Rampat, Chandr Toshan
(2018)
Reinforcement of ballasted railway tracks using 3D-geocomposite.
MPhil thesis, University of Nottingham.
Abstract
Recent growth in high-speed traffic, due to increased commercialism and modernisation, has driven the conventional ballasted track to its limit. The design of railway tracks has not significantly evolved for hundreds of years. Most attention has been paid to the superstructures, and less consideration was given to the substructure. Researchers have indicated that the major portion of maintenance budget is spent on the substructure. The development of a high-speed railway demands a more stable, durable and longitudinally homogenous substructure with higher strength, higher rigidity, and reduced strain. Track deterioration is directly influenced by the applied pressure distribution combined with high frequency, the entire track modulus, and the type of ballast material used. In this context, high-speed traffic increases the need to improve ballasted tracks structurally. The reinforcement of railway tracks has proven to be advantageous, but each specific reinforcement technique comes with its respective benefits. In this research two common materials, bitumen and rubber geocell, are combined to form a new reinforcing geocomposite where the weakness of one material is compensated by the strength of the other. Bitumen, being a visco-elastic material, has proven to have vibrational damping properties and can also consolidate ballast owing to its highly adhesive nature. Geocell confines the ballast and helps to increase shear strength, bending resistance, tensile strength and bearing capacity by allowing more excellent load distribution and improved frictional characteristics. Current codes are non-scientific, and a proper understanding of the phenomenon at the wheel-rail interface is needed. Using theories derived from contact mechanics, a load-time graph was plotted so that the load from different train speeds that acts on the ballast at each time step could be obtained for a range of different track profiles. A finite element model was set up to understand the change in mechanics of the model when speed was raised, and a parametric study was conducted to verify if components can be changed to account for higher speeds. The results were then used to perform a Discrete Element Analysis on the ballast since extremely high-frequency cyclic loading experimental tests are very limited. Ballast was modelled by agglomerating spheres to mimic the behaviour of breakage. The geocell was a clump of small spheres, while bitumen was modelled as visco-elastic spheres and settlement was observed for each reinforcement scheme. Experimental tests were also performed on a single geocell to check the damping ratio and the increase in bearing capacity. A small cyclic compressive test using a tensile machine was performed to check the optimum geometry of geocells and the increase in lateral confinement. A large box test was also performed in order to validate DEM results and to check the benefits of implementing the reinforcement. It was concluded that bitumen decreased settlement but did not contribute in reducing the lateral vibration or lateral load. Geocell served to enhance the properties of ballast by deducing the lateral force but settlement was much significant since interlocking between ballast was reduced. While geocell amplified the settlement and movement of ballast, bitumen decreased the damping properties of ballast.
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