Unsaturated free-standing mainline railway embankments – Part 1: Can you handle the awkward truth?

The presence of negative pore pressures within cuttings and embankments, and their benefit to the reduction of the likelihood of instability, have long been recognised by the Geotechnical profession. Negative pore pressures are usually a consequence of natural environmental influences upon soils, clay in particular, and are frequently (and perhaps misleadingly) termed “soil suction”.

The recognition of pore suctions in the assessment of potential instability of embankments by way of conventional stability analyses is not common. Tools are becoming available to conduct such analyses once the boundary conditions and parameters involved are understood. Measurement of suction in-situ is beginning to assist in the identification of reliable suction values suitable for use in such analyses.

In this paper, the authors develop a philosophy to include suction and present a defensible model for the analysis of free- standing embankments. The impetus for the work was the need to demonstrate via analysis that Main Line Railway embankment infrastructure within NSW was not in jeopardy, as was indicated by “the usual” saturated soil mechanics approach. Discussion includes the challenges posed to realistically implement and consider the benefits resulting from recognition of the “known” suctions.

The paper is intended to alert the profession to an assessment technique that includes the benefit of these suctions.