Research On Dynamic Stability Of Red Clay Subgrade Under Ballastless Track Of High-speed Railway

The dynamic characteristics of red clay in original structure, the dynamic stability of cutting bed under ballastless track of high-speed railway and its replacement thickness and so on were deeply researched by laboratory dynamic experiments, in-situ dynamic response tests, theoretical analysis and numerical simulations, combining with the construction of red clay cutting bed under ballastless track of Wuhan-Guangzhou high-speed railway and the following important research projects:Study on deformation behaviors and dynamic stability of red clay foundation under ballastless track of Dedicated Passenger Line(No:50778180) supported by the National Science Foundation of China and Experimental research on deformation behaviors of subgrade and cutting slope stability of the eluvial red clay along the Dedicated Passenger Line from Wuhan to Guangzhou(No:2005K002-B-2) supported by the key scientific research projects from Ministry of Railways. The main reseaches had been carried out as follows:(1) Based on a series of general cyclic triaxial tests indoors, the dynamic constructive relation, thhe characteristics of dynamic elastic modulus and dynamic strength on red clay in original structure under different test conditions were studied, and the influencing law for confining pressure, consolidation ratio and water content ratio to them. The experimental studies indicate that the dynamic constitutive relation can be described with the hyperbolic curves model presented by R. L Kondner. The dynamic modulus reduces nonlinearly with the accretion of dynamic strain and be of strain softening features obviously. Based on regression analysis, the hyperbolic model parameters, the empiric formula on dynamic modulus attenuation and fitting parameters were given.(2) Through free vibration column tests indoors, the relationship curves of Gd/Gdmax～γd andλ～γd of red clay in original structure were obtained, the parameters of Davidenkov model and empirical formula of damping ratio and fitting parameters of it were also given. Under low shear strain level, dynamic shear module increases and damping ratio reduces with the augment of confining pressure. Under high shear strain, dynamic shear module reduces and damping ratio increases with the increases of confining pressure.(3) Based on the principle and method of determining dynamic shear strain threshold using shear-strain-controlled resonant column device overseas, the reformed stress-controlled cyclic triaxial apparatus was firstly utilized to measure dynamic shear strain threshold of soil mass, replacing the shear-strain-controlled resonant column device not existing at home now. A complete set of test program and data processing methods were presented, it opens up a new test way of determining dynamic shear strain threshold. The proposed test technique is verified as feasible through the contrastive analysis.(4) Based on the techniques of stress-controlled short-time and fatigue cyclic triaxial tests presented in this paper, the values of short-time and fatigue dynamic shear strain threshold of red clay in original structure were firstly obtained under different test conditions. The empirical formulas of estimating short-time and fatigue dynamic shear strain thresholds were given by linear fitting, and the latter is 0.2-0.3 of the former. The Presentation of three empirical formulas above has provided a new empirical method.(5) Via analyzing test data, the nonliner trend features of accumulated plastic strainεp and vibration times N for red clay in original structure are revealed. There are three types ofεp-lgN curves followed by stable, critical and destructive ones. Values of critical dynamic stress of red clay in original structure under different test conditions and its simple practical empirical formula were given.(6) In view of convergence characteristics of accumulated plastic strain for stableεp-lgN curves of red clay in original struture, a new mathematical model about the accumulated plastic strain is presented. Compared with general model of logarithmic function, the new model with convergence can simulate better the variation of stable curves. The important theoretical research foundation was provided by of the proposed new model for the deep study of deformation fatigue characteristics of red clay in original struture. (7) Based on often-used model of exponential function for destructiveεp-lgN curves, the corresponding fitting parameters of it for red clay in original structure were obtained under different test conditions through regression analysis. The fitting formulas of parameters in model of exponential function are obtained, considering simultaneously factors of dynamic-stress ratio, water content ratio, confining pressure and consolidation ratio.(8) Through dynamic response test and analysis of red clay cutting bed under ballastless track of Wuhan-Guangzhou high-speed railway, the varying laws of dynamic velocity, dynamic acceleration, dynamic strain and dynamic stress in vertical and transverse test sections are obtained. Based on the contrastive analysis, the dynamic response features of roadbed under ballastless track unlike ballast track are known.(9) By way of numerical calculation of FEM, Variations of dynamic response in vertical and transverse test sections are gotten. The comparision results between calculated and measured values show that the two were basically in agreement.(10) Based on results of laboratory dynamic tests and field dynamic response testing, the dynamic stability of red clay cutting bed under ballastless track of Wuhan-Guangzhou high-speed railway was preliminary evaluated by methods of critical dynamic stress, effective vibration velocity and dynamic shear strain imultaneously, the suggestion values of roadbed replacement thickness applied conveniently in engineering were also given form a view of dynamic stability. The comparative analysis shows that method of dynamic shear strain is the best way of evaluating roadbed dynamic stability of high-speed railway. This provides new thought for evaluating dynamic stability and ensuring replacement thickness of roadbed under high-speed.