Copy of Discrete element modelling of geogrid-reinforced railway ballast

Railway ballast degrades under traffic loading. The broken fragments fill the pores, making drainage difficult. The ballast then has to be tamped, which involves inserting vibrating tines into the ballast and squeezing it to raise the sleeper back to the appropriate level. However tamping also causes particle breakage. Eventually the ballast is spent and has to be replaced. Five percent of spent ballast ends up in landfill, at a cost of millions of pounds per year. It is therefore desirable to increase ballast life and reduce maintenance costs. In order to do this, it is essential to gain an understanding of the fundamental micro mechanics.The behaviour of ballast can be characterised by triaxial testing; a large cell funded by EPSRC is now being used at the University of Nottingham. Both monotonic tests and cyclic loading tests are used; the cyclic loading tests are used to determine the behaviour of ballast under traffic stresses. In order to understand the particle mechanics, the discrete element method can be used. In the program PFC3D, there are two entities: a ball and a wall. A ballast particle can be modelled as a single uncrushable ball, or more realistically, as an agglomerate of many small balls bonded together. The aim is to model the triaxial testing of crushable ballast to gain insight into its behaviour. Geogrids are used to limit the deformation in ballast by providing interlock. This reduces settlement and therefore maintenance costs and prolongs life of the ballast. However, the ideal geogrid / ballast geometry is not known. The discrete element method will be used here to optimise the system.The outcome will be an improved understanding of ballast behaviour under monotonic and repeated loads, and the effect of geogrids, so that improved trackbeds can be designed such that maintenance costs are significantly reduced and ballast has a prolonged life.