More than that depth, the maximum horizontal stress can be estimated approximately by Helm formula. This is the most important rule of in-situ stress with depth. Based on this: If the buried depth of the tunnel is less than the transition depth, the supporting structure and construction method should be dynamically adjusted during the construction period. Because this is conducive to control the risk of high in-situ stress. If the buried depth of the tunnel is more than the transition depth, the in-situ stress of tunnel site can be estimated by Helm formula. According to the understanding, we can use the method of route selection to choose the better scheme of economy and technology. Finally, the route selection program of high in-situ stress area is established. The paper gives useful ideas for solving the problem of lack of quantitative evaluation method of in-situ stress in railway scheme selection stage.
2021, 29(2): 383-393.
Karst geothermal system is one of the most important geothermal types. The particularity of karst media makes geothermal groundwater easier to be influenced by the shallow circulating cold groundwater during its formation, as a result, differs the genetic mechanism of karst geothermal system from others. In present research, the karst geothermal system of Tongluoshan anticline at the margin of Sichuan Basin is taken as an example to reveal the potential misunderstanding of geothermal groundwater genesis caused by the mixing of cold and hot water. A comprehensive approach using of hydrogeochemistry and isotope coupled with field survey is performed to get insight into the mixing process of cold and hot water at the discharge zone of geothermal system. The effects of the mixing on the understanding of geothermal system are deeply discussed. A conceptual model of karst geothermal groundwater system with cold-hot water mixing was established. The results show that the mixing effect of cold and hot water is extremely strong at the discharge zone of geothermal system. The mixing ratio of cold water can be up to 73% ~89% in Tongluoshan geothermal system. The mixing of cold and hot water would potentially destroy the information of thermal groundwater for geothermal system, resulting in underestimating the recharge elevation and reservoir temperature. This underestimation reaches to 78% and 45% for recharge elevation and reservoir temperature, respectively, in Tongluoshan geothermal system. The true model of Tongluoshan karst geothermal groundwater system is recognized by eliminating the effects of cold and hot water mixing. The geothermal system is recharged by precipitation in the exposed karst mountain area at an elevation of 2100~2400 m at the north. Recharged water is heated by the terrestrial heat flow during its deep circulation. The reservoir temperature of carbonate aquifers of lower Triassic Jialingjiang formation(T1j) and middle Triassic Leikoupo formation(T2l) is estimated ranging from 128~172 ℃. Thermal groundwater is controlled by the karst and geological structure of anticline, and flows from the northeast to the southwest. Thermal water mixes with the local shallow circulating cold water during its discharge. As a result, the temperature of exposed thermal water declines to 38~62 ℃.
Karst geothermal system is one of the most important geothermal types. The particularity of karst media makes geothermal groundwater easier to be influenced by the shallow circulating cold groundwater during its formation, as a result, differs the genetic mechanism of karst geothermal system from others. In present research, the karst geothermal system of Tongluoshan anticline at the margin of Sichuan Basin is taken as an example to reveal the potential misunderstanding of geothermal groundwater genesis caused by the mixing of cold and hot water. A comprehensive approach using of hydrogeochemistry and isotope coupled with field survey is performed to get insight into the mixing process of cold and hot water at the discharge zone of geothermal system. The effects of the mixing on the understanding of geothermal system are deeply discussed. A conceptual model of karst geothermal groundwater system with cold-hot water mixing was established. The results show that the mixing effect of cold and hot water is extremely strong at the discharge zone of geothermal system. The mixing ratio of cold water can be up to 73% ~89% in Tongluoshan geothermal system. The mixing of cold and hot water would potentially destroy the information of thermal groundwater for geothermal system, resulting in underestimating the recharge elevation and reservoir temperature. This underestimation reaches to 78% and 45% for recharge elevation and reservoir temperature, respectively, in Tongluoshan geothermal system. The true model of Tongluoshan karst geothermal groundwater system is recognized by eliminating the effects of cold and hot water mixing. The geothermal system is recharged by precipitation in the exposed karst mountain area at an elevation of 2100~2400 m at the north. Recharged water is heated by the terrestrial heat flow during its deep circulation. The reservoir temperature of carbonate aquifers of lower Triassic Jialingjiang formation(T1j) and middle Triassic Leikoupo formation(T2l) is estimated ranging from 128~172 ℃. Thermal groundwater is controlled by the karst and geological structure of anticline, and flows from the northeast to the southwest. Thermal water mixes with the local shallow circulating cold water during its discharge. As a result, the temperature of exposed thermal water declines to 38~62 ℃.
2021, 29(2): 404-415.
Snow avalanche is one of the most common geological disasters ubiquitously distributed in the Qinghai-Tibet Plateau. Snow avalanche susceptibility mapping is critical for disaster prevention and mitigation of major engineering construction such as the Sichuan-Tibet Railway. This paper exemplifies the Parlung-Tsangpo catchment in south-eastern Tibet, collects 381 channelled snow avalanches that fell below the timberline through field surveys and remote sensing interpretation. First of all, 18 influence factors are selected and quantitatively extracted by using GIS, remote sensing, et al. Then, Principal Component Analysis(PCA)method is performed to obtain the key predisposing factors and assign weight coefficient to each factor. Thereafter, the Weighted Information Value Method(PCA-Ⅰ) and Weighted Certainty Factor Method(PCA-CF) are conducted to map snow avalanche susceptibility. Finally, the performance of two models is compared and evaluated based on ROC(receiver operating characteristic) curve and AUC(area under receiver). The results show that the susceptibility assessment is very sensitive to climate, macro-topography, micro-topography, and inhibition factors. Among them, the climate explained 30.61% of the data variation; topography explained 21.23% of the data variation. The PCA-Ⅰ model gives the susceptibility zoning index in the range of [-2.41, 1.365], and the PCA-CF model gives the susceptibility zoning index ranging in[-0.549, 0.424]. In addition, the susceptibility map of the PCA-Ⅰ model has a significant abnormal area in downstream at the Tongmai section. Thus, the zoning index of the PCA-CF model is more reasonable than that of the PCA-Ⅰ model. The overall results suggest that the high snow avalanche prone areas are mainly distributed at Ranwu-Yupu section along the National Road 318 around in the upper narrow valley section of Parlung-Tsangpo river, the ridges of the mountains on both sides of the middle and lower reaches, and the narrow valley sections of the tributaries.
Snow avalanche is one of the most common geological disasters ubiquitously distributed in the Qinghai-Tibet Plateau. Snow avalanche susceptibility mapping is critical for disaster prevention and mitigation of major engineering construction such as the Sichuan-Tibet Railway. This paper exemplifies the Parlung-Tsangpo catchment in south-eastern Tibet, collects 381 channelled snow avalanches that fell below the timberline through field surveys and remote sensing interpretation. First of all, 18 influence factors are selected and quantitatively extracted by using GIS, remote sensing, et al. Then, Principal Component Analysis(PCA)method is performed to obtain the key predisposing factors and assign weight coefficient to each factor. Thereafter, the Weighted Information Value Method(PCA-Ⅰ) and Weighted Certainty Factor Method(PCA-CF) are conducted to map snow avalanche susceptibility. Finally, the performance of two models is compared and evaluated based on ROC(receiver operating characteristic) curve and AUC(area under receiver). The results show that the susceptibility assessment is very sensitive to climate, macro-topography, micro-topography, and inhibition factors. Among them, the climate explained 30.61% of the data variation; topography explained 21.23% of the data variation. The PCA-Ⅰ model gives the susceptibility zoning index in the range of [-2.41, 1.365], and the PCA-CF model gives the susceptibility zoning index ranging in[-0.549, 0.424]. In addition, the susceptibility map of the PCA-Ⅰ model has a significant abnormal area in downstream at the Tongmai section. Thus, the zoning index of the PCA-CF model is more reasonable than that of the PCA-Ⅰ model. The overall results suggest that the high snow avalanche prone areas are mainly distributed at Ranwu-Yupu section along the National Road 318 around in the upper narrow valley section of Parlung-Tsangpo river, the ridges of the mountains on both sides of the middle and lower reaches, and the narrow valley sections of the tributaries.
2021, 29(2): 478-485.
Debris flow is one of the most developed disaster types in the Tibetan Plateau, and poses a certain threat to the construction of Sichuan-Tibet Railway and the safety of life and property due to its sudden outbreak, violent movement process, and strong destructive characteristics. Geological hazard assessment is part of the effective measures in the management and prevention of disaster prevention and mitigation. For reasonably quantifying the spatial distribution characteristics of debris flow hazard along the railway line, the paper takes the Zire-Bomi section of the Sichuan-Tibet Railway within Linzhi City as the experimental area. We apply the Bayesian optimization algorithm based random forest(TBOR) and gradient boosting tree model(TBOG)to quantify the debris flow hazard values of the route section and identify the hazard levels. The input information of the two models includes 172 historical hazard points and 11 feature parameters. The output information is the outbreak probability of debris flow for each prediction unit. Lastly, the ROC-AUC method is employed to test the evaluation outcomes of the two prediction algorithms. The findings reveal that in the TBOR and TBOG models, the overall debris flow hazard level of the Zire-Bomi line section of the Sichuan-Tibet Railway is relatively high. The proportion of hazard zones inside the high-higher hazard interval of the two models reaches 56.439% and 66.580% respectively, corresponding to the highest hazard point densities of 12.577/(102km2) and 12.940/(102km2)respectively. Compared to the ROC-AUC values derived from the TBOG model, the TBOR model computes 0.89, which is higher than that of TBOR at 0.83. Therefore, the TBOR model has better prediction accuracy. The research findings can provide necessary reference for the construction of disaster prevention and mitigation protection projects along Sichuan-Tibet Railway and the hazard evaluation of other line sections.
Debris flow is one of the most developed disaster types in the Tibetan Plateau, and poses a certain threat to the construction of Sichuan-Tibet Railway and the safety of life and property due to its sudden outbreak, violent movement process, and strong destructive characteristics. Geological hazard assessment is part of the effective measures in the management and prevention of disaster prevention and mitigation. For reasonably quantifying the spatial distribution characteristics of debris flow hazard along the railway line, the paper takes the Zire-Bomi section of the Sichuan-Tibet Railway within Linzhi City as the experimental area. We apply the Bayesian optimization algorithm based random forest(TBOR) and gradient boosting tree model(TBOG)to quantify the debris flow hazard values of the route section and identify the hazard levels. The input information of the two models includes 172 historical hazard points and 11 feature parameters. The output information is the outbreak probability of debris flow for each prediction unit. Lastly, the ROC-AUC method is employed to test the evaluation outcomes of the two prediction algorithms. The findings reveal that in the TBOR and TBOG models, the overall debris flow hazard level of the Zire-Bomi line section of the Sichuan-Tibet Railway is relatively high. The proportion of hazard zones inside the high-higher hazard interval of the two models reaches 56.439% and 66.580% respectively, corresponding to the highest hazard point densities of 12.577/(102km2) and 12.940/(102km2)respectively. Compared to the ROC-AUC values derived from the TBOG model, the TBOR model computes 0.89, which is higher than that of TBOR at 0.83. Therefore, the TBOR model has better prediction accuracy. The research findings can provide necessary reference for the construction of disaster prevention and mitigation protection projects along Sichuan-Tibet Railway and the hazard evaluation of other line sections.
2021, 29(2): 486-494.
The Quaternary deep overburden is developed in a proposed airport site on the southern margin of the Qinghai-Tibet Plateau, and induces a great restriction on the construction of the project. Drilling data shows that the deep overburden is generally distributed in the entire field area with a thickness of more than 30m, and the maximum hole depth of 105m has not been exposed. Vertically, the deep overburden can be divided into four layers from top to bottom: Holocene debris flow accumulation layer(Q4sef), Holocene alluvial accumulation layer(Q4al+pl), Holocene lacustrine accumulation layer(Q4l) and Late Pleistocene Alluvial accumulation layer(Q3al+pl). Among them, the debris flow is mainly composed of breccia and gravel, with a maximum thickness of about 13m, distributed on the west side of the research site; the Holocene alluvial strata is composed of complex materials, mostly silty fine sand and round gravel, without obvious layering rules, with a maximum thickness of about 7m; the lacustrine sedimentary silty clay is plastic-flow plastic, with sand in some parts, and the maximum thickness is about 43m; the late Pleistocene alluvial strata are dominated by fine sand and medium-coarse sand, with pebbles and gravels locally, and the thickness is greater than 42m. The analysis shows that the cause of the deep overburden is closely related to the rapid uplift of the Himalayas since the Late Pleistocene and the interglacial climate, while the surface overburden is closely related to the warming and the short-term alternate climate of cold and warm of the Qinghai-Tibet Plateau climate since the Holocene. The test results reveal that the physical and mechanical properties of the deep overburden are obviously different, and there are major engineering geological problems including uneven settlement, seepage failure, slope stability, and foundation damage induced by freezing and thawing.
The Quaternary deep overburden is developed in a proposed airport site on the southern margin of the Qinghai-Tibet Plateau, and induces a great restriction on the construction of the project. Drilling data shows that the deep overburden is generally distributed in the entire field area with a thickness of more than 30m, and the maximum hole depth of 105m has not been exposed. Vertically, the deep overburden can be divided into four layers from top to bottom: Holocene debris flow accumulation layer(Q4sef), Holocene alluvial accumulation layer(Q4al+pl), Holocene lacustrine accumulation layer(Q4l) and Late Pleistocene Alluvial accumulation layer(Q3al+pl). Among them, the debris flow is mainly composed of breccia and gravel, with a maximum thickness of about 13m, distributed on the west side of the research site; the Holocene alluvial strata is composed of complex materials, mostly silty fine sand and round gravel, without obvious layering rules, with a maximum thickness of about 7m; the lacustrine sedimentary silty clay is plastic-flow plastic, with sand in some parts, and the maximum thickness is about 43m; the late Pleistocene alluvial strata are dominated by fine sand and medium-coarse sand, with pebbles and gravels locally, and the thickness is greater than 42m. The analysis shows that the cause of the deep overburden is closely related to the rapid uplift of the Himalayas since the Late Pleistocene and the interglacial climate, while the surface overburden is closely related to the warming and the short-term alternate climate of cold and warm of the Qinghai-Tibet Plateau climate since the Holocene. The test results reveal that the physical and mechanical properties of the deep overburden are obviously different, and there are major engineering geological problems including uneven settlement, seepage failure, slope stability, and foundation damage induced by freezing and thawing.
2021, 29(2): 495-507.
The unstable rock mass develop in the intersection section of Qinghai-Tibet Railway, G109 National Highway and Beijing-Tibet Expressway, which pose a great threat to the construction and safe operation of traffic engineering. The geological background, development characteristics and deformation failure mechanism of the unstable rock mass are systematically analyzed in order to find out the instability conditions, the main controlling factors and to evaluate the stability. Using block theory and discrete element method, the stability and dynamic response of dangerous rock mass under dead weight condition and seismic load are studied respectively. The study shows that the unstable rock mass can be divided into three parts: the integral collapse source at the top, the fissure zone in the middle and the accumulation area of the block stone in the lower. and there are four groups of dominant structural planes. The calculation results show that the dangerous rock block is basically stable under the condition of dead weight. The unstable modes of dangerous rock mass under the action of seismic load are shear sliding, fracture and fall. Short-time violent earthquakes intensified the accumulation of internal damage in the unstable rock mass. To ensure safety, it is suggested to carry out targeted prevention and control, monitoring and early warning.
The unstable rock mass develop in the intersection section of Qinghai-Tibet Railway, G109 National Highway and Beijing-Tibet Expressway, which pose a great threat to the construction and safe operation of traffic engineering. The geological background, development characteristics and deformation failure mechanism of the unstable rock mass are systematically analyzed in order to find out the instability conditions, the main controlling factors and to evaluate the stability. Using block theory and discrete element method, the stability and dynamic response of dangerous rock mass under dead weight condition and seismic load are studied respectively. The study shows that the unstable rock mass can be divided into three parts: the integral collapse source at the top, the fissure zone in the middle and the accumulation area of the block stone in the lower. and there are four groups of dominant structural planes. The calculation results show that the dangerous rock block is basically stable under the condition of dead weight. The unstable modes of dangerous rock mass under the action of seismic load are shear sliding, fracture and fall. Short-time violent earthquakes intensified the accumulation of internal damage in the unstable rock mass. To ensure safety, it is suggested to carry out targeted prevention and control, monitoring and early warning.
2021, 29(2): 508-515.
Based on the consideration of the temperature and the number of cycles on the internal structure damage of the granite sample, we studied the mechanical properties and acoustic emission response characteristics of the granite after high temperature-rapid cooling cycles by the uniaxial compression test and the acoustic emission monitoring test. The results show the follows. As the temperature and number of cycles applied to the granite increase, the uniaxial compressive strength of the granite continued to decrease, and the peak strain corresponding to the uniaxial compressive strength increased accordingly. There was a clear difference in the intensity and distribution characteristics of the acoustic emission signal of granite between the normal temperature and the high temperature treatment. As the temperature applied to the granite increased, the maximum acoustic emission count rate and the cumulative ring count of the acoustic emission increased first and then decreased. With the increase of the number of heating-cooling cycles for granite, the maximum value of AE count rate and cumulative AE ring count tended to decrease, while the intensity of AE signal tended to increase.
Based on the consideration of the temperature and the number of cycles on the internal structure damage of the granite sample, we studied the mechanical properties and acoustic emission response characteristics of the granite after high temperature-rapid cooling cycles by the uniaxial compression test and the acoustic emission monitoring test. The results show the follows. As the temperature and number of cycles applied to the granite increase, the uniaxial compressive strength of the granite continued to decrease, and the peak strain corresponding to the uniaxial compressive strength increased accordingly. There was a clear difference in the intensity and distribution characteristics of the acoustic emission signal of granite between the normal temperature and the high temperature treatment. As the temperature applied to the granite increased, the maximum acoustic emission count rate and the cumulative ring count of the acoustic emission increased first and then decreased. With the increase of the number of heating-cooling cycles for granite, the maximum value of AE count rate and cumulative AE ring count tended to decrease, while the intensity of AE signal tended to increase.
2021, 29(2): 516-525.
Based on the background of Xi'an Metro Line 6 section tunnel with shallow excavation construction through the ground fissure site, and considering the particularity of the ground fissure site, the construction optimization method based on the traditional CRD method was constructed. The three-dimensional finite element numerical calculation was carried out to simulate the excavation process of traditional CRD and optimized CRD method. Combined with in-situ monitoring test data, the surface settlement deformation of the ground fissure site and the stress deformation characteristics of the metro tunnel were analyzed. The results show that the surface settlement deformation is anti-s-shaped, which can be roughly divided into three stages: small deformation before excavation, rapid settlement deformation during excavation and stable deformation after excavation. The closer the monitoring points are to the tunnel face, the higher the ground settlement. Compared with the traditional CRD method, the optimized CRD method can increase appropriately the number of locking feet and waist bolts, improve the concrete strength of primary support properly, and so as to simplify the temporary support, expand the construction surface of the lower stage, facilitate the removal of temporary support, and improve the speed of the closure of the initial support and the removal of the temporary middle diaphragm. The construction speed of the optimized CRD method is 1.37 times faster than that of the traditional CRD method. The maximum surface settlement is reduced by 52.96%, the influence range is reduced by 22.17%, and the settlement of the tunnel roof is reduced by 54.53%. The optimized CRD method has the advantages of rapid construction speed, lower influence range, easily controlled surface and structure settlement, which can not only improve engineering benefit, but also guarantee the construction safety and the reliability of the tunnel construction. The research results can provide a scientific reference for the underground excavation construction of metro tunnel through ground fissure site in Xi'an city and other areas.
Based on the background of Xi'an Metro Line 6 section tunnel with shallow excavation construction through the ground fissure site, and considering the particularity of the ground fissure site, the construction optimization method based on the traditional CRD method was constructed. The three-dimensional finite element numerical calculation was carried out to simulate the excavation process of traditional CRD and optimized CRD method. Combined with in-situ monitoring test data, the surface settlement deformation of the ground fissure site and the stress deformation characteristics of the metro tunnel were analyzed. The results show that the surface settlement deformation is anti-s-shaped, which can be roughly divided into three stages: small deformation before excavation, rapid settlement deformation during excavation and stable deformation after excavation. The closer the monitoring points are to the tunnel face, the higher the ground settlement. Compared with the traditional CRD method, the optimized CRD method can increase appropriately the number of locking feet and waist bolts, improve the concrete strength of primary support properly, and so as to simplify the temporary support, expand the construction surface of the lower stage, facilitate the removal of temporary support, and improve the speed of the closure of the initial support and the removal of the temporary middle diaphragm. The construction speed of the optimized CRD method is 1.37 times faster than that of the traditional CRD method. The maximum surface settlement is reduced by 52.96%, the influence range is reduced by 22.17%, and the settlement of the tunnel roof is reduced by 54.53%. The optimized CRD method has the advantages of rapid construction speed, lower influence range, easily controlled surface and structure settlement, which can not only improve engineering benefit, but also guarantee the construction safety and the reliability of the tunnel construction. The research results can provide a scientific reference for the underground excavation construction of metro tunnel through ground fissure site in Xi'an city and other areas.
2021, 29(2): 526-535.
Thousands of coseismic landslides were induced by the Jiuzhaigou MS7.0 earthquake on August 8th 2017, Sichuan, China. Thus, a tremendous volume of loose debris are extremely prone to convert into landslides and debris flow under rainfall. Therefore, it is necessary to conduct landslide susceptibility assessment in Jiuzhaigou Scenic area. Based on a large set of pre-and post-earthquake high-resolution satellite images and field investigation, a polygon-based interpretation of the coseismic landslides is carried out. In total, 1047 landslides are identified, covering an area of 3.88 km2. The relationship between the spatial distribution of landslides and its influencing factors is analyzed. Nine factors including tectonic, topographic, geological and other factors are selected to evaluate the landslide susceptibility by certainty factor model and logistic regression model, as well as coupled model. The result shows that slope, aspect, elevation and lithology are the main factors controlling the spatial distribution of landslides. The susceptibility of landslide is divided into low(60.72%), moderate(24.18%), high(9.89%) and extremely high areas(5.21%). The high and extremely high areas covering an area of 99 km2 including Tiger Lake and Panda Lake are distributed along the valleys. The assessment result of the coupled model is more reasonable than that of the single model and can be used as scientific reference for prevention of landslide hazard and segmented opening time of scenic area.
Thousands of coseismic landslides were induced by the Jiuzhaigou MS7.0 earthquake on August 8th 2017, Sichuan, China. Thus, a tremendous volume of loose debris are extremely prone to convert into landslides and debris flow under rainfall. Therefore, it is necessary to conduct landslide susceptibility assessment in Jiuzhaigou Scenic area. Based on a large set of pre-and post-earthquake high-resolution satellite images and field investigation, a polygon-based interpretation of the coseismic landslides is carried out. In total, 1047 landslides are identified, covering an area of 3.88 km2. The relationship between the spatial distribution of landslides and its influencing factors is analyzed. Nine factors including tectonic, topographic, geological and other factors are selected to evaluate the landslide susceptibility by certainty factor model and logistic regression model, as well as coupled model. The result shows that slope, aspect, elevation and lithology are the main factors controlling the spatial distribution of landslides. The susceptibility of landslide is divided into low(60.72%), moderate(24.18%), high(9.89%) and extremely high areas(5.21%). The high and extremely high areas covering an area of 99 km2 including Tiger Lake and Panda Lake are distributed along the valleys. The assessment result of the coupled model is more reasonable than that of the single model and can be used as scientific reference for prevention of landslide hazard and segmented opening time of scenic area.
2021, 29(2): 536-544.
Karst ground collapse is the result of many factors, and has the characteristics of concealment and abruptness. In order to predict karst ground collapse quickly and accurately, this paper presents a karst ground collapse prediction model which is based on factor analysis, genetic algorithm(GA) and extreme learning machine(ELM). Firstly, we identify eight typical influencing factors in the karst area. Then, we extract five common factors by factor analysis. Finally, we input the common factors into GA-ELM model and predict karst ground collapse. The sample data contains 20 groups of actual case data, of which 12 groups as training sets and the other 8 groups as test sets. The conclusions are as follows: Factor analysis can not only simplify the ELM model network structure, but also improve the prediction accuracy under the same number of hidden layer of neuron nodes. In the case of a small number of samples, the prediction accuracy of the ELM model can be improved by increasing the number of hidden layer neuron nodes. Compared with the ELM model, the prediction accuracy of GA-ELM model is improved significantly, with stronger learning and prediction ability. The prediction method of karst ground collapse based on Factor Analysis-GA-ELM model is simple, accurate and efficient.
Karst ground collapse is the result of many factors, and has the characteristics of concealment and abruptness. In order to predict karst ground collapse quickly and accurately, this paper presents a karst ground collapse prediction model which is based on factor analysis, genetic algorithm(GA) and extreme learning machine(ELM). Firstly, we identify eight typical influencing factors in the karst area. Then, we extract five common factors by factor analysis. Finally, we input the common factors into GA-ELM model and predict karst ground collapse. The sample data contains 20 groups of actual case data, of which 12 groups as training sets and the other 8 groups as test sets. The conclusions are as follows: Factor analysis can not only simplify the ELM model network structure, but also improve the prediction accuracy under the same number of hidden layer of neuron nodes. In the case of a small number of samples, the prediction accuracy of the ELM model can be improved by increasing the number of hidden layer neuron nodes. Compared with the ELM model, the prediction accuracy of GA-ELM model is improved significantly, with stronger learning and prediction ability. The prediction method of karst ground collapse based on Factor Analysis-GA-ELM model is simple, accurate and efficient.
2021, 29(2): 545-553.
According to the clear segmental characteristics of the stress curve of red sandstone under impact load, a constitutive model with the nonlinear body, Maxwell body and damaged body in parallel is established based on the Zhu-Wang-Tang(Z-W-T)constitutive model. According to the mechanical properties of red sandstone, the Z-W-T model is simplified, and another rate-type damage constitutive equation is derived based on the Lemaitre equivalent strain hypothesis. The applicability of two new constitutive equations is discussed and the influence of each parameter on the fitting result of the experimental data is compared and analyzed. The results show that the nonlinear viscoelastic damage constitutive equation based on Weibull distribution can well characterize the dynamic mechanical properties of pre-peak stress and the whole stress curves at room temperature and freezing temperature. However, the damage constitutive equation based on Lemaitre's hypothesis of equivalent strain can only be used to solve the pre-peak stress of red sandstone, and has some limitations in the characterization of the whole stress curves.
According to the clear segmental characteristics of the stress curve of red sandstone under impact load, a constitutive model with the nonlinear body, Maxwell body and damaged body in parallel is established based on the Zhu-Wang-Tang(Z-W-T)constitutive model. According to the mechanical properties of red sandstone, the Z-W-T model is simplified, and another rate-type damage constitutive equation is derived based on the Lemaitre equivalent strain hypothesis. The applicability of two new constitutive equations is discussed and the influence of each parameter on the fitting result of the experimental data is compared and analyzed. The results show that the nonlinear viscoelastic damage constitutive equation based on Weibull distribution can well characterize the dynamic mechanical properties of pre-peak stress and the whole stress curves at room temperature and freezing temperature. However, the damage constitutive equation based on Lemaitre's hypothesis of equivalent strain can only be used to solve the pre-peak stress of red sandstone, and has some limitations in the characterization of the whole stress curves.
2021, 29(2): 554-563.
Rockfall hazard is a common type of geological hazard in mountain areas. It has significant meaning to research the character of rockfall for geological hazard investigation and risk assessment. A rockfall hazard occurred on 2018.9.15 during a project in Nepal. We investigated it at site and analyzed its genesis to find out the reason of the accident. The investigation results showed that"9.15 event" is a natural geological disaster and is not affected by human activities. The main cause of the accident is that the scope of rockfall from super high slope exceeded the scope which was predicted in the early stage. There are so many uncertainties in the rockfall movement. The movement pattern of rockfall has been predicted through the imprint analysis at site. According to impact point surface morphology, slope deposit material and geometrical characteristics, we calculate and analyze the motion trail and motion process of rockfall by the kinematics theory. In addition, combined with the scale of rockfall source area and the size, shape, mechanical properties of the rockfall blocks, we apply the Rocfall software to simulate and analyze the rockfall trail of super high slope. The Rocfall can calculate the energy of rockfall and provide the reliable basis for protection measures. The opening curtain type net has a good effect on preventing from high and steep slope rockfall disaster. This paper is useful to the investigation and prevention of dangerous rock of other similar height and steep slope, as reference.
Rockfall hazard is a common type of geological hazard in mountain areas. It has significant meaning to research the character of rockfall for geological hazard investigation and risk assessment. A rockfall hazard occurred on 2018.9.15 during a project in Nepal. We investigated it at site and analyzed its genesis to find out the reason of the accident. The investigation results showed that"9.15 event" is a natural geological disaster and is not affected by human activities. The main cause of the accident is that the scope of rockfall from super high slope exceeded the scope which was predicted in the early stage. There are so many uncertainties in the rockfall movement. The movement pattern of rockfall has been predicted through the imprint analysis at site. According to impact point surface morphology, slope deposit material and geometrical characteristics, we calculate and analyze the motion trail and motion process of rockfall by the kinematics theory. In addition, combined with the scale of rockfall source area and the size, shape, mechanical properties of the rockfall blocks, we apply the Rocfall software to simulate and analyze the rockfall trail of super high slope. The Rocfall can calculate the energy of rockfall and provide the reliable basis for protection measures. The opening curtain type net has a good effect on preventing from high and steep slope rockfall disaster. This paper is useful to the investigation and prevention of dangerous rock of other similar height and steep slope, as reference.
2021, 29(2): 564-574.
The deformation of overburden strata in goaf is a long-term process, seriously affecting the construction of tunnels that pass through the mining-induced subsidence zone. The Sangzhang tunnel is a typical case. Accurate predicting the influence of the residual deformation of goaf to Sangzhang tunnel is crucial to ensure its engineering safety. In this paper, a Boltzmann function is used to optimize the probability integral model of equivalent variable mining thickness, and a time function is introduced to put forward an improved model. We employ this modified model to predict the residual deformation of the four mining-induced subsidence zones in the Yangquan No.2 Mine, Shanxi Province, and then analyse the influence of the residual deformation of goaf to Sangzhang tunnel. The study indicates that the maximum residual settlement on the planned line of Sangzhang tunnel is 325 mm, appearing in the middle of the tunnel; the maximum residual lateral displacement is 168 mm, occurring near the start of the tunnel. As the axis of the Sangzhang tunnel intersects with the long axis of the mining-induced subsidence zone at a small angle, the tunnel shows a compound deformation trend of lateral movement, vertical settlement and axial torsion. Results in this paper can provide a scientific basis for the construction and deformation treatment of Sangzhang tunnel, and the adopted method can provide a theoretical support for the construction of tunnels that pass through mining-induced subsidence zones.
The deformation of overburden strata in goaf is a long-term process, seriously affecting the construction of tunnels that pass through the mining-induced subsidence zone. The Sangzhang tunnel is a typical case. Accurate predicting the influence of the residual deformation of goaf to Sangzhang tunnel is crucial to ensure its engineering safety. In this paper, a Boltzmann function is used to optimize the probability integral model of equivalent variable mining thickness, and a time function is introduced to put forward an improved model. We employ this modified model to predict the residual deformation of the four mining-induced subsidence zones in the Yangquan No.2 Mine, Shanxi Province, and then analyse the influence of the residual deformation of goaf to Sangzhang tunnel. The study indicates that the maximum residual settlement on the planned line of Sangzhang tunnel is 325 mm, appearing in the middle of the tunnel; the maximum residual lateral displacement is 168 mm, occurring near the start of the tunnel. As the axis of the Sangzhang tunnel intersects with the long axis of the mining-induced subsidence zone at a small angle, the tunnel shows a compound deformation trend of lateral movement, vertical settlement and axial torsion. Results in this paper can provide a scientific basis for the construction and deformation treatment of Sangzhang tunnel, and the adopted method can provide a theoretical support for the construction of tunnels that pass through mining-induced subsidence zones.
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