Pore pressure generation in a poro-elastic soil under moving train loads

Abstract

During the passage of a train along a railway track, the underlying soil experiences repeated loading. If the soil is saturated, pore pressures will increase as the load passes, and these may or may not start to dissipate before each load is removed. To investigate the dynamic response and excess pore pressures generated in a saturated ground below the track, this study uses a 2.5D finite element model (FEM) of a coupled track-embankment-ground system. The saturated soil is modelled using Biot's theory of elastic wave propagation. The implementation of the method is verified by comparison with semi-analytical solutions for both single-phase elastic and poro-elastic media. It is then used to investigate the influence of load speed c, soil Darcy permeability kD and stiffness on the excess pore water pressures generated. It is found that the ratio c/kD determines the extent to which excess pore pressures build up during passage of the load, at any given depth. For a saturated soil of a particular stiffness, if c/kD is less than 104, the soil can be viewed as highly permeable in relation to the load speed and almost no excess pore pressure is developed. For a single moving load, there is a critical value of c/kD, above which the maximum pore pressure reaches a constant value; this critical value depends on the depth. Below the critical value, the pore pressure accumulated during the passage of a train depends on c/kD but is otherwise independent of the load speed. The pore pressure accumulated during the passage of a bogie pair is greatest for intermediate values of c/kD. For small values of c/kD (high permeability), the pore pressure build-up is small, whereas for large values of c/kD (low permeability) the pore pressure does not dissipate during the loading cycle. The variation in the maximum stress ratio, (τ/σ′)max, with permeability depends on the depth under consideration. The depth to which pore pressures are generated and the effects of soil stiffness are also discussed in this paper.