Contributions to the management of shallow failures in transportation cuttings

Abstract

Shallow cutting failures are the main cause of derailments on the UK railway lines. These failures are triggered by intense rainfall and occur fast, with no previous warning. Frequently the first person to notice them is the train driver. The derailment that took place at Watford on 16 September 2016 is a clear example of the risks that shallow cutting failures pose to the railway passengers. Network Rail uses a classification system to obtain a history record of cutting failures by type. A statistical analysis of these records was used in the past to calculate the probability of failure in cuttings accounting for the type of failure. If the classification system is prone to ambiguities, and the type of failures are difficult to allocate using only visual inspections, the estimation of the likelihood of failure will be inaccurate under these premises. Shallow cutting failures are in some cases triggered by runoff flowing along the face of cuttings. At present, there is not a recognised analytical method for the calculation of the stability of cuttings subject to runoff. Traditional limit equilibrium methods do not capture this type of failures since hydrodynamic forces are not considered. This thesis proposes the introduction of two measures that will help to improve the management of cuttings: A new classification system for shallow cutting failures, and a novel method to assess the stability of cuttings subject to runoff. It is expected that the new classification system will help to improve the accuracy of classifying cutting failures and to have a better understanding of the factors involved in each type of failure. A better knowledge of past failures will help to prevent future failures. The novel method to analyse the stability of runoff triggered cutting failures has been designed to account for runoff hydrodynamic forces by coupling computational fluid dynamics and the discrete element method in combination with the theory of sedimentology for the initiation of movement in particles. The resulting method establishes a relationship between the angle of the cutting and the critical shear stress that initiates the mass failure of the cutting. A design chart version of this method has also been introduced where the assessment of stability is carried out knowing the catchment area, the rainfall intensity and the angle of the cutting. The method has been validated using 17 cases and was successful in the assessment of the stability in all of them.

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

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