Effects of principal stress rotation and drainage on the resilient stiffness of railway foundations

Abstract

Railway foundations play an integral role in controlling the stability of the overlying track structure and the maintenance of the overall track geometry. Premature failures of railway track foundation are likely to result in frequent maintenance, which may entail significant costs since railway track foundations are less easily accessible than the other layers of railway track. Premature failures of track foundations may arise if the service loads exceed the design specifications, but may also develop as a result of the shortcomings of the design codes to simulate in situ stress paths, which involve cyclic stress changes in the horizontal as well as vertical direction, which result in principal stress rotation (PSR). Laboratory investigations have suggested that cyclic changes in the horizontal as well as vertical direction may result in a higher rate of plastic strain accumulation than cycling the vertical stress only. The effect of PSR on the soil stiffness is less certain however. Furthermore little consideration has been given to how the gradation of different soils may affect in situ drainage conditions and therefore influence the rate of railway track deterioration during PSR. A knowledge gap exists as to how cyclic changes in the directions of principal stresses may affect the pore pressure and stiffness of soils under different drainage conditions.
In order to improve our understanding of the effects of PSR on the long term performance of railway track foundations, a series of laboratory tests were conducted which investigated the effects of cyclic changes in the direction of principal stresses on the pore pressure, stiffness and susceptibility to failure of saturated railway track foundation soils under different drainage conditions. The investigated sand-clay mixes were selected so as to replicate the gradation of an in situ railway track foundation. It was found that even small additions of clay to the volume of a sand significantly affected the response of the mixes during cyclic changes in principal stress direction. Moderate additions of clay in the pore space of a sand reduced the susceptibility to principal stress rotation by reducing the tendency for excess pore pressure generation and by increasing the cyclic shear stress the mixes were able to sustain before rapid plastic strain accumulation occurred. Increases in principal stress rotation below the cyclic shear threshold increased the resilient stiffness of the sand-clay mixes, however once this threshold was exceeded rapid stiffness degradation occurred. Below the cyclic shear threshold, the response of the mixes was stable over a high number of loading cycles and no abrupt fatigue failures were observed. The sand-clay mixes were sensitive to even small changes in the magnitude of PSR near the cyclic shear threshold. Small increases in PSR could trigger the sudden collapse of a previously stable sand-clay mix. Under conditions where the rate of pore pressure dissipation was regulated by the permeability and the volumetric compressibility of the soil, the sand clay mixes with moderate additions of fines were stable over a range of cyclic increases in PSR which correspond to the maximum expected changes in magnitude within the depth of a ballasted railway track foundation.

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ORCID for W. Powrie: orcid.org/0000-0002-2271-0826

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