Numerical investigation of railway subgrade stiffening: Critical speed and free-field vibrations

For a train speed close to the speed of elastic waves in the soil, often referred to as “critical speed”, largely elevated vibration responses occur. This can be a practical problem for soft soil sites, where the phenomenon may cause excessive vibrations in the track and also at distances far from the track. To ensure the running safety of the train, the long-term quality of the track and to reduce the vibrations in the surroundings, such effects must be avoided. An effective counter-measure is to increase the stiffness of the soil underneath the track, thereby increasing the critical velocity. In this paper, a 2.5D finite element model is used for studying the critical velocity phenomenon and its mitigation through soil stiffening, for a ballasted track on a layered half-space with very soft soil. Soil improvement under the track, in the shape of a solid block or as various number of panels, with varying depth and stiffness is considered. The effect of the soil improvement is evaluated both in terms of the maximum rail and free-field displacements. It is shown that a shallow soil stiffening increases the critical velocity and reduces the rail and free-field response for load speeds near the shear wave velocity of the soft top soil layer. It is also demonstrated that a deep soil stiffening, by use of panels along the track direction, increases the critical velocity further, and may also be efficient in reducing the response for load speeds near the shear wave speed of the underlying half-space.