Experimental Study On The Influence Of Gradation On Consolidation Characteristics Of Debris Flow Sediments

After the debris flow rushes out of the gully,due to the gentle terrain,the energy consumption is exhausted,and the area formed by the accumulation of a large amount of sediment and gravel constitutes a geographical space with special environmental attributes that cannot be ignored.For mountainous areas lacking land resources,the development and utilization of debris flow accumulation areas is particularly important for the development of urbanization.However,from fresh debris flow sediments with high rheology,high compressibility,weak permeability and low strength to foundation soil with bearing capacity to carry out urban construction,it takes a long time to consolidate.In the consolidation process of debris flow sediment,solid particles constitute the basic skeleton of debris flow sediment,and pore channels are formed in the process.The gradation difference affects the size and number of pore channels in the sediment,and then affects the seepage velocity of fluid,the dissipation of pore water pressure and the growth rate of effective stress,which plays a crucial role in the consolidation of debris flow sediment.However,there are relatively few studies on the effect of solid phase on the consolidation characteristics of debris flow sediment,which cannot reveal the consolidation characteristics of debris flow sediment and support the development and utilization of debris flow accumulation area.This thesis takes Diebu County in the middle and upper reaches of Bailong River to Wudu District in the middle reaches of Bailong River as the research area.Based on field investigation,solid particle gradation characteristics of 4 groups of debris flow sediments are extracted and laboratory model tests of debris flow sediment consolidation are carried out to explore the influence of gradation on the consolidation characteristics of debris flow sediments.The main research contents and conclusions of this thesis are as follows:（1）According to the relationship between the characteristic parameters of each gradation and the structure,intergranular action,permeability and rheology of debris flow sediments during consolidation,the curvature coefficient C_c and the interlayer coefficient D₁₅/d₈₅ were selected as the gradation characteristic values of this thesis.Combined with field investigation sampling and indoor test,the debris flow sediment samples were divided into four groups according to the sampling location and gradation characteristics:Diebu County,Zhouqu County,Zhouqu-Wudu and Wudu District.Among them,the d_i,C_u,C_c and D₁₅/d₈₅ of the sediment samples from the upstream to the middle reaches all showed a decreasing trend,that is,the upstream sediment gradation was wider,the content of coarse particles in the material components was more,and the migration ability of fine particles was stronger.（2）Four groups of saturated debris flow sediments were prepared by taking the average percentage content of debris flow sediments in four sections of the study area as the average gradation characteristics,and the consolidation model test of debris flow sediments for more than 150 days was carried out.The three-dimensional laser scanner and point cloud processing were used to analyze the apparent morphological changes of debris flow sediments during consolidation.The dynamic strain gauge was used to monitor the internal stress changes of debris flow sediments,and the influence characteristics of gradation on the consolidation of debris flow sediments were analyzed.（3）The volume change and sedimentation of debris flow sediments are related to the changes of C_c and D₁₅/d₈₅.From the upper reaches to the middle reaches of the Bailong River,the content of coarse particles in the sediment components decreases and the proportion of fine particles increases,resulting in the decrease of C_c and D₁₅/d₈₅ values,which affects the porosity and permeability of the sediment by changing the intergranular arrangement of the sediment.The smaller the C_c and D₁₅/d₈₅,the smaller the porosity of the sediment,the lower the permeability coefficient,the slower the loss rate of pore water at the same time,and the smaller the volume change.For sediments with too many coarse particles,the connection between particles is strong,the skeleton structure is less damaged by slurry seepage,and the relative position of particles has no large displacement,so the settlement decreases.（4）The variation of pore water pressure and soil pressure is affected by C_c and D₁₅/d₈₅.The consolidation process of debris flow sediments is mainly divided into three stages:initial accelerated change,medium-term rapid change and later stable change.The larger the content of coarse particles in debris flow sediments,the larger the C_c and D₁₅/d₈₅,the larger the void ratio and permeability coefficient of sediments,and the faster the dissipation of pore water pressure.The increase of earth pressure is negatively correlated with the change of pore water pressure,and also increases with the increase of C_c and D₁₅/d₈₅.（5）Based on the influence of solid gradation in the consolidation process of debris flow sediments in this thesis,the nonlinear regression statistics of the test results in this thesis are carried out by using the grading characteristic values C_c and D₁₅/d₈₅,and the correction formula of the average excess pore water pressure and settlement of debris flow sediments under the influence of solid gradation characteristic values is proposed.