Study On Debris Composition And Related Effect On Vertical Grading Of Rock Avalanche Deposits

Laboratory experiments and discrete element numerical simulations were conducted on the simplified geological model of Yigong rock avalanche to investigate the relationship between the initial granular characteristics and the vertical structure of rock avalanches on the basis of granular flow theory.Spatial granular characteristics forming two typical sedimentary structures were analyzed by the description of sedimentary features in the laboratory experiments.Kinematics of particle in propagation process of granular flow were analyzed by reconstructing the analogue of laboratory experiments in the numerical simulations.In macro and micro perspective,the above comprehensive synthesis of analysis indicated the influence of the initial granular characteristics on the vertical structure of rock avalanches,as well as the kinematics in propagation process,which provide insights into the dynamics and kinematics of rock avalanches.Main conclusions as follow:(1)In planview,the debris flow deposits can be divided into two parts: the main depositional area and the front splash area,accompanied by typical fluidized morphologies of transverse ridges,lateral levees,and tongue shaped edges at the front due to the compressional process during emplacement.The transportation process of granular flow comprised four stages: initiation,acceleration,deceleration,and emplacement,accompanied by the dissipation of gravitational energy due to the friction in systematical interior and boundary.(2)Inverse grading with different degrees occurred under different fractal dimensions,however,with lager particles distributed in the surface and front.The results showed that discrete flow state of debris flow formed and a low velocity gradient in vertical occurred during the propagation process,resulting in inverse grading derived from lager particles migrating from base or interior to surface under the dispersive stress under smaller fractal dimensions.However,dense flow state formed and a relatively high velocity gradient occurred during the propagation process,resulting in segregation of sieving that smaller particle penetrating down through the gaps between lager ones.Overall,segregation deceased as fractal dimension increased.(3)Inverse grading in deposits resulted by original inverse grading and/or mixed grading,while normal grading resulted by initial normal grading.This indicated that without or little segregation occurred during the propagation of the original inverse grading and normal grading with relatively steady layers relationship,and initial mixed grading resulting in inversing grading was caused by segregation.Therefore,mixed material posed the smallest runout because of energy dissipation in the segregated process during the propagation of granular flow.(4)Retention of stratification formed obviously in granular flows under the condition original vertical sequence,accompanied by the forward deflection of the stratification boundary.Lower degree of reserved stratification occurred easily in the horizontal strata and bedding strata with thrust faults and folds in base.In general,granular size and original stratified structure are main factors that govern the reserved stratification.