remote sensing Article Retrospective InSAR Analysis of East London during the Construction of the Lee Tunnel Jennifer Scoular 1,* , Richard Ghail 2 , Philippa Mason 3 , James Lawrence 1, Matthew Bellhouse 4, Rachel Holley 5 and Tom Morgan 1 1 Department of Civil and Environmental Engineering, Imperial College London, London SW7 2AZ, UK; [email protected] (J.L.); [email protected] (T.M.) 2 Department of Earth Sciences, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UK; [email protected] 3 Department of Earth Science and Engineering, Imperial College London, London SW7 2AZ, UK; [email protected] 4 Soil Engineering Geoservices Limited, Foundation Court, Watchmoor Park, Camberley, Surrey GU15 3RG, UK; [email protected] 5 CGG Satellite Mapping, Crockham Park, Edenbridge TN8 6SR, UK; [email protected] * Correspondence: [email protected]; Tel.: +44-(0)-2075940700 Received: 7 February 2020; Accepted: 4 March 2020; Published: 6 March 2020 Abstract: The Lee Tunnel was constructed as the first part of the Thames Tideway Improvement scheme, between 2010 and 2016. With tunnelling for the East section of the main Thames Tideway Tunnel, which joins the Lee Tunnel at Abbey Mills Pumping Station, beginning in early 2020, this paper investigates patterns of deformation in East London during construction of the Lee Tunnel. An unexpected geological feature, later identified as a drift filled hollow, was discovered during tunnelling. This study demonstrates that had eight years of ERS Persistent Scatterer Interferometry (PSI) data been analysed prior to tunnelling, the unusual pattern of displacement may have been recognised and further targeted borehole investigations taken place before the launch of the tunnel boring machine. Results also show how areas of different land use, including cemeteries and historic landfill, exhibit differences in settlement behaviour, compared with surrounding terraced housing. This research highlights the challenges in interpreting PSI results in an urban area with ongoing construction and the value of a long archive of data, which now spans almost three decades in London, that can be used to establish a baseline prior to construction. Keywords: Persistent Scatterer Interferometry; InSAR; urban subsidence; London; tunnelling 1. Introduction The Lee Tunnel was constructed as the first part of the Thames Tideway Improvements Scheme. On completion of the main Thames Tideway Tunnel (TTT), the scheme aims to eliminate frequent combined sewage and stormwater discharges into the River Thames, to comply with European waste-water directives (91/271/EEC) [1]. The Lee Tunnel is a 7.2 m internal diameter, 6.9 km long transfer tunnel, from Abbey Mills Pumping Station to Beckton Sewage Treatment Works, London [2]. Construction began in September 2010 [3] and tunnelling was completed in 2014 [4]; it became operational in April 2016 [5]. The Lee Tunnel is the deepest tunnel ever built in London, with an average depth of 75 m. The main TTT, also known as London’s ‘super sewer’ is a 25 km tunnel of the same diameter, that runs from Acton in West London, to Abbey Mills Pumping Station, East London, where it joins the Lee Tunnel (Figure1). The West and Central sections of the TTT run directly beneath the River Thames. The East section consists of a 5.5 km tunnel that runs from Chambers Wharf, Bermondsey, to Abbey Remote Sens. 2020, 12, 849; doi:10.3390/rs12050849 www.mdpi.com/journal/remotesensing Remote Sens. 2020, 12, 849 2 of 19 Remote Sens. 2020, 12, x FOR PEER REVIEW 2 of 21 toMills Abbey Pumping Mills Pumping Station,and Station, the Greenwichand the Greenwich Connection Connection Tunnel, aTunnel, 5 m internal a 5 m diameter,internal diameter, 4.6 km long, 4.6 kmtunnel long, from tunnel Greenwich from Greenwich to Chambers to Chambers Wharf [6]. Wharf Tunnelling [6]. Tunnelling for the East for section the East is scheduledsection is scheduled to begin in toearly begin 2020 in andearly similarities 2020 and similarities between the between East works the East and theworks Lee and Tunnel the inLee terms Tunnel of size,in terms geology of size, and geologydepth, means and depth, that much means can that be learnt much from can thebe challengeslearnt from and the successes challenges of theand Lee succ Tunnelesses andof the applied Lee Tunnelto the ongoing and applied construction. to the ongoing construction. FigureFigure 1. Map showingshowing thethe location location of of major major tunnelling tunnelling projects projects in London in London since 2010.since The2010. area The studied area studiedin this paperin this is paper shown is byshown the black by the rectangle. black rectangle. InterferometricInterferometric Synthetic Synthetic Aperture Aperture Radar Radar (InSAR) (InSAR) is is used used to to investigate investigate patterns patterns of of ground ground deformationdeformation associated withwith the the Lee Lee Tunnel. Tunne InSARl. InSAR is a is remote a remote sensing sensing technique technique that provides that provides reliable reliableground ground surface surface measurements measurements over time, over with time, millimetric with millimetric precision, precision, along the along sensor’s the sensor’s ‘line-of-sight’ ‘line- of(LOS).-sight’ InSAR (LOS). has InSAR gained has recognitiongained recognition in thelast in the 10 last years, 10 asyears, a tool as fora tool ground for ground monitoring monitoring above abtunnellingove tunnelling projects, projects, in London in London particularly particularly because because of the post-constructionof the post-construction monitoring monitoring of the Jubileeof the JubileeLine Extension Line Extension (1993 to (1993 1999) andto 1999) the Crossrail and the project,Crossrail for project, which tunnelling for which took tunnelling place between took place May between2012 and May May 2012 2015 and and May led to2015 a clear and settlementled to a clear trough settlement aligned trough east–west aligned across east central–west Londonacross central [7–11]. LondonAlthough [7– much11]. Although of the Lee much Tunnel of constructionthe Lee Tunnel coincided construction with thatcoincided of Crossrail, with that it remained of Crossrail, largely it remainedunreported largely because unreported it is much because deeper it and is much narrower, deeper so and that narrower, the surface so deformation that the surface associated deformation with it associatedis much less with apparent it is much than Crossrail’sless apparent settlement than Crossrail’s trough. Beyond settlement the UK,trough. InSAR Beyond has notably the UK, been InSAR used hasto monitor notably tunnellingbeen used projects to monitor in China tunnelling [12–14 ],projects France [in15 China], Germany [12–1 [416], ],France Italy [ 17[1],5] Netherlands, Germany [1 [186],], ItalyRomania [17], Netherlands [19], Spain [20 [18,21],] Romania and USA [ [1922]]., Spain [20,21] and USA [22]. ThisThis study study looks looks mainly mainly at at East East London London over over the the period period Febru Februaryary 2010 2010 to to September September 2015, 2015, using using RadarsatRadarsat-2-2 PSI PSI data processed byby CGG’sCGG’sSatellite Satellite Mapping Mapping group. group. Additional Additional InSAR InSAR data data acquired acquired by byERS ERS (1992–2000), (1992–2000), ENVISAT ENVISAT (2002–2010), (2002–2010), TerraSAR-X TerraSAR (2011–2017)-X (2011–2017) and and Sentinel Sentinel 1A and1A and 1B (2015–2018) 1B (2015– 2018)satellites satellites are used are used in specific in specific case studies.case studies. 1.1. Geological Setting 1.1. Geological Setting At an average depth of 75 m, the majority of the Lee Tunnel lies within the Upper Cretaceous, At an average depth of 75 m, the majority of the Lee Tunnel lies within the Upper Cretaceous, Seaford Chalk Formation (Figure2). Overlying the Chalk are Palaeogene sediments, including the Seaford Chalk Formation (Figure 2). Overlying the Chalk are Palaeogene sediments, including the Thanet Sand, Lambeth Group (variable sands, silts and clays), Harwich Formation (variable sands, Thanet Sand, Lambeth Group (variable sands, silts and clays), Harwich Formation (variable sands, clays and occasionally gravel) and London Clay Formation, and above these are Quaternary superficial sediments, the river terrace deposits and alluvium. Certain lithologies are relatively Remote Sens. 2020, 12, 849 3 of 19 claysRemote and Sens. occasionally 2020, 12, x FOR gravel) PEER REVIEW and London Clay Formation, and above these are Quaternary superficial3 of 21 sediments, the river terrace deposits and alluvium. Certain lithologies are relatively homogeneous, suchhomogeneous, as London such Clay, as whilst London others Clay, are whilst characterised others are bycharacterised lateral and by vertical lateral variability and vertical [23 variability]. Further details[23]. Further on lithostratigraphy details on lithostratigraphy encountered duringencountered construction during canconstruction be found incan [24 be– 26found]. in [24–26]. Figure 2. GeologicalGeological cross cross-section-section of the Lee Tunnel (modified(modified after Newman et al. [[2255]).]). TwoTwo faultfault zoneszones areare recognisedrecognised alongalong thethe tunneltunnel alignment:alignment: thethe GreenwichGreenwich faultfault zone,zone, withinwithin whichwhich thethe BecktonBeckton shaftshaft isis sited,sited, andand thethe PlaistowPlaistow GrabenGraben faultfault zone,zone, whichwhich waswas discovereddiscovered