Deformation And Failure Mechanism Of The Red-bed Coarse Particle Modified Filler And The Evaulation Of Subgrade Settlement On The Sichuan-tibet Railway

With the rapid development of Strategy for Large-scale Development of Western China,the Sichuan-Tibet Railway project has become a key project to promote the intital of"One Belt,One Road"in China,and the reasonable usage of red beds filling in subgrade engineering has become an urgent problem to be solved.Red beds is widely appeared in Yunnan,Guizhou,Sichuan and Southeast Asia,with the characteristics of serious weathering,erosion and easy softening in water.Therefore,it should not be directly used directly as subgrade filler.Relying on the improvement project of red beds subgrade filling in Chengdu-Ya’an Railway,a section of the Sichuan-Tibet Railway,weakly weathered mudstone gravel is mixed into red beds clay in a certain proportion to improve its particle gradation and density,obtaining improved filling materials that meet the requirements of railway subgrade filling.The relevant experimental results are obtained through on-site sampling and indoor experiments.This time the experimental results combined with the particle flow software PFC,carries out detailed numerical simulation,and establishes the corresponding numerical test model and embankment settlement model.This experimental result explains the meso-mechanism of the strength and deformation of the red-bedded roadbed modified filler under static load and cyclic dynamic load.This research discusses the whole process of deformation and failure of the improved filling material in the red beds subgrade and its adaptability,which provides technical guidance and reference for the design of railway subgrade in the red-bedded area and similar projects.The main work and research results conducted in the paper are as follows:（1）The study expounds the basic assumptions,calculation principle and particle generation method,surrounding pressure servo system,loading method,improves the polyhedron construction method based on ellipsoidal surface form,and proposes a random model method of gravel geometry with concave and convex polyhedron shape,which can more accurately reflect the actual gravel shape and simulate the action of real gravel particles.That provides support for better studying the fine mechanical characteristics of red beds improved filling.（2）Based on the basis of the compression test of indoor red beds improved filling,by establishing the PFC^3D compression geometry model,the author carefully studied the proportion of different types of strong contact force chain and the probability density distribution law at peak strength,and explained the internal mechanism affecting the strength of red beds improved filler.The results show that the mechanics and deformation properties of red beds filling with 50%are the best,the best bite contact between the gravel and the mixed stone.Red beds clay bonding as a weak force chain can form an effective super strong chain to assist the filling in the support system,bearing the load;different stone contents are:50%>40%>60%in the improvement of Sichuan-Tibet railway.It is recommended to improve the red beds clay of the Sichuan-Tibet railway by mixing with 50%red mudstone graded gravel as subgrade filling.（3）Through the indoor shear experiment and PFC^2D particle flow simulation experiment with the red beds improvement coarse as the object,the author deeply studied the stress-strain characteristic curve,the morphological change of the shear surface and its evolution law and energy conversion law of the modified filler 2.The experimental results show that the stress-strain curves of the modified filler 2 under 250k Pa,500k Pa,750k Pa axial compression are in a strain-hardened state,and the axial compression increases,the wider the shear band,the greater the shear strength.The author tried to record the energy conversion of different types of soil and boulders in the sample.Then,from the energy point,the author divided the shear process of the modified filler 2 into three stages,which revealed the meso-mechanism of the red beds improvement filler during the large direct shear test.（4）The study uses PFC^3D particle flow analysis software,taking the red-bed modified filler 2 as the research object,and in-depth study of the hysteresis curve,cumulative deformation and dynamic elastic modulus of the modified filler 2 under the cyclic load of the train,and a more comprehensive analysis of the improved filler 2,the problem of dynamic characteristics under cyclic loading.The results show that with the increase of the dynamic stress amplitude,the hysteresis loop appears from a straight line to a shape similar to a"willow leaf";dynamic stress amplitude is increased,hysteresis loop similar to the performance of straight line from"Lancet"shape;dynamic elastic modulus of a sample,the greater the attenuation of strain exhibited more serious phenomenon;while the damping ratio of the sample increases with the addition of the dynamic strain;4 different dynamic stress amplitudes cause the sample to show different forms of deformation and failure;the increase of the confining pressure makes the sample The cumulative deformation decreases,indicating that the confining pressure can reduce the cumulative deformation rate of the specimen.（5）By studying the size and direction distribution probability of contact force chain of red beds modified filling 2 under dynamic load,it provides a clear and accurate internal force observation mechanism inside the sample,and reveals the transmission evolution law of contact force between particles in 2.The results show that the contact among mixed stones plays a main role on the load;the dynamic stress and surrounding pressure have obvious influence on the distribution of the contact chain.The shape of the average rose chart is roughly"peanut",which increases as the amplitude of the dynamic stress increases.The force is in the vertical direction increasing,the contact force in the horizontal direction is decreasing.With the amplitude of the vertical direction,the influence of the surrounding pressure is mainly shown in the horizontal direction with less influence on the vertical direction.（6）The study establishes the numerical model of the red beds embankment through PFC^2D,applied the corresponding train load,and studied the playing mechanism of the red beds improved filling subgrade from two perspectives.During the construction stage of the subgrade filling and the train load,the settlement is less than 20cm.Through analysis of the change of contact chain under the train load,the presence of clump block can increase the effect of transferring the load to the surroundings and internal stiffness of the basic body and enhance the internal stress distribution of the system,and make the stress distribution in the system more uniform,thus improves the settlement of the subgrade.