Formation Analysis And Stability Study Of The Chada 1# Accumulation In Luolong County,Tibet

A railway station is located on the Chada large-scale accumulation,which is covered by brown coarse boulders and characterized by steep slope,thus there exists high risk of overall slippage and local rockfall.In order to ensure the safety of the station,this paper carried out a systematic investigation and analysis on the accumulation to explore its spatial distribution characteristics,formation and stability.This paper ascertained the spatial distribution characteristics and material composition(including grain and lithology)of the Chada accumulation through on-site investigation,geological drilling,three-dimensional oblique photography of drones and satellite images interpretation.In addition,we determined the age of black silty soil below the valley section of accumulation to infer the formation and evolution process of the accumulation by performing the C-14 dating method.Otherwise,this paper used Geostudio software to analyze the overall stability of the accumulation and Roc Pro 3D software to simulate the movement trend of brown boulders on the surface of the accumulation.The main results are as follows:(1)Based on on-site geological investigation and three-dimensional oblique photographic images,we divide the accumulation into platform-I,II and III.Formation causes of each section are inferred that the deeper soil of the large-scale accumulation(above 3780 m elevation)belongs to the origin of early morainal process,the deeper soil of the large-scale accumulation(below 3780 m elevation)is attributed to the river alluvium,the coarse boulders on the surface of deposit are presumed to be the resultant of high-position rockfall,and the rock blocks located on the surface of the platform-II should result from the failure of the left ridge in the later period.(2)According to the slope characteristics of the accumulation surface,we utilize Geostudio software to analyze the overall stability of the accumulation below the platform-II,obtaining the positions of the most dangerous sliding surfaces and their stability coefficient under different conditions.The results show that the most dangerous slip surface of the accumulation is concentrated on the shallow layer of the slope,and the soil at the front edge of the platform-II is relatively loose and cohesionless,which is expected to failure under heavy rainfall conditions.(3)Roc Pro 3D software is performed to simulate the motion trajectory and velocity energy of the selected typical large boulders at different positions on the slope after failure.Combining with the on-site investigation,we have evaluated the impact of falling rocks on the railway and station.Brown boulders on the accumulation surface below the platform-II are more prone to fall,most of the blocks will strand on the flat surface of the slope or the negative topographic groove at the rear edge of the platform-I,and only a few blocks may cause impact on the auxiliary buildings of the station.In addition,once the dangerous rock masses at the rear edge of the accumulation failed,they will move along the lower channel to the toe of the slope,which has a certain impact on the proposed railway,thus protective measures are required to ensure the safety and stability of the line.