The effects of drainage on the behaviour of railway track foundation materials during cyclic loading

A Mamou,C Clayton,W Powrie,J A Priest

doi:10.1680/jgeot.15.p.278

Abstract

This paper presents the results of a series of hollow cylinder tests carried out to investigate the role of drainage conditions on the response of railway track foundation materials during cyclic loading. Three sand–clay mixes were tested. It was found that, below a certain cyclic shear stress threshold, and depending on the drainage conditions, changes in principal stress direction should not adversely affect the cyclic stability of a railway foundation. However, significant stiffness degradation and failure may occur if this cyclic shear stress threshold is exceeded. The cyclic shear stress threshold increased with moderate additions of clay per unit volume of sand, and reduced significantly when specimen drainage was prevented. For the materials tested, the cyclic shear stress threshold in free-to-drain conditions was generally similar to the cyclic shear stress in the soil immediately below a 0·3 m deep ballast bed, but comfortably greater than the cyclic stress at a depth of 1 m below the sleeper base. In undrained conditions, the cyclic shear stress threshold was generally similar to the cyclic shear stress at a depth of 1 m below the sleeper base. This has implications for the suitability of such materials for railway track foundations.

Highlights

Railway track foundation materials play an essential role in ensuring the stability of the track structure and maintaining track geometry
During their operational life they are subject to repeated loading cycles, which under unfavourable conditions can result in the accumulation of unacceptably large plastic strains and a reduction in resilient stiffness
The laboratory tests were carried out using a hollow cylinder apparatus (HCA)

Summary

BACKGROUND

Railway track foundation materials play an essential role in ensuring the stability of the track structure and maintaining track geometry During their operational life they are subject to repeated loading cycles, which under unfavourable conditions can result in the accumulation of unacceptably large plastic strains and a reduction in resilient stiffness. Brown et al, 1975; O’Reilly et al, 1991; Shahu et al, 1999; Chung Ip et al, 2012) It has generally been observed (Brown & Selig, 1991) that there is a threshold stress level above which significant accumulation of plastic strain and (in undrained conditions) generation of excess pore pressure occurs. A number of investigations have considered the effects of cyclic loading on soil behaviour, relatively few have addressed the combined effects of the horizontal and vertical stress changes to which railway track foundation soils are subjected, and result in principal stress rotation (PSR) (Brown, 1996). The aim of the research reported in this paper was to investigate these, with an emphasis on the relative magnitudes of the cycles of shear and normal stress and on the drainage conditions – the time available for drainage between successive loading peaks, as governed by the permeability of the material and the drainage path length

EXPERIMENTAL METHOD Apparatus

RESULTS AND DISCUSSION

CONCLUSIONS