Groundwater and Flood Risk in the London Rail Infrastructure Network: Building Resilience into Existing Masonry Infrastructure Assets

Lead Research Organisation: University College London
Department Name: Civil Environmental and Geomatic Eng

Abstract

The environmental hazard posed by increased precipitation presents an ever more significant risk for rail transport infrastructure owners and operators, as more frequent severe weather events increase the extent and recurrence of disruption to services through flooding and associated ground condition problems. This is an issue during and in the immediate aftermath of an event, as services are disrupted and in some extreme cases lines require reinstatement at significant cost to the operator, such as that required following the Dawlish line collapse in 2014 where Network Rail are investing upwards of £35 billion in re-instatement and improvement works (BBC News).
The hazards posed by groundwater fluctuations and excessive groundwater levels also pose a continual concern for those responsible for the ongoing maintenance and resilience planning for infrastructure networks and assets. Observed changes and potential further alterations to wetting regimes present an unknown risk for infrastructure operators, and similarly those providing consultancy services within infrastructure development have identified a need to increase their knowledge of these hazards and risks in order to continue to provide relevant advice to their clients.
The physical processes involved in these disruptive events are however not fully understood with respect to historic structures, and furthermore the long-term impact of these events on resilience of the network is yet to be determined. Whilst historic structures, by nature of their continued presence, show resilience to extreme climate events, we do not have a means of determining their response to conditions of specified severity. Achieving this would allow for accurate prediction of future risks, through analysis of the impact of changing frequency of such events on long-term resilience. This is especially important in light of the need identified by the industry to build resilience into the network; this can only be successfully achieved i we first understand the extent of additional mitigation investment work required.
The project will analyse using the skills and knowledge available at UCL CEGE the response of the historic infrastructure components tunnels, embankments and viaducts to changing groundwater conditions in terms of stability and movement within the structure, with tolerance's applicable to infrastructure operation forming the boundary conditions. From this vulnerability of historic infrastructure assets linked to moisture regime fluctuations associated with climate change will be quantified. Finally assessment of the impact of this response on the network resilience is measured using key indicators derived from ability of network to maintain its capacity function.
Transport for London-London Underground as the asset owner will set out the nature of the hazard and vulnerabilities within the asset that are of most concern, and hence should be studied first as part of this initial investigation. Arup, as a key contributor to the economic sector of infrastructure development will ensure the methods used and outputs generated are tailored to suit the wider professional services sector, and hence that industry applicability of the method is maximised.
The project seeks to address this through production of guidelines on future infrastructure management strategies concerned with developing a longer-term approach to maintenance of heritage assets within the rail network. The guidelines will identify the key mechanisms associated with vulnerability to decay and failure in these structures. Further, it will propose methods for not only repairing existing assets as a response to these vulnerabilities, but also for retrofitting them as a means of permanently mitigating against future risks. Through this increased resilience can be built into the network, ensuring risks of the future are prepared for now, reducing their potential impact.

Publications

10 25 50
 
Description The computational packages currently used by the industry are typically used to analyse either the soil or the structure in isolation, and do not account for a time-dependant assessment of groundwater flow (although some models might offer the potential).

The innovation of this project is the use of PLAXIS to develop a finite element model that computes the response of both the soil and the masonry to water level changes and the state of stress and strain at the interface between them.

To do this, the team referred to the information provided in TfL archives and in CIRIA report to input data relating to strength, stiffness and, where applicable, permeability of soil and masonry into the model. Additionally, they assigned a phreatic surface from which a seepage calculation determined the flow of groundwater around the structure.

This allowed the model to calculate stress and deformation conditions under differing groundwater levels. Such conditions were presented as fragility curves relating hazard levels to performance reduction which provided the assessor with a holistic view of the behaviour of the tunnel lining and surrounding geology.
Exploitation Route They can be applied to other railway networks by historic railways managers
Sectors Construction,Environment,Culture, Heritage, Museums and Collections,Transport

 
Description The provision of a finite element model that provides a holistic view of the pressures and uncertainties associated with the resilience of the line is the key output of the project. This helps TfL understand the nature and scale of the vulnerability of the asset and informs their decision making process. The direct impact of such a project is in the understanding of the response of the local section of the Circle Line to changing groundwater levels and the improvement of the maintenance and monitoring strategies. This would in turn minimise disruption to service and benefit the passengers. This impact could be extended through the application of the approach and the model to the individual locations where groundwater is a known concern for TfL, to the whole Circle Line or to other cut-and-cover tunnels within the London Underground network. The model could also be used by engineering consultancies, ie Arup, to assess other tunnels and heritage assets or in other engineering projects. Finally, having identified and set out in a systemic manner the parameters involved in understanding the resilience of these assets, and importantly the uncertainties associated with each of these parameters, the project has developed an approach that can be readily transferred to other networks and assets. In fact, through the repetition of this method using different datasets, it would be possible to extend its application to other infrastructure asset owners within the London basin, such as Network Rail, and to other urban locations where groundwater is a hazard known to affect infrastructure, such as Leeds in West Yorkshire.
First Year Of Impact 2015
Sector Construction,Environment,Culture, Heritage, Museums and Collections,Transport
Impact Types Economic,Policy & public services

 
Title Plaxis Groundwater Tunnel Interaction Model 
Description A finite element model of the tunnel, surrounding ground strata and drainage system. Used to model and measure the impact of fluctuating groundwater on the behaviour of the tunnel lining and the drainage system, and from this investigate the vulnerability of the tunnel asset to changing groundwater hazard. 
Type Of Material Computer model/algorithm 
Year Produced 2016 
Provided To Others? No  
Impact The model allowed more robust and detailed assessment of the vulnerability than would have been achieved by theoretical calculation alone. 
 
Description NERC cOllaboration with Arup 
Organisation Arup Group
Country United Kingdom 
Sector Private 
PI Contribution Use of qualitative /quantitative approach to knowledge and information to develop a phisycal finite element model on the basis of historical information
Collaborator Contribution expertise on the specific Plaxis software used, knowledge of archival sources
Impact recommendation for use of information to run sensitivity analyses on modelling with uncertain data
Start Year 2014
 
Description NERC project collaboration with tfL 
Organisation Transport for London
Country United Kingdom 
Sector Public 
PI Contribution development of an assessment procedure to determine the sensitivity of the underground network to ground water
Collaborator Contribution Data and expertise of operations in the London underground
Impact procedure report
Start Year 2014
 
Description British Hydrological Society Workshop 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Industry/Business
Results and Impact Presentation of the research by UCL/TfL to a professional institution. The talk was part of a programme of discussions surrounding the translation of risk based academic research into outputs for industry.
Year(s) Of Engagement Activity 2016