Retrospective Insar Analysis of East London During the Construction of the Lee Tunnel

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 showing showing the the location location of ofmajor major tunnelling tunnelling projects projects in London in London since 2010. since The 2010. 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 with with the the Lee Lee Tunnel. Tunne InSARl. InSAR is a isremote 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 (19931999) and to 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 duringduring groundground investigationsinvestigations forfor thethe LeeLee TunnelTunnel [[2277,,2828].]. TheThe GreenwichGreenwich faultfault [[2929]] isis aa north–eastnorth–east south–westsouth–west trendingtrending feature that has has as as much much as as 40 40 m m vertical vertical displacement displacement across across it, it, with with strata strata downthrown downthrown to tothe the north north–west–west [30 []30. The].The Plaistow Plaistow Graben Graben is a negative is a negative relief relief fault faultzone, zone,approximately approximately 900 m 900 wide, m wide,in which in which the ground the ground is progressively is progressively downthrown downthrown towards towards the the centre, centre, resulting resulting in in Thanet Thanet Sands beingbeing broughtbrought intointo thethe tunneltunnel horizonhorizon [[2828,,3311].]. Crossing any fault during tunnelling cancan bebe hazardoushazardous becausebecause theythey can cause sudden changes in lithology, disturbed ground, groundwater andand mechanical propertiesproperties ofof rocks,rocks, leadingleading toto tunnellingtunnelling problemsproblems suchsuch asas flowingflowing ground,ground, tunneltunnel overbreakoverbreak andand steeringsteering problemsproblems forfor aa TunnelTunnel BoringBoring MachineMachine (TBM)(TBM) [[2255].]. ItIt isis thereforetherefore importantimportant thatthat thethe locationlocation andand naturenature ofof anyany faultingfaulting areare knownknown priorprior toto construction.construction. AA feature with with unexpected unexpected geological geological characteristics characteristics was was encountered encountered during during the the progress progress of the of theLee LeeTunnel Tunnel drive. drive. In Inaddition, addition, a ‘yellowing’ a ‘yellowing’ of ofthe the cha chalklk and and rounded rounded flints flints were were reported reported in thethe slurryslurry treatment treatment plant plant [4 ,[244,2].4 Additional]. Additional boreholes boreholes were were drilled drilled to understand to understand the nature the nature of the feature, of the whichfeature, was which later was defined later as defined a drift as filled a drift hollow filled (DFH) hollow [24 (DFH)]. DFH’s, [24]. also DFH’s, known also as known scour hollows, as scour rockheadhollows, rockhead anomalies anomalies and buried and hollows, buried are hollows, steep-sided are steep geological-sided geological depressions depressions in the rockhead in the surface.rockhead They surface. can They be up can to 75be mup deepto 75 andm deep 90 toand 475 90 m to wide, 475 m narrowing wide, narrowing with depth with depth [29,32 ,[3329].,31 Infill,33]. depositsInfill deposits are typically are typically a mélange a mélange of alluvial of sand, alluvial gravel sand, and gravel some clayey and some beds. clayey Several beds. hypotheses Several havehypotheses been proposed have been for proposed their formation for their including formation fluvial including scour [fluvial32], periglacial scour [32 activity], periglacial (pingos) activity [34], excess(pingos) pore [34] water, excess pressures pore water [35 pressures], structural [35 control], structura by basementl control by transcurrent basement transcurrent faults [36]and faults faults [36] originatingand faults originating from pressurised from pressurised water lenses water during lenses glacial during periods glacial [ 37periods]; with [3 all7]; considering with all considering them as periglacialthem as periglacial phenomena. phenomena. DFH’sDFH’s areare aa substantialsubstantial hazardhazard toto tunnellingtunnelling projects,projects, oftenoften containingcontaining perchedperched waterwater leadingleading toto variablevariable groundwater groundwater conditions, conditions, potential potential slumping, slumping, shearing shearing of the sides of the and sidesdifferential and compaction differential ofcompaction material [ 29of, material38]. Adjustments [29,38]. Adjustments to the TBM mayto the be TBM required may be in responserequired in to response such variable to such conditions, variable suchconditions, as an increasesuch as an in TBMincrease thrust in TBM or a th decreaserust or a in decrease cutting wheelin cutting rotation wheel speed rotation [30 ].speed This [3 can0]. leadThis can lead to an increased risk for operators and project delays because of reduced speed, therefore resulting in additional costs. Additionally, their distribution across the region is largely unknown. In terms of hydrogeology, both of London’s two main aquifers were encountered during the Lee Tunnel project. The Lower Aquifer was recorded in the Seaford Chalk, in hydraulic connection with the overlying Thanet Sand and Upnor formations. The Upper Aquifer was recorded in the river Remote Sens. 2020, 12, 849 4 of 19 to an increased risk for operators and project delays because of reduced speed, therefore resulting in additional costs. Additionally, their distribution across the region is largely unknown. In terms of hydrogeology, both of London’s two main aquifers were encountered during the Lee Tunnel project. The Lower Aquifer was recorded in the Seaford Chalk, in hydraulic connection with the overlying Thanet Sand and Upnor formations. The Upper Aquifer was recorded in the river terrace deposits and alluvium across the whole tunnel alignment [25]. The piezometric profile for the Lower Aquifer indicated groundwater pressures of up to 800 kPa at Beckton and 450 kPa at Abbey Mills [25]. The Lower Aquifer (chalk) is a high transmissivity aquifer comprising dual porosity, with flow and storage occurring in both the fractures and the matrix [39]. The majority of flow occurs in the fractures, with water stored in the matrix released to the fractures when groundwater levels fall [39]. Groundwater levels in London are managed by the Environment Agency. During the 19th century and early 20th century, the Lower Aquifer in London was exploited as a result of increased industrialisation. From the mid-1960s, there was a significant reduction in groundwater abstraction from the Lower Aquifer, which caused groundwater levels to rise. In order to prevent potential structural damage, such as that to the London Underground and building formations, since the 1990s, the aquifer has been artificially managed by controlling changes in abstraction. The London Clay confines the Lower Aquifer in most of London and changes in groundwater can lead to uneven deformation of the London Clay as well as consolidation of the underlying sandy units. The Upper Aquifer is unconfined.

1.2. Tunnel Construction Five permanent large-diameter shafts were constructed as part of the Lee Tunnel, three at Beckton and two at Abbey Mills [40]. Tunnelling was carried out using an 8.85 m Slurry Pressure Balance System (SPBS) TBM, chosen because of the significant depth and high water pressures [4]. The TBM was launched from the Beckton Overflow Shaft in February 2012 [41] and completed tunnelling at Abbey Mills in January 2014 [4]. Most tunnel construction results in ground settlements at the Earth’s surface. The ground deforms towards the tunnel face due to stress relief. This settlement is usually described by an inverted bell curve and is the mechanism that can lead to ground strains being transmitted to tunnel structures [42]. Other movements tend to be more localised and erratic, resulting from specific failures in the tunnel, sudden changes in the hydrogeological regime and unpredicted geological features [43]. The main factors affecting ground movements are tunnel diameter and shape, depth, construction method and ground properties. Empirical settlement equations tend to provide good results for ‘greenfield’ areas, i.e., those without road surfaces, hard standings, foundations or other subsurface infrastructure, but are less reliable for brownfield areas due to soil–structure interaction. Ground deformation, both as a result of tunnelling, other construction projects and geological movements, can be measured using InSAR. Understanding the causes of deformation observed in InSAR results is important to improve our knowledge of potential unforeseen ground conditions for future engineering projects. In addition to the Thames Tideway East Tunnel, further construction is planned: the Silvertown Tunnel, which will carry a new road under the Thames to connect the Greenwich Peninsula to West Silvertown (Royal Docks).

2. Materials and Methods The primary Synthetic Aperture Radar (SAR) data used in this study are from Radarsat-2 (RS2), chosen due to its availability over the time period of interest (2010 to 2015). It has been processed by CGG, using their PSI algorithm. RS2 is a jointly funded mission between the Canadian Space Agency (CSA) and MacDonald Dettwiler Associates Ltd. (MDA). It was launched in December 2007 and is still operational. RS2 is a C-band (wavelength 5.6 cm) sensor, with a revisit period of 24 days. The RS2 processed data have been derived from 58 ascending images acquired during the 5.5 years from 2 February 2010 to 28 September 2015. Displacements are given along the LOS which in this case has Remote Sens. 2020, 12, 849 5 of 19

an inclination of approximately 33◦, and the minimum coherence is 0.72. The dataset contains, on average, 2200 points per km2 over an area of 89 km2. Displacement along the LOS can be measured to better than 1 mm on PS characterised by very consistent radar returns [44]. The RS2 PSI results used here have a mean estimated uncertainty of 0.4 mm/y for the displacement rate, calculated using the covariance-weighted standard deviation of residual phase components for each point. However, these data-derived empirical estimates may potentially underestimate uncertainties in real-world use-cases. Validation against conventional land survey monitoring across a similar urban environment as part of a large European Space Agency (ESA) PSI validation activity showed the typical accuracy for the PSI average annual motion rates was 1 to ± 2 mm/y, with individual time series displacements having a typical RMS error of 4-6 mm [45], and we consider these values more representative. Additional SAR data used are from ERS, ENVISAT, TerraSAR-X (TSX) and Sentinel 1A and 1B (Table1). Together, the ERS, ENVISAT, Radarsat-2 and Sentinel-1 data provide a near-continuous record of ground displacement from 1992 through to the present day. In addition, the TerraSAR-X data provide considerable overlap with that from Radarsat-2, providing an opportunity for cross-comparison between two fully independent datasets.

Table 1. SAR datasets used in the study.

No. of Processing PS Density Satellite Start Date End Date Asc/Desc Band Images Method (per km2) ERS 19/06/1992 31/07/2000 27 Asc PSI, CGG C 1400 ENVISAT 13/12/2002 17/09/2010 45 Desc PSI, CGG C 1500 Radarsat-2 02/02/2010 28/09/2015 58 Asc PSI, CGG C 2200 SqueeSARTM, TerraSAR-X 01/05/2011 28/04/2017 150 Desc X 2400 TRE Altamira PSInSAR, Sentinel-1 06/05/2015 28/12/2018 163 Asc ENVI C 7200 (A and B) SARScape

A Persistent Scatterer InSAR (PSI) dataset derived from 27 irregularly spaced ERS SAR images (19 June 1992 to 31 July 2000), and from 45 irregularly spaced ENVISAT ASAR scenes (13 December 2002 to 17 September 2010), is used here to assess long-term deformation patterns. ERS and ENVISAT were European Space Agency (ESA) C-band radar missions. ERS-1 was operational between 1991 and 2000 and ERS-2 1995 to 2011. ENVISAT was the successor to ERS, launched in 2002 and ended in 2012. Sentinel-1 (A and B) ascending data were processed using ENVI (v5.5) SARScape (v5.4.1) PSI module. Sentinel-1 (A and B) are ESA-owned C-band satellites, with a repeat cycle of 6 days when both are in operation. The time period processed is from 6 May 2015 to 28 December 2018 and the LOS inclination is approximately 35◦. The minimum coherence used was 0.8, which leads to an average density of 7200 PS points per km2. The velocity precision, calculated using the acquisition temporal baseline and the multi-temporal coherence is 0.1 mm/y. TSX data has been used as a comparison to RS2 data because the TSX dataset was validated with levelling data (Building Research Establishment (BRE) levelling sockets and conventional levelling studs/plates) from Crossrail Elizabeth Line monitoring at Bond Street [46]. TSX is an X-band (wavelength 3.1 cm) satellite, operated by the German Aerospace Centre (DLR) and launched in 2007. It has a revisit period of 11 days and the LOS inclination of the sensor is 37◦. The TSX dataset contains 150 descending images, taken in StripMap mode, over the period May 2011 to April 2017 (6 years) and was processed by TRE Altamira using their SqueeSARTM method [47]. In this part of East London, it has an average of 2400 PS points per km2. Direct comparison of the RS2 and TSX datasets, which have different start and end dates and a different temporal frequency of measurements, was made from the first common date, 1 May 2011, to Remote Sens. 2020, 12, 849 6 of 19 the same end date, 28 September 2015. This was achieved by subtracting the displacement value on the first date, from the whole time series, to make 01/05/2011 zero. Each dataset was then interpolated toRemote derive Sens. a20 daily20, 12, x value FOR PEER for REVIEW displacement, to adjust for the different temporal frequencies6 and of 21time gaps in the data. The two datasets use different reference locations, which can result in a systematic shift3. Results in displacement and Discussion through time, corresponding to any differential displacement between the two reference locations. Several interesting features have been identified in the Radarsat dataset. One of the most notable 3.is Resultsat the Limmo and Discussion Peninsula (Figure 3), where up to -40 mm of displacement, over the 5.5-year period, has been caused by dewatering for Crossrail construction. This phenomenon was similarly observed in theSeveral TSX results, interesting described features in detail have by been Bischoff identified et al. [ in7] the. Radarsat dataset. One of the most notable is at theThe Limmo case studies Peninsula presented (Figure in this3), wherepaper are up labelled to 40 mmon Figure of displacement, 3 and include: over Newham the 5.5-year Hospital period, − has(1), been Custom caused House by dewatering residential area for Crossrail (2), A13 construction. Newham Way This (3), phenomenon East London was Crematorium similarly observed and inCemetery the TSX (4a) results, and describedEast Ham Jewish in detail Cemetery by Bischo (4b),ff et and al. Beckton [7]. Sewage Treatment Works (5).

FigureFigure 3. Extent of of the the Radarsat Radarsat-2-2 dataset dataset (20 (201010 to to2015), 2015), showing showing average average velocity velocity in mm/ in mmy overlain/y overlain onon ArcGIS ArcGIS Online Online World World Imagery. Imagery. The The locations locations of of the the Lee Lee Tunnel Tunnel route route and and its its main main shafts shafts are are shown, inshown, addition in addition to the caseto the study case study sites at:sites 1—Newham at: 1—Newham Hospital; Hospital 2—Custom; 2—Custom House House Residential Residential Area; Area; 3—A13 Newham Way; 4a—East London Crematorium and Cemetery; 4b—East Ham Jewish 3—A13 Newham Way; 4a—East London Crematorium and Cemetery; 4b—East Ham Jewish Cemetery; Cemetery; 5—Beckton Shafts (Pumping and Connection). 5—Beckton Shafts (Pumping and Connection).

3.1. NewhamThe case Hospital studies presented in this paper are labelled on Figure3 and include: Newham Hospital (1), CustomNewham House Hospital residential and Boundary area (2), Lane A13 Newham(to the east Way of the (3), hospital) East London exhibit Crematorium substantial settlement and Cemetery (4a)during and the East period Ham 2010 Jewish to 2015, Cemetery and this (4b), can and be seen Beckton clearly Sewage in Figure Treatment 3, Location Works 1. To (5). understand the cause of this settlement, historic maps [48] have been analysed (Figure 4). Until the 1910s, the hospital 3.1.site Newhamand surrounding Hospital area were occupied by marshland and agricultural fields with the most notable manNewham-made feature Hospital being and the Boundary east–west Lane embankment (to the east of of the the Northern hospital) Outfall exhibit Sewersubstantial which settlement first appears in the 1870 map. In 1910, a gravel pit is first shown just south of the site. By 1930, there are during the period 2010 to 2015, and this can be seen clearly in Figure3, Location 1. To understand the substantial housing developments to the north of the Northern Outfall Sewer and to the West of the cause of this settlement, historic maps [48] have been analysed (Figure4). Until the 1910s, the hospital hospital site. In 1950, further embankments appear on the site of the current hospital and, by 1960, site and surrounding area were occupied by marshland and agricultural fields with the most notable there is an extensive network of embankments with tracks on the top. Environment Agency records man-made feature being the east–west embankment of the Northern Outfall Sewer which first appears reveal that these embankments were constructed for the historic Prince Regent Lane landfill site. The in the 1870 map. In 1910, a gravel pit is first shown just south of the site. By 1930, there are substantial extent of that landfill site was obtained from the Department of Environment Food and Rural Affairs housing[49]. Thedevelopments landfill lies beneath to the part north of the of thecurrent Northern Newham Outfall Hospital, Sewer Cumberland and to the School, West of Gateway the hospital site.Surgical In 1950, Centre further and Newham embankments Centre appear for Mental on the Health. site of The the geographical current hospital extents and, and by ground 1960, there-level is an measurement time series of the landfill site and its surrounding area are shown in Figure 5. Remote Sens. 2020, 12, 849 7 of 19 extensive network of embankments with tracks on the top. Environment Agency records reveal that these embankments were constructed for the historic Prince Regent Lane landfill site. The extent of that landfill site was obtained from the Department of Environment Food and Rural Affairs [49]. The landfill lies beneath part of the current Newham Hospital, Cumberland School, Gateway Surgical Centre and Newham Centre for Mental Health. The geographical extents and ground-level measurement time seriesRemote of Sens. the 20 landfill20, 12, x FOR site PEER and REVIEW its surrounding area are shown in Figure5. 7 of 21

FigureFigure 4. 4.Historic Historic mapsmaps ofof NewhamNewham Hospital (red rectangles) rectangles) and and its its surroundings. surroundings. The The maps maps are are fromfrom Ordnance Ordnance Survey Survey historicalhistorical mapmap database:database: ( (aa)) 1870, 1870, (b (b) )1930, 1930, (c ()c )1960 1960 and and (d ()d 1990) 1990.. Available Available fromfrom Edina Edina Digimap Digimap online. online.© © Crown Copyright Copyright and and Landmark Landmark Information Information Group Group Limited Limited 2019. 2019. All All rightsrights reserved. reserved.

This historic landfill is further evidenced in borehole records (accessed through the British Geological Survey’s (BGS) GeoIndex database), by an approximately 6 m thickness of Made Ground containing brick, glass, metal, clinker, pottery and wood (Figure6: TQ38SW 2081, 2082, 2083, 2084, 2085, 2087, 2088, 2090, 2091). From the time series in Figure5, it is clear that the area of historic landfill settles more quickly than the surrounding area; ca. 7 mm more, over the five-year time period (2010–2015). Looking back at the ERS data from 1992 to 2010 (Figure5b,c), it is apparent that this pattern of deformation has been ongoing prior to tunnelling, for at least three decades, with the landfill area accumulating ca. 90 mm − of settlement between 1992 and 2015 compared to ca. 40 mm for the surrounding area. − The same pattern of deformation can also be identified in the TSX data, from 2011 to 2015 (Figure7). In this comparison, the landfill area (red) contains 841 PS points for RS2 and 889 for TSX, and the surrounding area (green) contains 1735 PS points for RS2 and 1779 for TSX. The correlation between each dataset has been tested using a Pearson Product Moment Correlation Coefficient. The correlation for the landfill area between RS2 and TSX is 0.984 and for AOI 2 it is 0.980, therefore there is a strong positive correlation between the two datasets. Remote Sens. 2020, 12, 849 8 of 19

Remote Sens. 2020, 12, x FOR PEER REVIEW 8 of 21

Figure 5. (a) High-resolution image (ArcGIS Online World Imagery, dated 07/03/2014) indicating the Figure 5. (a) High-resolution image (ArcGIS Online World Imagery, dated 07/03/2014) indicating the location and extents of the historical Prince Regent Landfill Site (red hatched) and the area location and extents of the historical Prince Regent Landfill Site (red hatched) and the area surrounding surrounding the Newham landfill (green striped). (b–e) Ground displacement curves over the past the Newham27 years: (b landfill) 1992–2000 (green ERS; striped). (c) 2002–2010 (b– eENVISAT;) Ground (d displacement) 2010–2015 Radarsat curves-2; overand (e the) 2015 past to 2018 27 years: (b) 1992–2000Sentinel-1. ERS; The red (c) 2002–2010line represents ENVISAT; the landfill (d )area, 2010–2015 and the Radarsat-2;green line the and surrounding (e) 2015 toarea. 2018 Sentinel-1. The red line represents the landfill area, and the green line the surrounding area. This historic landfill is further evidenced in borehole records (accessed through the British TheGeological settlement Survey’s appears (BGS) to GeoIndex cease in database), the first few by an months approximately of 2015 (Figure6 m thickness5e). This of Made is assumed Ground to be a resultcontaining of a cease brick, in dewateringglass, metal, clinker, for Crossrail pottery at and the wood Limmo (Figure Peninsula 6: TQ38SW Cross 2081, Passage 2082, 2083, 13 (CP13, 2084, [50]) and in2085, local 2087, chalk 2088, basin 2090, abstractions 2091). [39]. TheFrom area ofthe high time settlementseries in Figure extends 5, it is wellclear beyondthat the area the of historic historiclandfill, landfill settles to the more north quickly and east. Boreholethan recordsthe surrounding reveal anarea; unusual ca. 7 mm thickness more, over of the gravels five-year and time sands period extending (2010–2015). to a Looking depth of back at least 20 mat AODthe ERS (above data from ordnance 1992 to datum), 2010 (Figure which 5b, isc), the it is base apparent of the that boreholes, this pattern on theof de easternformation edge has of the − been ongoing prior to tunnelling, for at least three decades, with the landfill area accumulating ca. hospital and in the car park (Figure6). The descriptions are consistent with that of DFH’s [ 29,32,35]. −90 mm of settlement between 1992 and 2015 compared to ca. −40 mm for the surrounding area. NewmanThe et al. same [25] pattern and Bellhouse of deformation et al. [can24] also reported be iden atified DFH in here the TSX identified data, from during 2011 constructionto 2015 (Figure of the Lee Tunnel.7). In this A comparison, mixture of the sand landfill and area weathered (red) contains flint was 841 PS found points in for the RS2 slurry and treatment889 for TSX, plant, and the over a distancesurrounding of approximately area (green) 4 contains m, before 1735 the PS TBM points was for stoppedRS2 and 1779 to allow for TSX. further The correlation borehole investigationbetween ahead of the tunnel face to go ahead [25]. At this point approximately halfway along the Lee Tunnel, the depth is approximately 40 m, so the DFH must be at least 40 m deep, extending into the chalk, to be intersected by the TBM. Remote Sens. 2020, 12, x FOR PEER REVIEW 9 of 21

each dataset has been tested using a Pearson Product Moment Correlation Coefficient. The Remotecorrelation Sens. 2020, 12for, 849 the landfill area between RS2 and TSX is 0.984 and for AOI 2 it is 0.980, therefore there9 of 19 is a strong positive correlation between the two datasets.

Remote Sens. 2020, 12, x FOR PEER REVIEW 10 of 21

Rates of subsidence are variable across the area. The greatest subsidence rates are at the eastern edge of the hospital and in the hospital car park, which can be seen in the cross-section in Figure 8. The cross-section was constructed using mean PS displacement measurements, averaged within a 25 FigureFigure 6. BGS 6. BGS boreholes boreholes (accessed (accessed from from BGS BGS GeoIndex online) online) in in the the area area surrounding surrounding Newham Newham m buffer Hospital.of the profile Logged line,records with from nodes boreholes spaced within every the black 10 dashed m. This circle area evidence is consistent materials typical with ofthe area in Hospital. Logged records from boreholes within the black dashed circle evidence materials typical of Figure 6 containingscour hollows. boreholes Borehole colours that evidence represent depth a DFH. below surface. scour hollows. Borehole colours represent depth below surface. The settlement appears to cease in the first few months of 2015 (Figure 5e). This is assumed to be a result of a cease in dewatering for Crossrail at the Limmo Peninsula Cross Passage 13 (CP13, [50]) and in local chalk basin abstractions [39]. The area of high settlement extends well beyond the historic landfill, to the north and east. Borehole records reveal an unusual thickness of gravels and sands extending to a depth of at least −20 m AOD (above ordnance datum), which is the base of the boreholes, on the eastern edge of the hospital and in the car park (Figure 6). The descriptions are consistent with that of DFH’s [29,32,35]. Newman et al. [25] and Bellhouse et al. [24] reported a DFH here identified during construction of the Lee Tunnel. A mixture of sand and weathered flint was found in the slurry treatment plant, over a distance of approximately 4 m, before the TBM was stopped to allow further borehole investigation ahead of the tunnel face to go ahead [25]. At this point approximately halfway along the Lee Tunnel, the depth is approximately 40 m, so the DFH must be at least 40 m deep, extending into the chalk, to be intersected by the TBM.

Figure 7 7.. TimeTime series comparison showing the the relatively relatively close close fit ﬁt between the the TerraSAR TerraSAR-X-X ( (TSX)TSX) and Radarsat ground displacement datasets for for the the area of landfill landﬁll (red) and its surroundings (green), as shown in Figure 55.. TheThe solidsolid lineline representsrepresents Radarsat-2Radarsat-2 (RS2)(RS2) andand dasheddashed lineline TSX.TSX.

The western and southern edge of this DFH are well constrained. Boreholes TQ48SW2091, TQ48SW2081, TQ48SW2082 and TQ48SW2083 encounter London Clay at an average depth of −2.2 m AOD (Figure 6). However, there are no boreholes deeper than 10 m within 250 m to the north or east, so the true lateral extent is unknown. The extent of the DFH and similarities to the DFH at the Blackwall Tunnel is evident in a colour map of the rockhead in Figure 9. This model consists of depth to rockhead of more than 1000 boreholes, accessed from the BGS and imported into Midland Valley’s Move 3D software, with Ordinary Kriging used to generate a 3D surface. Ordinary Kriging was found to create the most realistic surface and the option to honour all data points was selected. It should be noted that the densities of boreholes in the Blackwall area have enabled greater constraint of this DFH, whilst the data sparsity surrounding the Newham Hospital DFH inhibit recognition of its geometric extent. The settlement mechanism(s) around Newham Hospital are unclear. Consolidation is evident, potentially being attributable to both anthropogenic (landfill) and natural (DFH) layers and appears to be a long-term phenomenon as settlement is evident since 1992. Dewatering, and to a less extent tunnelling, likely accelerated settlement. Now, these activities have ceased, the area has stabilised with a small amount of heave (ca. 3 mm, Figure 5e) attributed to groundwater recharge. Remote Sens. 2020, 12, 849 10 of 19

FigureFigure 8. (a 8). Map(a) Map showing showing the the location location of of thethe profileprofile A A (West) (West) to to A’ A’ (East) (East) across across the thehospital hospital and and landfill site. (b) Profile showing variable subsidence, with the greatest displacement to the East of the landfill site. (b) Profile showing variable subsidence, with the greatest displacement to the East of the hospital building and in the car park (indicated by the black square). hospital building and in the car park (indicated by the black square).

The western and southern edge of this DFH are well constrained. Boreholes TQ48SW2091, TQ48SW2081, TQ48SW2082 and TQ48SW2083 encounter London Clay at an average depth of 2.2 m − AOD (Figure6). However, there are no boreholes deeper than 10 m within 250 m to the north or east, so the true lateral extent is unknown. The extent of the DFH and similarities to the DFH at the Blackwall Tunnel is evident in a colour map of the rockhead in Figure9. This model consists of depth to rockhead of more than 1000 boreholes, accessed from the BGS and imported into Midland Valley’s Move 3D software, with Ordinary Kriging used to generate a 3D surface. Ordinary Kriging was found to create the most realistic surface and the option to honour all data points was selected. It should be noted that the densities of boreholes in the Blackwall area have enabled greater constraint of this DFH, whilst the data sparsity surrounding the Newham Hospital DFH inhibit recognition of its geometric extent. Remote Sens. 2020, 12, 849 11 of 19 Remote Sens. 2020, 12, x FOR PEER REVIEW 12 of 21

Remote Sens. 2020, 12, x FOR PEER REVIEW 12 of 21

FigureFigure 9. 9.Modelled Modelled surface surface of rockhead of rockhead elevation elevation in East in London, East London, constructed constructed using Move using 3D, Move showing 3D, twoshowing noticeable two noticeable depressions depressions which represent which represent scour hollows: scour hollows: a larger a larger one inone the in south–westthe south–west at theat Blackwallthe Blackwall Tunnel Tunnel and aand smaller a smaller onein one the in north–north–east the north–north– ateast Newham at Newham Hospital. Hospital. The dashedThe dashed black circleblack is circle the area is the shown area shown in Figure in Figure7. 7.

3.2. TheCustom settlement House Residential mechanism(s) Area around Newham Hospital are unclear. Consolidation is evident, Figure 9. Modelled surface of rockhead elevation in East London, constructed using Move 3D, potentially being attributable to both anthropogenic (landﬁll) and natural (DFH) layers and appears Toshowing the south two noticeableof Newham depressions Hospital which, there represent is another scour area hollows: of subsidence a larger one with in the a distinct south–west north at –east to be a long-term phenomenon as settlement is evident since 1992. Dewatering, and to a less extent to souththe –Blackwallwest trending Tunnel boundaryand a smaller (Figure one in 10thea) north and– nFigureorth–east 3, atLocation Newham 2. Hospital. The overall The dashed pattern of tunnelling, likely accelerated settlement. Now, these activities have ceased, the area has stabilised with subsidenceblack circle, calculated is the area by shown averaging in Figure the 7.PS points within Figure 10a (1) and (2) is similar on either aside small of amountthis boundary of heave but (ca. the 3 mm,south Figure-eastern5e) side attributed subsides to groundwater9 mm more over recharge. the same time period (3.2.Figure Custom 10 b ).House Residential Area 3.2. Custom House Residential Area To the south of Newham Hospital, there is another area of subsidence with a distinct north–east To the south of Newham Hospital, there is another area of subsidence with a distinct north–east to to south–west trending boundary (Figure 10a) and Figure 3, Location 2. The overall pattern of south–westsubsidence trending, calculated boundary by averaging (Figure the 10a) PS and points Figure within3, Location Figure 2.10 Thea (1) overall and (2) pattern is similar of subsidence, on either calculatedside of this by boundary averaging but the the PS south points-eastern within side Figure subsides 10a (1) 9 andmm (2)more is similarover the on same either time side period of this boundary(Figure 10 but b).the south-eastern side subsides 9 mm more over the same time period (Figure 10b).

Figure 10. (a) Map of average ground displacement (mm/y) at Persistent Scatterer (PS) points (RS2) showing sites (1) and (2), straddling a change in ground displacement behaviour in the Custom House residential area. (b) Time series displacement averaged over the area of boxes at sites (1) and (2) in 10a. At (2) (green), the ground has subsided by 9 mm more than at (1) (blue) over the 5.5 years.

In the ERS data, total displacement from 1992 to 2000 was −6 mm for (1) and −1 mm for (2), a −5 FigureFigure 10. 10.( a(a)) Map Map of of average average groundground displacementdisplacement (mm/ (mm/yy)) at at Persistent Persistent Scatterer Scatterer (PS (PS)) points points (RS2) (RS2) mm difference. From 2002 to 2010, (1) subsides 13 mm and (2) 8 mm ca. 5 mm more. The same pattern showingshowing sites sites (1) (1) and and (2), (2), straddling straddling aa changechange in ground displacement displacement behaviour behaviour in in the the Custom Custom House House is not observed in the Sentinel-1 (1A and 1B) data from 2015 to 2018. residentialresidential area. area. ( b(b) Time) Time series series displacement displacement averaged averaged over over the the area area of of boxes boxes at at sites sites (1) (1) and and (2) (2) in in (a ). No large-scale redevelopment that aligns with the footprint of this area took place during the At10a. (2) At (green), (2) (green) the ground, the ground has subsided has subsided by 9 mmby 9 moremm more than than at (1) at (blue) (1) (blue) over over the 5.5the years.5.5 years. study period; the area is occupied by terraced housing, built between the 1960s and the 1990s. WeIn the suggest ERS data, this boundary total displacement is a north from–east 1992trending to 2000 fault. was Firstly, −6 mm it foris consistent (1) and −1 with mm thefor (2),trend a −5 of amm major difference. fault set From identified 2002 to by 2010, Morgan (1) subsides et al. (in 13 press) mm and [31 (2)]. Secondly, 8 mm ca. 5it mm is proximal more. The to same the Plaistow pattern is not observed in the Sentinel-1 (1A and 1B) data from 2015 to 2018. No large-scale redevelopment that aligns with the footprint of this area took place during the study period; the area is occupied by terraced housing, built between the 1960s and the 1990s. We suggest this boundary is a north–east trending fault. Firstly, it is consistent with the trend of a major fault set identified by Morgan et al. (in press) [31]. Secondly, it is proximal to the Plaistow Remote Sens. 2020, 12, 849 12 of 19

In the ERS data, total displacement from 1992 to 2000 was 6 mm for (1) and 1 mm for (2), a − − 5 mm difference. From 2002 to 2010, (1) subsides 13 mm and (2) 8 mm ca. 5 mm more. The same − pattern is not observed in the Sentinel-1 (1A and 1B) data from 2015 to 2018. No large-scale redevelopment that aligns with the footprint of this area took place during the study period; the area is occupied by terraced housing, built between the 1960s and the 1990s. RemoteWe Sens. suggest 2020, 12 this, x FOR boundary PEER REVIEW is a north–east trending fault. Firstly, it is consistent with the trend13 of 21 a major fault set identified by Morgan et al. (in press) [31]. Secondly, it is proximal to the Plaistow Graben faultGraben zone fault (Figure zone 10 (Figure), which 10 is), 750which m to is the 750 north–west m to the north and a– suspectedwest and a product suspected of major product fault of linkage, major implyingfault linkage, the presence implying of the an unmarked presence of fault an locally unmarked [31]. Thirdly,fault locally the north–east [31]. Thirdly, continuation the north of–east this boundarycontinuation intersects of this boundary the DFH at intersects Newham the Hospital DFH at and Newham DFH’s Hospital are postulated and DFH’s to have are an postulated association to withhave faultingan association [35,38], with since faulting faults provide[35,38], pathwayssince faults for provide groundwater pathways flow. for groundwater flow. Another possiblepossible cause of this boundary is variability in the thicknessthickness and composition of alluvium,alluvium, that underlies this area. However, However, t thehe lack of boreholes in the vicinity of this boundary inhibit verificationverification by examiningexamining geological evidence in borehole borehole log log records. records. If this didifferentialfferential settlement is associated with faulting, it would support observations by Mason et al. [[5511] that these major structures are actively creepingcreeping inin London.London.

3.3. Newham Way A13 The road surface of Newham Way (A13), between Junctions 9 and 10, subsided by up to 45 mm between February 2010 to December 2015 (Figure3 3,, LocationLocation 33 andand Figure Figure 11 11).). TheThe causecause ofof thisthis subsidence is uncertain. It is known that there were ongoing improvements works to the A13 during this period.period. TheThe UKUK PowerPower Networks Networks (UKPN) (UKPN) replaced replaced a 1.6a 1.6 km km length length of cableof cable under under the the road road [52 ],[52 the], lengththe length of which of which appears appears to coincide to coincide with with the sectionthe section of road of road with with the greatestthe greatest subsidence subsidence (Figure (Figure 12). The12). The exact exact nature nature of these of these works works is unknownis unknown due due to to the the sensitive sensitive nature nature of of thethe project.project. Historically Historically,, this area has been occupied by marshland, as labelled on historic maps between the 1870s and 1890s (Figure4 4),), and and this this sectionsection ofof roadroad ﬁrst first appears appears on on the the 1930 1930 mapmap [[48].]. Subsidence here may therefore be a product of both natural and anthropogenic processes.processes.

Figure 11.11. ((aa)) Map Map of of average ground ground displacement (mm/ (mm/yy),), from RS2, at PS Points 1 to 5, along Newham Way (A13)(A13) betweenbetween JunctionsJunctions 99 andand 10;10; andand ((bb)) timetime seriesseries displacementdisplacement atat PointsPoints 11 toto 5.5. Remote Sens. 2020, 12, 849 13 of 19 Remote Sens. 2020, 12, x FOR PEER REVIEW 14 of 21

FigureFigure 12. 12.Map Map of of the the extent extent of of cables cables laid laid under under the the A13 A13 during during 2013 2013 to 2014. to 2014. The locationThe location of PS of points PS frompoints Figure from 11 Figure are also 11 are shown. also shown. Modiﬁed Modified from Winch from Winch [52]. [52].

3.4.3.4. East East London London Crematorium Crematorium andand CemeteryCemetery and East Ham Jewish Cemetery Cemetery TheThe Lee Lee Tunnel Tunnel drive drive passes passes just just North North of theof the East East London London Cemetery Cemetery (Figure (Figure3, Location 3, Location 4a). Over4a). theOver 5.5-year the 5.5 period,-year period, 27.7 mm 27.7 of mm the of settlement the settlement was observed was observed at the at cemetery, the cemetery, when when 167 PS 167 points PS points in the cemeteryin the cemetery are averaged. are averaged. On the On opposite the opposite side ofside the of tunnel the tunnel drive, drive, there there are are terraced terraced houses, houses, and and the averagethe average settlement settlement of 221 of points221 points is 6.8 is mm6.8 mm in total. in total. The The extent extent of of the the areas areas analysed analysed on on either either side side of theof tunnelthe tunnel drive drive is approximately is approximately equal, equal, but but the the comparison comparison area area of terraced of terraced housing housing to theto the north–west north– ofwest the cemeteryof the cemete areary was area selected was selected to exclude to exclude Lister Lister Gardens, Gardens, because because only only areas areas of Madeof Made Ground Ground are requiredare required for the for comparison the comparison against against the cemeteriesthe cemeteries (Figure (Figure 13, Site13, Site 1). 1). AA similar similar disparity disparity inin thethe settlementsettlement can be observed at at East East Ham Ham Jewish Jewish Cemetery Cemetery (Figure (Figure 33, , LocationLocation 4b), 4b), further further east eastalong along thethe routeroute (Figure(Figure 13, Site 2). On On the the north north side side of of the the route route where where the the cemeterycemetery is, is, there there has has been been anan averageaverage totaltotal settlement of 12.5 12.5 mm, mm, whereas whereas south south of of the the tunnel tunnel in in the the terraced housing, the settlement is only 8 mm. terraced housing, the settlement is only 8 mm. In the cemeteries, the PS points are located either directly on the ground surface or on the gravestones, whereas in the areas of terraced housing, PS points can be located anywhere from the roof to the pavement. Soil–structure interaction, stiffness of existing structures and consolidation from the loading of buildings reduce the magnitude of tunnelling induced displacements and produce a shallower and flatter settlement profile (Figure 14)[53,54]. This could explain why the PS points on the unconsolidated ground of the cemetery subside more than those on the terraced housing. However, Walker [4] reported that, in general, surface settlement as a result of tunnelling for the Lee Tunnel was less than 2 mm, therefore it is unlikely that tunnelling is the only contributing factor to this deformation. Prior to 2010, the cemeteries and surrounding houses (Figure 13b,c) subside collectively, with a total displacement of 11 mm; the differing rates of − deformation did not begin until approximately January 2011. More recent Sentinel-1 data (post-2015, Figure 13e) shows that the East London Crematorium and Cemetery is still subsiding, by ca. 13 mm from May 2015 to December 2018, whereas the area of houses is stable, settling by just 1 mm over the 3.5 years (Figure 13e). Remote Sens. 2020, 12, 849 14 of 19 Remote Sens. 2020, 12, x FOR PEER REVIEW 15 of 21

FigureFigure 13. 13.( a()a) Map Map of of average average ground ground displacementdisplacement (velocity, mm/ mm/yy)) at at PS PS points, points, with with the the locations locations ofof the the East East LondonLondon Crematorium and and Cemetery Cemetery (1) (1) and and East East Ham Ham Jewish Jewish Cemetery Cemetery (2) indicated; (2) indicated; (b) (bto–e ()e Time) Time series series displacement, displacement, averagedaveraged over over the the area area of of boxes boxes shown shown in inthe the map map in 13a, in 13a, selected selected as asrepresentative representative of of local local terrace terrace housing: housing: (b (b) )1992 1992–2000–2000 ERS; ERS; (c ()c )2002 2002–2010–2010 ENVISAT; ENVISAT; (d (d) )2010 2010–2015–2015 Radarsat;Radarsat; and and (e ()e) 2015 2015 to to 2018 2018 Sentinel-1.Sentinel-1. The line colours colours in in (b (b) –toe) (e correspond) correspond to to the the outlined outlined areas areas of theof samethe same colour colour in the in mapthe map (a). (a).

In the cemeteries, the PS points are located either directly on the ground surface or on the gravestones, whereas in the areas of terraced housing, PS points can be located anywhere from the roof to the pavement. Soil–structure interaction, stiffness of existing structures and consolidation from the loading of buildings reduce the magnitude of tunnelling induced displacements and produce a shallower and flatter settlement profile (Figure 14) [53,54]. Remote Sens. 2020, 12, 849 15 of 19 Remote Sens. 2020, 12, x FOR PEER REVIEW 16 of 21

FigureFigure 14. 14.Greenfield Greenfield settlementsettlement profileprofile compared to building settlement settlement during during tunnelling tunnelling works, works, wherewhere DR DR is is the the deflection deflection ratio ratio of of hogginghogging andand sagging.sagging. Modified Modified after after Farrell Farrell etet al.al. [[5353].].

3.5. DewateringThis could at explain Beckton why the PS points on the unconsolidated ground of the cemetery subside more than those on the terraced housing. However, Walker [4] reported that, in general, surface The Beckton site comprises three shafts, the overﬂow shaft, from which the TBM was launched, settlement as a result of tunnelling for the Lee Tunnel was less than 2 mm, therefore it is unlikely that the connection shaft, which is sited on the line of the main tunnel and the pumping shaft, which tunnelling is the only contributing factor to this deformation. Prior to 2010, the cemeteries and is connected to the main tunnel via a smaller diameter sprayed concrete lining tunnel (Figure3, surrounding houses (Figure 13b,c) subside collectively, with a total displacement of −11 mm; the Location 5). differing rates of deformation did not begin until approximately January 2011. More recent Sentinel- At Beckton, dewatering took place at the pumping and connection shafts for three to four months, 1 data (post-2015, Figure 13e) shows that the East London Crematorium and Cemetery is still achieving a drawdown of 80 m, before it was switched oﬀ due to drawing in contaminants [55]. Three subsiding, by ca. 13 mm from May 2015 to December 2018, whereas the area of houses is stable, PS points close to the shafts demonstrate the ground response to this dewatering. Pumping began on 9 settling by just 1 mm over the 3.5 years (Figure 13e). September 2011, which coincides with the initial decrease in the ground level illustrated in Figure 15.

Pumping3.5.Remote Dewatering Sens. ceased 2020, 1at2 at, xBeckton theFOR start PEER of REVIEW February 2012, after which the ground stabilises. 17 of 21 The Beckton site comprises three shafts, the overflow shaft, from which the TBM was launched, the connection shaft, which is sited on the line of the main tunnel and the pumping shaft, which is connected to the main tunnel via a smaller diameter sprayed concrete lining tunnel (Figure 3, Location 5). At Beckton, dewatering took place at the pumping and connection shafts for three to four months, achieving a drawdown of 80 m, before it was switched off due to drawing in contaminants [55]. Three PS points close to the shafts demonstrate the ground response to this dewatering. Pumping began on 9th September 2011, which coincides with the initial decrease in the ground level illustrated in Figure 15. Pumping ceased at the start of February 2012, after which the ground stabilises.

FigureFigure 15. 15.( a(a)) High High resolutionresolution imageimage (ArcGIS(ArcGIS Online World World Imagery, Imagery, 2013) 2013) of of the the tunnel tunnel works works at at Beckton,Beckton, showing showing the the locations locations ofof PSPS pointspoints (RS2) at the southernmost, Beckton Beckton Connection Connection Shaft Shaft (1, (1, greengreen and and 2, 2, blue), blue), and and at at the the northernmost northernmost BecktonBeckton Pumping Station Station Shaft Shaft (3, (3, gold); gold); (b (b) )Time Time series series displacementdisplacement at at sites sites 1, 1, 2 2 and and 3.3. TheThe greengreen dasheddashed line indicates the the start start of of dewatering dewatering (09 (09-09-2011)-09-2011) andand the the red red dashed dashed line line the the end end (02-2011). (02-2011).

4. Conclusions The case studies in this paper demonstrate a range of causes of ground movement, as well as the challenges in interpreting InSAR data in an urban area where there is a long history of ongoing development. If the 2010 to 2015 data from Radarsat-2 only were considered, it could be reasonably concluded that subsidence at and around Newham Hospital was a result of the Lee Tunnel TBM encountering the DFH. Once the InSAR results for the previous 18 years are included, it becomes clear there is a much longer-term and more complex pattern of deformation occurring in this area. Had the 1992–2010 ERS and ENVISAT data been analysed prior to tunnelling, the unusual and longer-term deformation would likely have been noted and a more intense, targeted desk study and further borehole investigation being undertaken prior to TBM launch. The findings highlight the importance of using InSAR to establish a reliable baseline prior to tunnelling (or other construction). There are currently 27 years of archive InSAR data available for London, from multiple satellites, with different resolutions, coverage and availability, including ERS, ENVISAT, Radarsat-2, Sentinel 1A and 1B, COSMO Sky-Med and TerraSAR-X. No conventional ground-based in-situ measurements can provide a comparable legacy of historical deformation at the same temporal and spatial resolution. Indeed, there are no other, more cost-effective methods for deriving such accurate and widespread measurements of ground movement. Additionally, we have also shown the InSAR resolution requirements are dependent on the intention of the data. For example, the ERS data contains 27 images over eight years, which is approximately three images a year and is sufficient to define long-term deformation trends. However, during active construction, if InSAR is intended to be used as a monitoring tool, a higher temporal resolution and accuracy of displacement may be required. Aside from TerraSAR-X data, all the other InSAR data is C-band, which has an adequate resolution for these case studies. Therefore, the motivation for the InSAR data being acquired should always be considered when deciding if the higher spatial resolution X-band data is necessary. Despite the differences in resolution and processing between Radarsat-2 and TerraSAR-X, we have shown a strong correlation in their results. Furthermore, multiple sources of data should be considered to fully understand the cause of deformation. Historical maps and optical imagery should be used to complement the measurement data, to better understand human activities as well as the geomorphology of an area. Ground modelling, geological maps and open-source borehole data are essential to assist in the identification of anomalous ground conditions. Even with all this information, causes cannot always be confirmed without local knowledge, as illustrated by the deformation at the A13.

4. Conclusions The case studies in this paper demonstrate a range of causes of ground movement, as well as the challenges in interpreting InSAR data in an urban area where there is a long history of ongoing development. If the 2010 to 2015 data from Radarsat-2 only were considered, it could be reasonably concluded that subsidence at and around Newham Hospital was a result of the Lee Tunnel TBM encountering the DFH. Once the InSAR results for the previous 18 years are included, it becomes clear there is a much longer-term and more complex pattern of deformation occurring in this area. Had the 1992–2010 ERS and ENVISAT data been analysed prior to tunnelling, the unusual and longer-term deformation would likely have been noted and a more intense, targeted desk study and further borehole investigation being undertaken prior to TBM launch. The findings highlight the importance of using InSAR to establish a reliable baseline prior to tunnelling (or other construction). There are currently 27 years of archive InSAR data available for London, from multiple satellites, with different resolutions, coverage and availability, including ERS, ENVISAT, Radarsat-2, Sentinel 1A and 1B, COSMO Sky-Med and TerraSAR-X. No conventional ground-based in-situ measurements can provide a comparable legacy of historical deformation at the same temporal and spatial resolution. Indeed, there are no other, more cost-effective methods for deriving such accurate and widespread measurements of ground movement. Additionally, we have also shown the InSAR resolution requirements are dependent on the intention of the data. For example, the ERS data contains 27 images over eight years, which is approximately three images a year and is sufficient to define long-term deformation trends. However, during active construction, if InSAR is intended to be used as a monitoring tool, a higher temporal resolution and accuracy of displacement may be required. Aside from TerraSAR-X data, all the other InSAR data is C-band, which has an adequate resolution for these case studies. Therefore, the motivation for the InSAR data being acquired should always be considered when deciding if the higher spatial resolution X-band data is necessary. Despite the differences in resolution and processing between Radarsat-2 and TerraSAR-X, we have shown a strong correlation in their results. Furthermore, multiple sources of data should be considered to fully understand the cause of deformation. Historical maps and optical imagery should be used to complement the measurement data, to better understand human activities as well as the geomorphology of an area. Ground modelling, geological maps and open-source borehole data are essential to assist in the identification of anomalous ground conditions. Even with all this information, causes cannot always be confirmed without local knowledge, as illustrated by the deformation at the A13.

Author Contributions: Conceptualization, J.S, R.G., J.L. and P.M.; methodology, J.S.; software, J.S. and R.H.; validation, J.S., M.B.; formal analysis, J.S.; investigation, J.S. and M.B.; resources, J.S., M.B. and R.H.; data curation, J.S. and R.H.; writing—original draft preparation, J.S.; writing—review and editing, J.S., R.G., J.L., P.M. and T.M.; visualization, J.S.; supervision, R.G., J.L. and P.M.; project administration, R.G., J.L. and P.M.; funding acquisition, R.G., J.L. and P.M. All authors have read and agreed to the published version of the manuscript. Funding: This work was funded by the Engineering and Physical Sciences Research Council, grant number EP/L016826/1. The APC was funded by Research Councils UK (RCUK). Acknowledgments: This research has been conducted as part of a PhD, funded by EPSRC (UK) Award EP/L016826/1 and Thames Tideway East (Costain Vinci Bachy JV). The authors would also like to thank CGG and TRE Altamira for providing data for this project and to PETEX for the academic licence for Midland Valley’s Move 3D software. Conﬂicts of Interest: The authors declare no conﬂicts of interest.

References

1. EC. Council Directive 91/271/EEC of 21 May 1991 Concerning Urban Waste-Water Treatment. J. Eur. Commun. 1991, 34, 40. 2. Anamoah, M.; Feeney, M. The Lee Tunnel—TBM Launch. Wastewater Treatment & Sewerage. 2013, pp. 107–111. Available online: http://www.ukwaterprojectsonline.com/case_studies/2013/Thames_Lee_Tunnel_2013.pdf (accessed on 20 August 2019). Remote Sens. 2020, 12, 849 17 of 19

3. Mitchell, R.; Jewell, P. The Lee Tunnel. Wastewater Treatment & Sewerage. 2011, pp. 57–62. Available online: http://www.ukwaterprojectsonline.com/case_studies/2011/Thames_Lee_Tunnel_2011.pdf (accessed on 20 August 2019). 4. Walker, A. The Lee Tunnel. London’s deepest sewer—The main TBM drive and tunnel lining. Wastewater Treatment & Sewerage. 2014, pp. 1–5. Available online: http://www.ukwaterprojectsonline.com/case_studies/ 2014/Thames_Lee_Tunnel_2014.pdf (accessed on 20 August 2019). 5. Costes, E.; Jewell, P.; Michel, C.; Pogu, F. Lee tunnel project—The first step toward a cleaner River Thames. Civ. Eng. 2018, 171, 69–76. [CrossRef] 6. Tideway. Annual Report 2018/19. Available online: https://www.tideway.london/corporate-info/financial- publications/ (accessed on 20 November 2019). 7. Bischoff, C.A.; Ghail, R.C.; Mason, P.J.;Ferretti, A.; Davis, J.A. Photographic feature: Revealing millimetre-scale ground movements in London using SqueeSAR™. Q. J. Eng. Geol. Hydrogeol. 2020, 53, 3–11. [CrossRef] 8. Giardina, G.; Milillo, P.; DeJong, M.J.; Perissin, D.; Milillo, G. Evaluation of InSAR monitoring data for post-tunnelling settlement damage assessment. Struct. Control Health Monit. 2018, 26, e2285. [CrossRef] 9. Marti, J.G.; Nevard, S.; Sanchez, J. The use of InSAR (Interferometric Synthetic Aperture Radar) to complement control of construction and protect third party assets. In Crossrail Learning Legacy Report; Crossrail Ltd.: London, UK, 2017. 10. Milillo, P.; Giardina, G.; DeJong, M.J.; Perissin, D.; Milillo, G. Multi-Temporal InSAR Structural Damage Assessment: The London Crossrail Case Study. Remote Sens. 2018, 10, 287. [CrossRef] 11. Robles, J.G.; Black, M.; Gomar, B. Correlation Study between In-Situ Auscultation and Satellite Interferometry for the Assessment of Nonlinear Ground Motion on Crossrail London. In Crossrail Learning Legacy Report; Crossrail Ltd.: London, UK, 2016. 12. Chen, W.-F.; Gong, H.-L.; Chen, B.-B.; Liu, K.-S.; Gao, M.; Zhou, C.-F. Spatiotemporal evolution of land subsidence around a subway using InSAR time-series and the entropy method. Giscience Remote Sens. 2017, 54, 78–94. [CrossRef] 13. Ge, D.; Zhang, L.; Li, M.; Liu, B.; Wang, Y. Beijing subway tunnellings and high-speed railway subsidence monitoring with PSInSAR and TerraSAR-X data. In Proceedings of the Geoscience and Remote Sensing Symposium (IGARSS), Beijing, China, 10–15 July 2016; pp. 6883–6886. 14. Wang, H.; Feng, G.; Xu, B.; Yu, Y.; Li, Z.; Du, Y.; Zhu, J. Deriving spatio-temporal development of ground subsidence due to subway construction and operation in delta regions with PS-InSAR data: A case study in Guangzhou, China. Remote Sens. 2017, 9, 1004. [CrossRef] 15. Koudogbo, F.; Urdiroz, A.; Robles, J.G.; Chapron, G.; Lebon, G.; Fluteaux, V.; Priol, G. Radar interferometry as an innovative solution for monitoring the construction of the Grand Paris Express metro network–First results. In Proceedings of the World Tunnel Congress, Dubai, UAE, 21–25 April 2018. 16. Liu, D.; Sowter, A.; Niemeier, W. Process-related deformation monitoring by PSI using high resolution space-based SAR data: A case study in Düsseldorf, Germany. Nat. Hazards Earth Syst. Sci. Discuss. 2014, 2, 4813–4830. [CrossRef] 17. Arangio, S.; Calò, F.; Di Mauro, M.; Bonano, M.; Marsella, M.; Manunta, M. An application of the SBAS-DInSAR technique for the assessment of structural damage in the city of Rome. Struct. Infrastruct. Eng. 2014, 10, 1469–1483. [CrossRef] 18. Korff, M.; Maccabiani, J. Building damage related to Amsterdam subway; comparison of traditional monitoring with satellite monitoring results. In Proceedings of the 4th International Conference on Computational Methods in Tunnelling and Subsurface Engineering, Innsbruck, Austria, 18–20 April 2017. 19. Poncos, V.; Teleaga, D.; Boukhemacha, M.A.; Toma, S.A.; Serban, F. Study of urban instability phenomena in Bucharest city based on Ps-InSAR. In Proceedings of the Geoscience and Remote Sensing Symposium (IGARSS), Quebec City, QC, Canada, 13–18 July 2014; pp. 429–432. 20. García, A.J.; Marchamalo, M.; Martínez, R.; González-Rodrigo, B.; González, C. Integrating geotechnical and SAR data for the monitoring of underground works in the Madrid urban area: Application of the Persistent Scatterer Interferometry technique. Int. J. Appl. Earth Obs. Geoinf. 2019, 74, 27–36. [CrossRef] 21. Serrano-Juan, A.; Pujades, E.; Vázquez-Suñè, E.; Crosetto, M.; Cuevas-González, M. Leveling vs. InSAR in urban underground construction monitoring: Pros and cons. Case of la sagrera railway station (Barcelona, Spain). Eng. Geol. 2017, 218, 1–11. [CrossRef] Remote Sens. 2020, 12, 849 18 of 19

22. Eppler, J.; Kubanski, M. Subsidence monitoring of the Seattle viaduct tunnelling project with Homogeneous Distributed Scatterer InSAR. In Proceedings of the Ninth Symposium on Field Measurements in Geomechanics, 9–11 September 2015; pp. 303–313. 23. Skipper, J.; Edgar, J. The Harwich formation in London—The legacy of Chris King. Proc. Geol. Assoc. 2019. [CrossRef] 24. Bellhouse, M.; Skipper, J.; Sutherden, R. The engineering geology of the Lee Tunnel. In Proceedings of the Geotechnical Engineering for Infrastructure and Development: XVI European Conference on Soil Mechanics and Geotechnical Engineering, Edinburgh, UK, 13–17 September 2015; pp. 419–424. 25. Newman, T.; Bellhouse, M.; Corcoran, J.; Sutherden, R.; Karaouzene, R.; Shirlaw, J.N. Discussion: TBM performance through the engineering geology of the Lee Tunnel. Proc. Inst. Civ. Eng. Geotech. Eng. 2017, 170, 559–560. [CrossRef] 26. Newman, T.; Hadlow, N.W. Evaluation of chalk rock mass properties in subcrop and outcrop in London; examples from the Tideway and Lee tunnels. In Proceedings of the Engineering in Chalk 2018, Imperial College London, London, UK, 17–18 September 2018; pp. 721–727. 27. Newman, T. Ground modelling for the Lee Tunnel. In Engineering Geology an Experience in the East London and Thames Gateway Area; Geological Society: London, UK, 2008. 28. Skipper, J.; Newman, T.; Mortimore, R. The Engineering Geology of the Lee Tunnel; Euroengeo International Association of Engineering Geology: London, UK, 2008. 29. Ellison, R.; Woods, M.; Allen, D.; Forster, A.; Pharaoh, T.; King, C. Geology of London: Special Memoir for 1: 50000 Geological Sheets 256 (North London), 257 (Romford), 270 (South London), and 271 (Dartford)(England and Wales); British Geological Survey: London, UK, 2004. 30. Newman, T. Engineering Geology of the Thames Tideway Tunnel The Story So Far. In Proceedings of the 14th Annual BGA Conference, Institution of Civil Engineers, London, UK, 21 June 2017. 31. Morgan, T.; Ghail, R.C.; Lawrence, J. Major faulting in London: Evidence for inherited basement faults in the London Basin. Q. J. Eng. Geol. Hydrogeol. 2020, in press. [CrossRef] 32. Berry, F. Late Quaternary scour-hollows and related features in central London. Q. J. Eng. Geol. Hydrogeol. 1979, 12, 9–29. [CrossRef] 33. Flynn, A.; Collins, P.; Reading, P.; Banks, V.; Anguilano, L. The role of chalk in the development of buried (“drift-filled”) hollows. In Proceedings of the Engineering in Chalk, Imperial College London, London, UK, 17–18 September 2018; pp. 309–314. 34. Hutchinson, J. Possible late Quaternary pingo remnants in central London. Nature 1980, 284, 253. [CrossRef] 35. Banks, V.J.; Bricker, S.H.; Royse, K.R.; Collins, P.E.F. Anomalous buried hollows in London: Development of a hazard susceptibility map. Q. J. Eng. Geol. Hydrogeol. 2015, 48, 55–70. [CrossRef] 36. Ghail, R.; Mason, P.; Skipper, J. The geological context and evidence for incipient inversion of the London Basin. In Proceedings of the Geotechnical Engineering for Infrastructure and Development: XVI European Conference on Soil Mechanics and Geotechnical Engineering, Edinburgh, UK, 13–17 September 2015. 37. Collins, P.E.; Banks, V.J.; Royse, K.R.; Bricker, S.H. Superficial hollows and rockhead anomalies in the London Basin, UK: Origins, distribution and risk implications for subsurface infrastructure and water resources. In Engineering Geology for Society and Territory-Volume 6; Springer: Berlin/Heidelberg, Germany, 2015; pp. 663–666. 38. Toms, E.; Mason, P.J.; Ghail, R.C. Drift-filled hollows in Battersea: Investigation of the structure and geology along the route of the Northern Line Extension, London. Q. J. Eng. Geol. Hydrogeol. 2016, 49, 147–153. [CrossRef] 39. Environment Agency. Management of the London Basin Chalk Aquifer Status Report 2018. 2018. Available online: https://www.gov.uk/government/publications/london-basin-chalk-aquifer-annual-status-report (accessed on 20 August 2019). 40. Jewell, P.; O’Connor, M. The Lee Tunnel Shafts. Wastewater Treatment & Sewerage. 2012, pp. 33–37. Available online: http://www.ukwaterprojectsonline.com/case_studies/2012/Thames_Lee_Tunnel_2012.pdf (accessed on 20 August 2019). 41. Tunnel Talk. TBMs Ready for London Sewerage Drives. Available online: https://tunneltalk.com/Lee-Tunnel- Feb12-Beckton-sewage-works-extension-TBM-drive.php (accessed on 11 July 2019). 42. Attewell, P. An overview of site investigation and long-term tunnelling-induced settlement in soil. Geol. Soc. Lond. Eng. Geol. Spec. Publ. 1988, 5, 55–61. [CrossRef] Remote Sens. 2020, 12, 849 19 of 19

43. Moss, N.; Bowers, K. Settlement due to tunnelling on the CTRL London Tunnels. In Proceedings of the Geotechnical Aspects of Underground Construction in Soft Ground—Proceedings of the 5th International Conference of TC28 of the ISSMGE, Amsterdam, The Netherlands, 15–17 June 2005. 44. Ferretti, A.; Prati, C.; Rocca, F. Permanent scatterers in SAR interferometry. IEEE Trans. Geosci. Remote Sens. 2001, 39, 8–20. [CrossRef] 45. Crosetto, M.; Monserrat, O.; Jungner, A.; Crippa, B. Persistent scatterer interferometry: Potential and limits. In Proceedings of the 2009 ISPRS Workshop on High-Resolution Earth Imaging for Geospatial Information, Hannover, Germany, 2–5 June 2009. 46. Bischoff, C. Monitoring Ground Movements and Infrastructure in London, UK, Using Permanent Scatterer Interferometry; Imperial College London: London, UK, 2019. 47. Ferretti, A.; Fumagalli, A.; Novali, F.; Prati, C.; Rocca, F.; Rucci, A. A new algorithm for processing interferometric data-stacks: SqueeSAR. IEEE Trans. Geosci. Remote Sens. 2011, 49, 3460–3470. [CrossRef] 48. EDINA Historic Digimap Service: 1870–1996, Ordnance Survey, UK. Available online: http://edina.ac.uk/ digimap (accessed on 20 August 2017). 49. Department of Environment Food and Rural Affairs. Historic Landfill Sites. Available online: https: //environment.data.gov.uk/DefraDataDownload/?mapService=EA/HistoricLandfill&Mode=spatial (accessed on 8 July 2019). 50. Semertzidou, K. Crossrail Project Dewatering Work—Close-Out Report; Crossrail Learning Legacy: 2016. Available online: https://learninglegacy.crossrail.co.uk/documents/crossrail-dewatering-works-close-report/ (accessed on 20 August 2019). 51. Mason, P.J.; Ghail, R.; Bischoff, C.; Skipper, J. Detecting and Monitoring Small-Scale Discrete Ground Movements Across London, Using Persistent Scatterer InSAR (PSI). In Proceedings of the Geotechnical Engineering for Infrastructure and Development: XVI European Conference on Soil Mechanics and Geotechnical Engineering, Edinburgh, UK, 13–17 September 2015. 52. Winch, C. East London LPN Regional Development Plan. UK Power Networks. Available online: https://library.ukpowernetworks.co.uk/library/en/RIIO/Asset_Management_Documents/Regional_ Development_Plans/ (accessed on 20 August 2019). 53. Farrell, R.; Mair, R.; Sciotti, A.; Pigorini, A. Building response to tunnelling. Soils Found. 2014, 54, 269–279. [CrossRef] 54. Leca, E.; New, B. Settlements induced by tunneling in soft ground. Tunn. Undergr. Space Technol. 2007, 22, 119–149. 55. Warren, C.; Newman, T.; Hadlow, N.W. Comparison of earth pressure balance and slurry tunnel boring machines used for tunnelling in Chalk. In Proceedings of the Engineering in Chalk 2018, Imperial College London, London, UK, 17–18 September 2018; pp. 617–628.

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).