APPLICATION OF RECYCLED BALLAST FOR GEOTECHNICAL IMPROVEMENT OF RAIL TRACKS by Yao Yao LI
Supervisor: Dr. Behzad Fatahi
25th June 2010
Traditional railway foundations or substructure have become increasingly overloaded in recent years due to the introduction of faster and heavier trains. The increasing axle loads increase the track maintenance frequency caused by differential track settlement and ballast degradation. Railway ballast breaks down and settles progressively under train wheel loading so that it requires the replacement of ballast in certain period. On purpose of reducing rail track maintenance cost, it requires to reduce the ballast replacing cycle or reuse the recycled ballast. The projects mainly focus on understanding the mechanism of ballasted railway track structure and the mechanical characteristics of ballast. The recycled ballast is introduced to use in the rail track design via economical purpose. This project would discuss the application of blended ballast in Australian railway construction. Also the application of geosynthetics to improve track condition and reduce maintenance is discussed. The plane strain finite element analysis (PLAXIS) is used to carry out the rail track condition based on changing the property of ballast layer. Different methods of railway improvement could be applied with various types of ballast. This project analyses the way of using recycled ballast to produce reasonable railway tracks. The two types of ballast, fresh and recycled ballast, will be checked with conceivable combinations. This project also addresses the potential use of geosynthetics for improving the deformation characteristics of rail ballast and formation soil. The different location of geosynthetics in rail track substructure is examined. According to the design of finite element analysis, some recommendations are made for the improvement of rail track performance.
Bg c = breakage index = effective cohesion
dBg dε dε
= the increment of breakage index. = increment of major principal strain. = increment of volumetric strain. = elastic normal (axial) stiffness = secant stiffness at 50% strength for loading condition = tangent stiffness for primary oedometer loading = triaxial unloading/reloading stiffness
E E E GCP GFM GGR GMB GNT GPP GTX HS K
= Geocomposites = Geofoam = Geogrids = Geomembranes = Geonets = Geopipe = Geotextiles = Hardening soil Model = coefficient of earth pressure at rest for normal consolidation
m MC n p’ Pref q Rf SN ΔWk γsat γunsat
= stress-dependent stiffness factor = Mohr-Coulomb model = dimensionless constant = mean effective stress. = reference confining pressure = the deviator stress. = failure ratio = ballast settlement at N cycles = the change in percentage retained on each sieve size = saturated unit weight = unsaturated unit weight 10
ν νur σ σc σ φ ψ τf
= Poisson’s ratio = Poisson’s ratio for unloading/reloading condition = principle stress = uniaxial compressive strength of parent rock in point load test = effective normal stress = effective friction angle = dilatancy angle = failure shear strength
Chapter 1 1 Introduction
Conventional ballasted railway tracks are founded on a number of layers including rails, sleepers, ballast, sub-ballast and subgrade. The function of ballast is to distribute the load from the railroad sleeper and transmit the train load to the subgrade. Sub-ballast is to facilitate drainage of water and protect the subgrade soil from softening and mud pumping. The deviation of track alignment and vertical profile from the design geometry due to progressive degradation of ballast and consolidation of soft formation often leads to costly track maintenance. In order to enhance the competition with other types of transportation, minimising the track maintenance...