| The proposed Sichuan-Tibet railway is an important step in the country’s implementation of the Western Development Strategy,which will greatly promote the economic development of Sichuan Tibet region.However,the complex environment has made the Sichuan-Tibet railway the most challenging line project in the world to date,crossing seven rivers and eight mountains and dozens of active faults,resulting in significant geological safety issues along the railway line.Linzhi Tongmai section of Sichuan-Tibet railway is located in the north of the Eastern Himalayan Syntaxis.Affected by regional crustal uplift,seismicity and climate change,gravity geological disasters occur frequently on both banks of regional rivers,often blocking rivers to form weir dams.Yigong Landslide at the beginning of this century formed weir dams,and the dam body burst,causing disaster to hundreds of residents in China and India along the lower reaches of Yarlung Tsangpo River,Economic losses amounted to hundreds of billions of yuan.Therefore,the research on the formation of weir dam by potential landslide blocking the river has become an important research topic in the comparison and selection of regional lines,the location of tunnel inlet and outlet,the safety of river crossing bridge and camp station.The research on the identification,genetic mechanism,dynamic process and parameter analysis of early damming dams will provide reference significance for the prevention,risk assessment and comprehensive treatment of potential landslide blocking the river,and the long-standing large damming dams also have a far-reaching impact on the river landform.In order to achieve the above research purposes,this study selects the weir dam in the basin of Lulang Tongmai section of Sichuan Tibet Railway as the research object,and carries out the following work through on-site detailed geological survey and indoor numerical simulation experiment.Firstly,combined with field geological survey and remote sensing interpretation,the geomorphic characteristics,material characteristics and distribution types of weir plug dams in the study area are identified,and their temporal and spatial influence laws are summarized.Secondly,since the river geomorphology often records the important impact of large-scale weir dams on the region,based on the river dynamic erosion theory,the impact of regional weir dams is analyzed from the aspects of reference concavity selection,river crack point identification,Channel Ancient profile reconstruction and crack point evolution.Then,based on the inversion of ancient landslide by discrete element software PFC3D,the parameter characteristics and dynamic characteristics of landslide process are studied.Finally,taking the landslide river blocking prediction on the opposite bank of Tongmai station of Sichuan Tibet Railway as the background,the characteristics identification,deformation analysis and river blocking scale prediction of potential accumulation are studied.The main research results are as follows:(1)According to the analysis of landform,characteristics of damming dam residual materials,characteristics of lacustrine sediments and river terraces,the characteristics of three large deposits in the study area are judged:Jiaobulong landslide and Delun landslide form damming dam to block the river,and the damming Lake ranges of the two are estimated to be 7.12 km~2 and 3.56 km~2 respectively based on Arc GIS.However,the Baimu old deposit did not block the river for a long time,but caused the river diversion.There are 22 damming events in the interpretation area.The formation mechanism of damming dams can be divided into four main types,among which the damming events formed by large-scale landslide blocking the river have the widest temporal and spatial impact range.(2)A total of 52 knickpoints of the Layue basin were identified by slope-area analysis and integral analysis using Matlab.The results show that most of the knickpoints in the basin are distributed above an elevation of 3 km and are concentrated near 4 km,thus distinguishing two different types of knickpoints,the 4k knickpoint group with homogeneity and mobility and the damming type fixed in spatial location.The normalized steepness index was used to distinguish between relict and modern reach upstream and downstream of the knickpoint,and it was found that the distribution of damming events responded to abrupt changes in the steepness index.Both the Jiaobunong landslide and the Delun landslide correspond to this abrupt change,with abrupt changes exceeding 200,whereas the change for the Baimu deposit is significantly less than this value and is therefore considered to have failed to block the river for a long time.(3)Reconstruction of the paleo-profile using the Lulang River as an example showed a difference of 4.6%between the two analytical methods.The reconstruction of 15 channel ancient profiles quantifies the river cutting history of the Layue River Basin,and estimates the regional river erosion and paleotopographic relief.The results indicate that the Layue River basin experienced a net uplift(downcutting erosion)of approximately 1500 m,resulting in an increase in relief of 35.3%to 214.3%within the basin,with an average increase of 104%.(4)The obvious damming-type knickpoint indicate that damming event play a controlling role in the evolution of the river geomorphology.Dating data and time volume substitution method determined that the onset of erosion of the 4k knickpoint group in the Layue basin was approximately 1-6 Ma ago.A celerity model was applied to restore the headward migration of the 4k knickpoint group in the Layue basin,and the average migration rate was estimated to be 10.8～51.7 km/Ma.Based on the above study,it is inferred that the Jiaobunong knickpoint is a damming-type knickpoint,and it is proposed that within a short geological time scale(<10~4 yr),damming events would have had a significant impact on the river profile similar to that driven by climatic and tectonic effects.(5)The dynamic process of Jiaobulong landslide forming a dam is simulated by using the discrete element software.The peak value of the average velocity of the landslide material in 45s is 51 m/s.The whole of the deposit basically stabilises at around 100s,forming a dam with a maximum height of 250 m and a length of 5.4 km along the river.(6)Parameter analysis is an important result of landslide numerical inversion.The parameter calibration results of direct shear test show that the mean tensile strength and cohesion of parallel bonding model are positively correlated with the shear strength of moraine.The effective modulus increases first,then decreases and tends to be stable.The normal-to-shear stiffness ratio is between 1～2,and the result is the most reasonable.And the micro parameters affect each other.By comparing the peak landslide velocity and geomorphological parameters,the wall and ball friction coefficients have some influence on the landslide morphology,but the former has a smaller variation interval than the latter.The contact strength has the most obvious control on landslide morphology.High strength contact can make it difficult to break up landslide material and greatly reduce the peak velocity.(7)The results of the field survey indicate that the Tongmai old deposit exists within 1.6 km of the left bank of the Parlung Tsangpo River and that the main material is moraine,which probably formed after the last ice age and destroyed or buried the tertiary river terrace below during its formation.The temporary deformation rate in the area where the Tongmai old deposit is located was analysed by TCP-In SAR technique up to 200 mm/yr,with an average deformation rate of 53.99 mm/yr.Based on the results of the deformation analysis,the sliding thickness was selected as the variable for the prediction of different scales of river blocking in the potential sliding zone of the Tongmai old deposit,and the numerical simulation results were compared using both SFLOW and PFC3D methods.The results show that the maximum thickness of the blocked river predicted to form a dem below the critical sliding thickness is 95～145 m.Although there is no direct impact on Tongmai station,the ripple effect is inevitable.In the extreme case,the sliding of the Tongmai old deposit would create a 152 m high dem that would have a direct impact on Tongmai station. |
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