1 OPTIMISING SOIL STIFFNESS ON HIGH SPEED RAIL LINES TO PREVENT VIBRATION Kaitai Dong, Omar Laghrouche Heriot Watt University, Scotland, UK David P. Connolly, Peter K. Woodward Institute of High Speed Rail & System Integration, University of Leeds, England, UK e-mail: d.connolly@leeds.ac.uk Pedro Alves Costa University of Porto, Portugal The fast movement associated with high speed trains can cause significant dynamic effects within the supporting railway track structure. The speed at which maximum dynamic response occurs is known as the 'critical velocity' and is undesirable because large rail vibrations are generated when travelling close to it. These vibrations can cause a safety concern, and also propagate to the free-field where they disturb nearby buildings. A method to minimise these vibrations is to stiffen the soil directly below the track either via soil replacement or soil improvement, however both options are expensive. Their cost can be reduced though if either the depth or stiffness magnitude of the replacement is optimised. Therefore this work develops a track-ground model using the thin-layer method, which is capable of assessing the effect of different combinations of soil improvement on track vibration lev- els. It is shown that if improvement is carefully designed, performance can be maximised for mini- mum cost. Similarly, if improvement is poorly chosen, it can result in marginal improvement, and in some cases even amplify track vibration. Keywords: Railway vibration, soil stiffening, thin-layer element method 1. Introduction Rail vibration is a growing problem due to increasingly stringent international standards and increased lengths of track infrastructure under construction/operation near buildings [1]. A range of numerical (e.g. 2.5D modelling [2], [3]) and experimental approaches (e.g. [4], [5]) have been proposed to predict such vibrations. The vibration spectrum from railways is comprised of low frequency ‘quasi-static’ vi- bration and also high frequency vibration generated due to wheel-rail irregularities ([6], [7]). This re- search focuses on the low frequency vibration generated when the train moves at a velocity greater than ≈50% of the natural wave speeds of the track-ground structure. When the train moves at 100% of this speed, it is known as the critical velocity. In an attempt to simulate track dynamics at critical velocity, [8] used closed-form expressions to model the problem as a moving load traversing a homogenous and infinite elastic medium. The problem was then better tailored towards railway applications by [9] and [10] who used closed-form expressions for the track and Green’s functions for the soil response.