110 RG PNC 00000 000557 EOR AbbeyMillsRoute

7/27/2019 110 RG PNC 00000 000557 EOR AbbeyMillsRoute

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Summer 2011

Engineering optionsreportAbbey Mills route


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Please note:

Further details are provided in the Final Report on SiteSelection Process (doc ref: 7.05) that can be found onthe Thames Tideway Tunnel section of the PlanningInspectorates web site.


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110-RG-PNC-00000-000557 | Summer 2011

Engineering options

reportAbbey Mills route


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Engineering options reportAbbey Mills route


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Thames Tunnel

Engineering options report

Abbey Mills routeList of contents

Page number

1 Executive summary ......................................................................................... 12 Introduction ...................................................................................................... 3

2.1 Purpose of report ..................................................................................... 32.2 Engineering design development ............................................................ 4

3 System design and engineering requirements .............................................. 63.1 System design and engineering assumptions ......................................... 63.2 Health and safety considerations ............................................................. 63.3 System requirements ............................................................................... 63.4 Engineering geology .............................................................................. 133.5 Tunnel engineering and construction requirements ............................... 183.6 CSO engineering and construction requirements .................................. 28

4 Main tunnel dr ive options .............................................................................. 334.1 Introduction ............................................................................................ 334.2 Main tunnel engineering options preparation ...................................... 334.3 Main tunnel engineering options assessment ..................................... 47

5 Connection tunnel drive options .................................................................. 565.1 CSO connection options ........................................................................ 565.2 Connection tunnel drive options ......................................................... 62

6 Conclusions and recommendations ............................................................ 66

The following appendices can be found in the accompanying document Engineeringoptions report Abbey Mills route Appendices:

Appendix A Assumptions register

Appendix B Drawings

Appendix C Time chainage

Appendix D Geology


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List of figures

Page number

Figure 3.1 Routes considered ................................................................................... 10Figure 3.2 Typical CSO interception arrangements .................................................. 11Figure 4.1 Main tunnel site types .............................................................................. 34Figure 4.2 Main tunnel site zones for all three routes ............................................... 35Figure 4.3 Main tunnel site zones for the Abbey Mills route ..................................... 35Figure 5.1 Type A CSO connection .......................................................................... 57Figure 5.2 Type B CSO connection .......................................................................... 58Figure 5.3 Type C CSO connection .......................................................................... 59Figure 5.4 Type D CSO connection .......................................................................... 60Figure 5.5 Type E CSO connection .......................................................................... 61

List of tables

Page number

Table 3.1 Control of CSOs .......................................................................................... 7Table 3.2 Geology of London Basin ......................................................................... 14Table 3.3 Chalk aquifer groundwater levels 2008 and imposed pressure at

tunnel invert (east of Shad) ....................................................................... 18Table 4.1 Grouping of shortlisted main tunnel sites for the Abbey Mills route

post-phase one consultation ..................................................................... 36Table 4.2 Drive options consideration of practical drive lengths ............................ 40Table 4.3 Initial provisional main tunnel drive options .............................................. 41Table 4.4 Interim main tunnel drive options .............................................................. 45Table 4.5 Interim list of main tunnel drive options ..................................................... 46Table 4.6 Programme assumptions for comparison of options ................................. 53Table 4.7 Summary of construction durations for main tunnel drive options ............ 54Table 4.8 Final list of main tunnel drive options ........................................................ 55Table 5.1 Frogmore Connection Tunnel drive options ........................................... 63Table 5.2 Greenwich Connection Tunnel initial drive options ................................ 64Table 5.3 Greenwich Connection Tunnel final drive options .................................. 65Table 5.4 North East Storm Relief Type A CSO connection tunnel drive options

matrix ........................................................................................................ 65


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List of abbreviations

AOD above Ordnance Datum

ATD above tunnel datumCSO combined sewer overflow

Defra Department of Environment Food and Rural Affairs

EA Environment Agency

EU European Union

EPB earth pressure balance

GWT groundwater table

m/s metres per second

m3

NESR North East Storm Relief

/s cubic metres per second

OD Ordnance Datum (mean sea level at Newlyn in Cornwall)

Ofwat Water Services Regulatory Authority

PLA Port of London Authority

PS pumping station

SMP System master plan

SR storm reliefSTW sewage treatment works

TBM tunnel boring machine


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1 Executive summary


Page 1

1 Execut ive summary

1.1.1 This report has been prepared for Thames Water as part of the process tosupport the creation of the preferred list of main tunnel sites andpreferred scheme for phase two consultation. It is specific to the ThamesTunnel project, but takes cognisance of the Lee Tunnel project. The needfor an engineering options report, and the process that it is part of, isoutlined in the Site selection methodology paper.

1.1.2 It is intended that this report is read as a technical document and, as such,the content has been kept brief with the understanding that the reader hastechnical familiarity with the subject matter.

1.1.3 The report begins by defining the overall engineering requirements thatare to be considered as part of the development of engineering options.These are largely summarised without providing any in-depth justification;the main aim of the report being the identification of tunnel drive options.

1.1.4 Three main tunnel routes between west London and Beckton SewageTreatment Works (STW) were identified as part of the design developmentand the Abbey Mills route was chosen as the preferred route for phaseone consultation. The Report on phase one consultation reports on thefeedback from this consultation phase and concludes that afterconsultation, the Abbey Mills route remains the preferred route.Therefore, only the Abbey Mills route is taken forward for furtherevaluation in this report.

1.1.5 The second part of the report presents a methodology for determining

possible options to construct the main tunnel on the Abbey Mills route.This is based on engineering requirements and the list of shortlisted maintunnel sites provided by the site selection process, which identifies sitespotentially suitable for use as either main tunnel drive, intermediate orreception sites to facilitate the construction of the main tunnel and itssubsequent operation. Drive options for the connection tunnels that bringtogether two or more CSOs in association with the shortlisted CSO sitesare also considered in this report.

1.1.6 To build the scheme it is necessary to drive a tunnel, or series of tunnels,connecting a number of main tunnel sites. Possible permutations of tunneldrive scenarios (drive options) for the presented sites are established in a

systematic manner to permit evaluation.

1.1.7 The relative desirability of the feasible drive options are then examined interms of engineering factors. These and the other discipline factors, suchas planning, environment, community and property, will ultimately be usedin conjunction with the site suitability reports to determine preferred sitesand the preferred scheme, by being addressed in subsequent workshopsand presented in the Phase two scheme development report.

1.1.8 This report shows that appropriate engineering options are available todrive the main tunnel. These are presented as a schedule of feasiblemain tunnel drive options to be taken forward to the next stage in the siteselection methodology process.


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1 Executive summary


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1.1.9 Finally, engineering factors that will be used to provide content forconsultations and for determining the preferred sites and associated driveoptions for the main tunnel are also presented. These are the factors thatwill be used in the Phase two scheme development report to examine theadvantages and disadvantages, including engineering risk, programme

and cost.


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2 Introduction


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2 Introduction

2.1 Purpose of report

2.1.1 The Engineering options report Abbey Mills route has been prepared as

part of the process for the creation of the preferred list of main tunnelsites and preferred scheme as set out in the Thames Tunnel projectsSite selection methodology paper. The Site selection methodology paperstates that the Engineering options report will consider:

a. how sites work in combination, and options for the main tunnelalignment and combined sewer overflow (CSO) connections

b. how options for tunnel alignment and CSO connection points would berefined, having regard to the availability of and spacing of suitablemain tunnel sites, as well as to the potential for combined use of sites.Cost considerations associated with engineering options, transport

and energy will be reported, balanced and taken into account.

2.1.2 This report identifies and refines possible main tunnel drive options for theAbbey Mills route, giving consideration to the overall location and groupingof the main tunnel sites that have been shortlisted for site suitabilityassessment. The establishment of preferred sites, and hence preferredscheme will be being addressed in subsequent workshops and presentedin the Phase two scheme development report.

2.1.3 Stage 1 of the site selection process, from identification of the long list tothe preferred list of sites for phase one consultation, was carried out in2009 and 2010. As part of that process, the Engineering options report

(100-RG-ENG-00000-900006 Spring 2010) was prepared whichconsidered the drive options associated with the shortlisted sites for threedifferent tunnel routes: The River Thames route, the Rotherhithe routeand the Abbey Mills route.

2.1.4 The shortlisted sites and three tunnel routes were consulted on at phaseone consultation (September 2010 to January 2011) by presenting thepreferred sites and preferred route along with the other sites and otherroutes that had been discounted. The phase one consultation feedbackhas been collated into the Report on phase one consultation. Analysis ofthe consultation feedback received concluded that the Abbey Mills route

remains the preferred route. As a consequence of consultation feedbackand a number of emerging factors, a series of site selection back-checkswere carried out, which led to a number of site changes and thereforeopened up new drive options.

2.1.5 This Engineering options report Abbey Mills route therefore considersthe latest short list of sites, which includes amendments resulting from therecent site selection back-checks, for the Abbey Mills route only.

2.1.6 The findings of this Engineering options report Abbey Mills route will helpinform the next stage of preferred scheme selection process that will bepresented in the Phase two scheme development report.


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2 Introduction


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2.1.7 The Engineering options report Abbey Mills route is divided into twoparts:

Part 1: System design and engineering requirements

2.1.8 This part sets out at high level the system, geological, tunnelling and CSOengineering requirements to be considered as part of the development ofengineering options, and subsequent selection of a preferred drive optionand its associated preferred list of main tunnel sites for the Abbey Millsroute. As such, this will largely state and summarise requirements withoutproviding an in-depth justification for the system and engineeringrequirements.

Part 2: Main tunnel and connection tunnel drive options

2.1.9 This part summarises the tunnel options considered and the analysis andrefinement of these options. Included in the analysis is consideration of

the relationship of the tunnel options to the available groups of shortlistedsites.

2.1.10 The report only considers the development of options from an engineeringperspective. The suitability of each site have not been referred to in thepreparation of this Engineering options report Abbey Mills route, but willbe presented in thesite suitability reports.

2.1.11 In considering tunnel drive options, this report does not identify preferredtunnel routes or preferred sites. The selection of the preferred tunnelalignments, preferred CSO sites and preferred main tunnel sites are to beassessed at later stages in the process (selection of the preferred sites

and preferred scheme). These stages will be carried out by a broadermultidisciplinary team and reported in the Phase two scheme developmentreport. The considerations in this Engineering options report Abbey Millsroute, along with site suitability reports, will feed into and inform thesestages.

2.2 Engineering design development

2.2.1 The Thames Tunnel project would comprise a main tunnel, running fromwest to east London, integrated with the existing sewerage system viaconnection tunnels, to control 34 of the most polluting CSOs. These

tunnels would store and transfer the intercepted flows to Beckton SewageTreatment Works (STW).

2.2.2 The Lee Tunnel, a tunnel connecting Abbey Mills Pumping Station toBeckton STW to control the Abbey Mills Pumping Station CSO, has beenconsented and construction started in 2010.

2.2.3 The Thames Tunnel projects site selection process recognises that theengineering design will need to proceed in parallel with the site selectionprocess, and that there is an iterative relationship between the two.


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2 Introduction


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2.2.4 Design development activities have included:

a. engineering designs and studies of various components of thescheme, and identification of possible feasible tunnel routes

b. system master planning to define the sewage system operation

changes and facilities needed to control and limit overflows from thescheme

c. construction, transportation and river navigational logistics studies

d. field investigations, including ground investigations and surveys.

2.2.5 This Engineering options report Abbey Mills route draws on the relevantaspects of these studies and investigations, as well as the results from thesite selection shortlisting process.


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3 System design and engineering requirements


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3.1 System design and engineering assumpt ions

3.1.1 The assumptions made for the preparation of this report are identified and

listed in an assumptions register in Appendix A (which can be found in theaccompanying document, Engineering options report Abbey Mills route Appendices). These assumptions and further requirements are discussedin the following sections.

3.2 Health and safety considerations

3.2.1 Through risk assessment and management, the Thames Tunnel project isworking in accordance with industry codes and project standards, with theaim to achieve world-class health and safety objectives. The project has aplan and policies in place to ensure compliance with the Construction

(Design and Management) Regulations 2007.

3.3 System requirements

3.3.1 The need and hence the overarching requirements for the Thames Tunnelproject is described in the Needs Report,100-RG-PNC-00000-900007.The Needs Report provides the legal and regulatory context and the needfor a solution to meet the regulatory drivers.

3.3.2 The concept of the Thames Tunnel project is to:

a. control discharges from 34 CSOs

b. store the CSO dischargesc. transfer CSO discharges for treatment.

3.3.3 The engineering requirements to be taken forward in assessingengineering tunnel route and alignment options are summarised andbriefly discussed in the following sections. Design development is ongoingso it is noted that the implications of any future changes would need to befurther assessed and reviewed.

3.3.4 This section of the report focuses on system requirements relevant to theselection of sites and tunnel engineering alignments.

Developments in design requirements

3.3.5 Developments in the design have updated the scheme requirements suchthat 18 CSOs are now required to be directly intercepted while theremaining CSOs are to be controlled by other measures.

3.3.6 Table 3.1 lists the control measures proposed for all 34 CSOs.


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Table 3.1 Control of CSOs

CSO ref Combined sewer overflow Method of overflow control

CS01X Acton Storm Relief Interception

CS02X Stamford Brook Storm Relief Control measures at other CSOsindirectly controls this CSO

CS03X North West Storm Relief

Interception and pumping stationoperation changes at HammersmithPumping Station indirectly controlsthis CSO

CS04XHammersmith PumpingStation

Interception and pumping stationoperation changes

CS05X West Putney Storm Relief Interception

CS06X Putney Bridge InterceptionCS07A

CS07B

Frogmore Storm Relief BellLane CreekFrogmore Storm Relief Buckhold Rd

Interception

CS08A

CS08B

Jews Row Wandle ValleyStorm ReliefJews Row Falconbrook StormRelief

Modifications already in place soCSO is indirectly controlled**

CS09X Falconbrook Pumping Station Interception

CS10X Lots Road Pumping Station Interception

CS11X Church StreetControlled indirectly by works atRanelagh CSO

CS12X Queen StreetControlled indirectly by works atRanelagh CSO

CS13ACS13B

Smith Street Main LineSmith Street Storm Relief

Controlled indirectly by works atRanelagh CSO

CS14X RanelaghInterception and additional sewer

connection relief*

CS15X Western Pumping StationControlled indirectly by works atRanelagh and Regent Street CSOs

CS16X Heathwall Pumping Station Interception

CS17X South West Storm Relief Interception

CS18X Kings Scholars PondControlled indirectly by works atRanelagh and Regent Street CSOs

CS19X Clapham Storm Relief Interception

CS20X Brixton Storm Relief Interception


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CSO ref Combined sewer overflow Method of overflow control

CS21X Grosvenor DitchControlled indirectly by works atRanelagh, Regent Street and FleetMain CSOs

CS22X Regent Street Interception and additional sewerconnection relief*

CS23X Northumberland StreetControlled indirectly by works atRegent Street and Fleet MainCSOs

CS24X Savoy StreetControlled indirectly by works atRegent Street and Fleet MainCSOs

CS25X Norfolk Street

Controlled indirectly by works at

Regent Street and Fleet MainCSOs

CS26X Essex StreetControlled indirectly by works atRegent Street and Fleet MainCSOs

CS27X Fleet MainInterception and additional sewerconnection relief*

CS28XShad Thames PumpingStation

Pumping station modifications**

CS29X North East Storm Relief InterceptionCS30X Holloway Storm Relief Local modifications**

CS31X Earl Pumping Station Interception

CS32X Deptford Storm Relief Interception

CS33X Greenwich Pumping StationInterception and pumping stationoperation changes

CS34X Charlton Storm ReliefPumping station operation changesat Greenwich Pumping Station andimprovements at Crossness STW

* Interceptions at Ranelagh, Regent Street and Fleet Main CSOs includeconnections into the northern Low Level Sewer No.1.** Planned to be controlled via interception at phase one consultation stage


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3.3.7 Further elements that the scheme should provide as a minimum are listedbelow:

a. The westerly start point of the scheme should connect to the ActonStorm Relief CSO.

b. The easterly end point of the tunnel is to connect to the Lee Tunnel atAbbey Mills pumping station (this is only associated with the AbbeyMills route).

c. Relieving flows in the northern Low Level Sewer No.1 at theRanelagh, Regent Street and Fleet Main CSO sites, gives sufficientcontrol to reduce local CSO spills so that direct interception is nolonger required on the Northumberland Street, Church Street, SmithStreet, Kings Scholars Pond, Grosvenor Ditch, Savoy Street, NorfolkStreet and Essex Street sewers.

d. A system that ensures the health and safety of operatives, public and

other third parties. This includes providing, during both theconstruction and operational phases, a hydraulically safe and robustsystem without the risk of flooding or adverse transient conditions;secure and resilient facilities, appropriate levels of ventilation and airtreatment, and safe methods and facilities for access and egress intoand from the main and connection tunnels.

Main tunnel routes

3.3.8 Design development identified three tunnel routes: The River Thamesroute, Rotherhithe route and Abbey Mills route.

3.3.9 The River Thames route largely follows the route of the Thames, while thetwo other routes provide respectively an alignment that cuts across theRotherhithe Peninsula and a route that connects to the Lee Tunnel atAbbey Mills. The latter became feasible due to an increase in depth of theLee Tunnel shaft at the Abbey Mills PS end to avoid difficult geologicalconditions on the Lee Tunnel route. This enables a continuous gradientwith the Thames Tunnel projects main tunnel, satisfying the designconstraints for the overall vertical tunnel alignment and system hydraulicrequirements.

3.3.10 The three routes were consulted on at phase one consultation, with the

Abbey Mills route presented as the preferred route. Analysis of theconsultation feedback received concluded that the Abbey Mills routeremains the preferred route. Only the Abbey Mills route is taken forwardfor further evaluation in this report.

3.3.11 The three routes are displayed in Figure 3.1.


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Figure 3.1 Routes cons idered


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Control and interception of CSO flows

3.3.12 The CSOs to be controlled by interception are outlined in Table 3.1. Table3.1

3.3.13 The interception of CSO flows and connection to the main tunnel typicallycomprises four major elements: A CSO interception chamber, connectionculvert, drop shaft and connection tunnel, as shown in Figure 3.2 below. Adescription of the construction elements is provided in the Site selection,background technical paperand discussed further in Section 3.6.

Figure 3.2 Typical CSO interception arrangements

Tunnel hydraulic requirements

3.3.14 The tunnel system is to store and convey flow, with the purpose ofreducing CSO discharge. The background for sizing the tunnels isprovided in Section 3.5.3.

3.3.15 The tunnel system has to be self-cleansing. A gradient in excess ofapproximately one in 850 has been found to generate self-cleansingconditions, with velocities exceeding 1m/s during event cycles. Based oninternational experience, the self-cleansing velocity above this is sufficientto move detritus without further flushing requirements.

3.3.16 The gradient of the connection tunnels is generally in the range of one in

400 to one in 500 in order to achieve flow capacities while not exceedingmaximum peak velocities.


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3.3.17 Large tunnel systems are potentially prone to hydraulic pressure effects,due to the generation of transient (temporary surge flow) conditions.Control features therefore need to be incorporated into the tunnel designand mode of operation to manage these transient pressure effects.

System functional and operational requirements3.3.18 In order to ensure safe operation, access, inspection and maintenance of

the tunnel, design development has been based on the following criteriaand features:

a. The main tunnel and connection tunnels are to be designed to begenerally maintenance free, and have a design life of 120 years.Tunnel entry for inspection and maintenance is only planned to takeplace approximately every ten years.

b. The ten-year inspection would be a major undertaking in its own right,which would involve extensive planning and temporary works provisionto permit entry.

c. Main tunnel shafts, and CSO drop shafts that are on line with themain tunnel, would be the designated access points to the tunnelsystem. The spacing of the main tunnel shafts is controlled by therequirements for maintenance access on the basis that theconstruction access demands would be met by the extensivetemporary work facilities associated with the tunnelling. These shaftswould allow the installation and removal of specialist inspection andmaintenance vehicles during the ten-yearly inspection of the tunnel.At this stage of design development, it is assumed that the spacing

between permanent access points should not exceed 9km. Long,large diameter connection tunnels shall have similar accessprovisions.

d. The main tunnel shafts and on-line CSO drop shafts shall be providedwith large access openings to permit inspection plant to be loweredinto/removed from the tunnel and emergency access/egress to beeffected. CSO and main tunnel sites are to be selected to ensurespace for two mobile cranes to service the shafts.

e. The provision of permanent air management facilities, includingventilation and monitoring of the exhaust air quality, along with air

treatment facilities (odour control).

f. The provision of control gates to isolate the tunnel system and preventflow from entering. These gates would be controlled from a centralcontrol room to permit overview of the system from a single point.They would also be used to isolate the tunnel from inflows during themaintenance inspections, currently envisaged to be every ten years.

g. Integrating the operating regime for the tunnel with the operatingregimes at pumping stations, particularly Abbey Mills and Greenwich,along with Beckton STW and Crossness STW.

h. Fixed ladders and access ways would not be provided to the bottom ofshafts or the main tunnel due to the likelihood of damage during surge


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events and corrosion, as has occurred on other projects. Specificarrangements would be developed for safe access to undertakeinspection and maintenance of the CSO drop shafts, connectiontunnel and main tunnel structures. Fixed ladder access would beprovided to subsurface MEICA equipment, odour control or other

equipment requiring routine inspection and maintenance.3.3.19 When considering the main tunnel shaft spacing for the completed system,

and based on the experience from other major CSO systems, it isassumed maintenance and inspection teams would travel through themain tunnel by inspection vehicle supported by a backup standby vehicle.This reduces the transit time and permits a wider range of equipment to becarried with relative ease, and would facilitate access to the internalcircumference of the tunnel for inspection. Vehicular access is practicablefor this system, given the main tunnel diameter and that the system wouldbe dry when inspection is undertaken, with all penstocks controlling flow

into the system locked off securely.3.3.20 Access to the connection tunnels would also be required during

inspection. The length of the connection tunnels is highly variabledepending on location, and varies from 16m up to 4,600m. Provision foremergency egress would be made at the drop shafts, by the provision ofsuitable access openings and space for cranes to operate a man-rider.The connection tunnel to Greenwich PS would be inspected using asimilar inspection vehicle as used for the main tunnel.

3.4 Engineering geology

Route geology

3.4.1 The route geology has been established using the British GeologicalSurvey (BGS) Lithoframe50 model, from which geological long sectionshave been prepared. This has been supplemented by project specific siteinvestigations, including a seismic refraction survey, ground and overwater boreholes and field and laboratory testing, as well as the installationof piezometers to establish water levels.

3.4.2 The geological long section, derived from the model, is provided for theAbbey Mills main tunnel route in Appendix D of the Engineering optionsreport Abbey Mills route Appendices.

3.4.3 The basic geological descriptions within in the London Basin geologicalsequence are given in Table 3.2.


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Table 3.2 Geology of London Basin

Era Group FormationBrief description of

formation

Approximaterange of

thickness

(m)Recent

AlluviumSoft clays, silts, sands andgravels. May contain peat.

0 5

FloodplainTerrace

KemptonParkTerrace

Medium to dense sand andflint and chert graveloccasional cobbles andboulders.

0 10

Tertiary Thames LondonClay

Very stiff, fissured silty locallyfine to medium sandy clay.

>100

Harwich

Swanscombe member:

Sandy clay to clayey sand(< 2m) with some fine tomedium black roundedgravel.

Blackheath member:

Dense to very dense flintgravel (with occasionalcobbles) in silty or clayey,

glauconitic, fine to mediumsand matrix.

Oldhaven member:

Very dense clayey sand withgravel and shells oftencemented as limestone.

0 10

LambethGroup

Woolwich

Stiff, dark grey to black claywith locally abundant shelldebris and strong limestonebeds (100 to 200mm thick).

10 20

Reading

Very stiff to hard,multicoloured (light blue greymottled red, orange, brownand purple), locally sandyclay.

UpnorGravel, glauconitic andorganic sand, silt and clay.

5 7


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Era Group FormationBrief description of

formation

Approximaterange of

thickness(m)

Thanet Sand Formation(incl. Bullhead Bed atbase 100mm thick).

circa 40

Lewes*Heterogeneous nodularchalk with nodular flint

horizons and marl seams.

circa 50

Notes: *Limited to those formations of the White Chalk subgroup expected withinthe Thames Tunnel project. (Upper and Middle Chalk are now knowncollectively as White Chalk.)

3.4.4 The distribution of strata along the route is largely controlled by theLondon Basin Syncline, which plunges gently eastwards. Thus, beneath acover of made ground and recent deposits, the succession of tertiarydeposits is gradually exposed west to east along the river until the Chalkoccurs at outcrop around Greenwich.

3.4.5 The anticipated geology at the proposed main tunnel invert is as follows:

a. London Clay Formation western end of the tunnel to just west ofAlbert Bridge (Harwich at the base approximately between CremorneWharf and Albert Bridge).

b. Lambeth Group Starts to enter tunnel invert just east of AlbertBridge, forming lower third of the face by Chelsea Bridge, formingfull-face by Tideway Walk. Tunnel continues in full-face Lambeth tojust east of London Bridge.

c. Thanet Sand Formation Within invert and lower third of face between

Blackfriars Bridge and London Bridge, becoming full-face to just eastof London Bridge to just west of Tower Bridge.

d. White Chalk subgroup Downstream from just east of Tower Bridge.

3.4.6 Faulting at London Bridge is expected to repeat the sequence, and mixed-face conditions in the Lambeth Group and Thanet Sand Formation areexpected from Chelsea Bridge through to Tower Bridge, with only a shortsection wholly in Thanet Sand Formation, close to Tower Bridge.

3.4.7 Various structural geological models provide different interpretations of thestructural setting across the London Basin, but they all generally indicate

regular faulted block groundmass in the Chalk and NW by SE trendingfaults cutting the basic eastwest main synclinal form.


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3.4.8 The dominant structural geological features are:

a. The Hammersmith Reach Fault Zone, a series of north-northwest south-southeast trending faults beneath and adjacent to the east sideof Hammersmith Bridge. A 5m displacement to the east is noted.

b. The Putney Bridge Fault, a series of southeast northwest trendingfaults on the syncline with axis to the west of Putney Bridge, withvertical displacement of top of Lambeth Group strata on the easternhanging wall of approximately 2m.

c. The Chelsea Embankment (Albert Bridge) Fault Zone, a series ofnorth south and south-southwest north-northeast trending faultsbetween Battersea and Chelsea bridges, intersecting the tunnelalignment at near to perpendicular. Up to 5m vertical displacement ofstrata has been noted over this zone, resulting in uplift of the top ofLambeth Group deposits on the east side of Albert Bridge.

d. The Lambeth Anticline, north-northwest south-southeast trendingfaulted anticline between Vauxhall and Lambeth Bridges, intersectingthe tunnel alignment at an oblique angle with a difference in stratalevel of approximately 5m.

e. The London Bridge Fault Graben, southeast northwest trendinggraben-type feature arranged between Cannon Street and TowerBridges, with known vertical displacement in excess of 10m.

f. The Greenwich Fault Zone, southwest northeast trending. Thisfeature was investigated in detail during the Lee Tunnel groundinvestigation (2008) and up to 20m downthrow is anticipated to the

northwest in a series of stepped faults. The fault runs generallyparallel with the main syncline, southwest northeast from Greenwichto Beckton, crossing the River Thames downstream of the ThamesBarrier, and is in close proximity to the Greenwich PS.

3.4.9 Other structural features include the North Greenwich Syncline (now moregenerally known as the Plaistow Graben), Millwall Anticline and BecktonAnticline, all of which have a NE SW trend, contrary to main basin axis.

3.4.10 There is a risk of scour hollows that are located on previous drainagechannels formed by the River Thames, often found at the confluence withthe existing tributaries, eg, at the Fleet, Lee and Wandle. The features

usually contain a variety of granular deposits and/or disturbed naturalmaterials and are localised and steep-sided.

3.4.11 The scour hollow in the vicinity of the Blackwall Tunnel is the only scourhollow known to penetrate into the Chalk; elsewhere, the hollows onlyaffect the tertiary deposits and, more particularly, the London Clay. Basaldepths are normally 5m to 20m below ground level, exceptionally 33m atBattersea Power Station and Hungerford Bridge.

3.4.12 Of the known scour hollows, only the hollow at Hungerford Bridge is closeto the main tunnel. This feature attains a base level of 72mATD in LondonClay near the south bank, equivalent to only 10m above the tunnel crown.

Such features may, however, have implications for the shallowerconnection tunnels in other locations.


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3.4.13 Known scour hollow locations affect the following potential main tunneland CSO sites:

a. S68WH (Battersea Power Station base 72mATD)

b. S94WH (Heathwall base 80mATD)

c. C22XA (Regents base 90mATD)

d. C27XA (Fleet base 90mATD).

3.4.14 The likely presence of flints within the Chalk may cause excessive wear tothe tunnel boring machine (TBM), causing frequent interventions forinspection and maintenance, so an important part of the current groundinvestigations comprises the investigation of the Chalk structure, Chalkpermeability and characteristics of any flint band features.

3.4.15 A number of flint bands are present within the Chalk. Within the SeafordChalk, two well-defined flint bands used as marker horizons, but not

necessarily the thickest seams, are the Bedwells Columnar and SevenSisters. The Bedwells typically comprise a discontinuous layer of verylarge, irregular flints, up to approximately 500mm high by 300mm indiameter, and on previous projects have been found to have acompressive strength of 600mPa. The Seven Sisters is a continuousband, with flints between 100mm and 150mm thick.

3.4.16 The selection of the appropriate TBM is important in this respect and aslurry machine is preferred for the section of the route in Chalk. A slurryTBM was used successfully on the Channel Tunnel Rail Link Thamescrossing next to the QE2 Bridge. An advantage is the ability to deal withwater-bearing fissures in the Chalk.

Hydrogeology

3.4.17 The major aquifer of the London Basin lies in the Chalk, the aquifer beingwholly unconfined to the east but confined to the west below the tertiarystrata and the London Clay Formation in particular. The Chalk aquifer isgenerally in hydraulic continuity with the overlying Thanet Sand Formationand sometimes also the base of the Lambeth Group, particularly thegravel part of the Lower Mottled Beds and the Upnor Formation. The EArefers to this combined aquifer as the Chalk-Basal Sands aquifer.

3.4.18 Local aquicludes can exist in the overlying Lambeth Group, in particularthe Woolwich Formation Laminated Beds, leading to perched groundwatertables. Historical records of engineering schemes have described theseperched features as retaining hydrostatic pressures of up to 40m, whichmay result in high inflows at tunnel levels and particularly in shafts duringconstruction.

3.4.19 The Harwich Formation (Blackheath Member) is also known to containhigh groundwater levels in places, which cause problems during tunnelconstruction.

3.4.20 A minor regional aquifer lies within the floodplain and river terrace deposits

and because of the connection to the Thames, this aquifer is generallytidal, with an average level of 100mATD (0mAOD) +/- 2.5m.


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3.4.21 Regional monitoring of the Chalk aquifer is reported by the EA and specificmonitoring data is available over the years 2000 to 2008. These indicate adepressed groundwater table in central London at 60mATD, withgroundwater levels close to Blackfriars Bridge at 62mATD (refer to thegroundwater level contour plan of the London Basin in Appendix D).

However, the latest ground investigations undertaken by the projectindicate that groundwater levels in the Chalk from Rotherhithe to Charltonare 10m higher than the reported EA levels.

3.4.22 Groundwater pressure in the Chalk would have an important bearing ontunnelling and especially the construction of junctions between the maintunnel and the connection tunnels. Table 3.3 shows the 2008 levels in theChalk aquifer eastwards from Tower Bridge, using the data obtained fromthe EA.

Table 3.3 Chalk aquifer groundwater levels 2008 and imposedpressure at tunnel invert (east of Shad)

Tunnel sectionTowerBridge

NESRAbbey

Mills

Approx tunnel invert mATD 50 45 40

Approx GWT level 2008 mATD 72 78 92

Approx GWT pressure bar 2.5 3.5 4.0

Note: * Highest levels indicated in Lee Tunnel and Thames Tunnel projectmonitoring holes.

3.4.23 Short-term effects of pumping can still have a demonstrable impact on the

regional contours. For example, levels decreased significantly due toabstractions in supply wells at Battersea/Brixton commencing in 2002, thegroundwater level being drawn down some 18m local to the wells, by 10min central London near Fleet and by approximately 6m respectively in thevicinity of Tower Bridge and the Battersea Power Station area.

3.4.24 The EA reports that the groundwater feeding the Chalk aquifer from thesoutheast interacts with the River Thames from Greenwich to Woolwich asit flows northwest to Stratford, then west to central London. In theGreenwich to Woolwich area, there is potential for/evidence of salineintrusion within the aquifer.

3.5 Tunnel engineering and construction requirements

Risk management considerations

3.5.1 The British Tunnelling Societys and the Association of British InsurersJoint Code of Practice for Risk Management of Tunnel Works in the UKrecommendations should be adopted for all significant tunnelling projectsin the UK, including the Thames Tunnel. The objective of the code is topromote and secure best practice for the minimisation and management ofrisks associated with tunnelling works and to set out best practices thatshould be adopted. At the core of the code is an obligation that owners,designers and contractors should have processes in place to identify andmanage risks throughout the life of the project.


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3.5.2 The project has a risk management plan and procedures in place tomanage and control risks and comply with the requirements of the JointCode of Practice for Risk Management of Tunnel Works in the UK. Referalso to Health and safety engineering risk considerations in Section 4.

General tunnel considerations

Tunnel diameters

3.5.3 Tunnels should be sized to suit the hydraulic performance of the systemand the storage capacity requirement. This indicates that the majority ofthe main tunnel needs to be 7.2m internal diameter, but the most westerlytunnel drive section may be smaller depending on the length: the westerlysection between a site in Zone 1 Acton and a site in Zone 2 Barn Elmsand Zone 3 Wandsworth Bridge would be 6m and 6.5m respectively.

3.5.4 Connection tunnels would connect CSOs to the main tunnel via

interception chambers/drop shafts. These tunnels should be sized to carrythe design flows from the CSOs at gradients to limit maximum flowvelocities to 5m/s. The size of the connection tunnels would vary,depending on the flow, from 2.2m to 5m internal diameter. The minimumtunnel size for safe man access is assumed to be 2.2m internal diameter.

Vertical tunnel alignments

3.5.5 The overriding criteria controlling the gradient (vertical tunnel alignment)that can be achieved are the hydraulic functional performance, theconstraints imposed by existing and proposed third-party infrastructureand the tunnel tie-in connection level at Abbey Mills PS in order tomaintain gravity flow throughout. The main constraints are the ThamesWater Ring Main Barnes to Barrow, the Thames Water Lee Valley WaterTunnel near Hammersmith Bridge, the proposed National Grid Wimbledonto Kensal Green cable tunnel and the need to connect to the Lee TunnelShaft F at Abbey Mills pumping station.

3.5.6 The vertical distance separating the Lee Valley Raw Water Tunnel and themain tunnel crossing above is approximately 3m. Other existing deeplevel service tunnels, including National Grids Richmond to Fulham highpressure pipeline tunnel and a number of BT Openreach tunnels, alsopresent constraints on the alignment. In addition to these, the planned

National Grid Wimbledon to Kensal Green tunnel is also noted asrequiring co-ordination to ensure that possible interference between thesefuture projects is minimised. The distance between the tunnel and otherexisting third-party underground tunnels is less critical to the vertical tunnelalignment.

3.5.7 The potential connection tunnel connecting Earl PS, Deptford SR andGreenwich PS CSOs to the main tunnel would be restricted vertically bythe Jubilee underground line that crosses the Rotherhithe Peninsula andthe proposed UKPN cable tunnel New Cross to Wellclose SquareScheme.

3.5.8 The tunnels cross 42 tunnels and 72 bridges.


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3.5.9 The alignment would be designed to minimise the impact on third-partystructures. A programme of work is underway to quantify the impacts onthird-party infrastructure including bridges, tunnels, buildings and utilities.

Horizontal tunnel alignments

3.5.10 The alignment options are identified and compared in Section 4 of thisreport. These must all satisfy the hydraulic flow regime requirements.

3.5.11 The minimum horizontal radius for the main tunnel is 600m for practicableconstruction purposes, but is reduced to 500m when constrained. Smallerdiameter, segmental lined, connection tunnels are typically of a minimumradius of 300m, although techniques can be employed to achieve lowerradii.

3.5.12 In order to minimise the effect of tunnelling on third-party infrastructure,the tunnel should, so far as is practicable:

a. pass under the centre of the mid-deck span of bridges to maximise theclearance to the bridge foundations

b. avoid interfaces with sensitive existing structures, such as the originalThames Tunnel (Brunels Thames Tunnel, now carrying theOverground railway line) and the Rotherhithe road tunnel

c. avoid passing beneath tall buildings on deep piles

d. maximise clearance to third-party infrastructure.

3.5.13 The alignment of CSO connection tunnels would generally be based onthe location of the main tunnel and its shafts, along with hydraulic

considerations.

Tunnel lining

3.5.14 The lining for the main tunnel is assumed to comprise a reinforcedconcrete, tapered, segmental primary lining ring, approximately 350mmthick, and a 300mm-thick concrete secondary lining1

Shaft sizes

to provide therequired finished tunnel internal diameter. The connection tunnels arealso assumed to have a secondary lining for the purposes of this report.

3.5.15 The main tunnel drive shafts are anticipated to be between 25m and 30minternal diameter, with depths ranging from about 30m in west London to65m in east London. Shafts of 25m diameter are considered to be theminimum size required to both ensure that a TBM can be launched andthat all equipment required for safe construction of the tunnel can beaccommodated. Shafts of 30m diameter may be required toaccommodate multiple hydraulic drop structures or for use as double driveshafts.

1

The decision about whether secondary lining is required has not be made at the time of writing this report, butthis report has been based on the assumption that it is required, as that represents the worst case for programmeconsiderations.


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3.5.16 The intermediate shafts and reception shafts for the main tunnel areassumed to have an internal diameter of between 15m and 25m.

3.5.17 The internal diameter of CSO shafts range from 6m to 24m to suit thehydraulic requirements, although at some locations, it may beadvantageous to connect the CSO connection culvert directly into a maintunnel shaft.

Tunnelling and shaft construction methods

Tunnelling construction methods

3.5.18 In order to construct the project within the construction period, severalTBMs would be required to operate at the same time. In addition to this,managing construction risk and the suitability of TBM types for the varyingground conditions along the route would also affect the determination ofthe number of TBMs to be used.

3.5.19 The geology and hydrogeology along each tunnel alignment wouldinfluence the selection of the TBM type. Full-face TBMs would be requiredto support the ground during tunnelling to prevent excess excavation,groundwater inflow, and to minimise ground movement.

3.5.20 Full-face TBMs can be either the earth pressure balance (EPB) or slurry/mixshield type. However, convertible TBMs, which have been used in thepast, can operate as either an EPB or slurry machine but result inadditional plant, equipment and impact to programme, to allow forchanges to the operational method. For the purpose of this report, it hasbeen assumed that specific machines would be tailored to the ground

conditions. These would typically be EPB type TBMs for the tunnel drivesthrough the London Clay and Lambeth Group west of Tower Bridge, andslurry type TBMs for the eastern drives through the Chalk.

Shaft construction methods

3.5.21 The geology, hydrogeology, depth and size of shaft would influence themethod of shaft construction. Various methods of construction can beused, such as:

a. precast concrete segmental lined caisson or underpinned construction

b. sprayed concrete lined

c. reinforced concrete sunk caisson

d. secant piled wall

e. diaphragm wall.

3.5.22 The construction of shafts in the London Clay is likely to be byconventional methods, with segmental lining sunk either as a caisson orunderpinned. Sprayed concrete linings may also be used in conjunctionwith sheet piles for support of any groundwater-bearing superficialdeposits.

3.5.23 Where the shafts are very deep, constructed through mixed groundconditions and under high groundwater pressures, diaphragm wall type


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construction is the most likely method of construction due to the highervertical accuracy of the method. A secant piled wall method could also beused. In general, the diaphragm wall type of construction requires a largerworking area than other methods of shaft construction. A diaphragm wallshaft is a reinforced concrete lined shaft, comprising individually installed,

abutting vertical concrete wall panels, constructed in the ground usingspecialist plant, prior to the excavation of the ground within the centre ofthe shaft.

Ground treatment and control of groundwater

3.5.24 For all methods of shaft construction, the control of groundwater would berequired to enable both safe excavation and sinking of the shaft and baseslab construction.

3.5.25 In some locations, ground treatment may be required to improve thenatural state of the ground in advance of shaft construction or tunnelling.

The term ground treatment covers a variety of techniques to strengthenor stabilise the ground, including:

a. injection of chemical or cementitious grouts to form blocks that can beexcavated without collapse. The method used would be dependent onthe ground encountered.

b. ground freezing, where injection pipes circulate brine or liquid nitrogento freeze the groundwater and produce a stable block that can beexcavated. Ground freezing is costly and takes a long time toimplement.

c. compressed air, where a section of tunnel at the face has the airpressure increased, using air locks and compressors. The airpressure is increased to resist the inflow of groundwater. Thistechnique has several health and safety implications and, with the8.8m-high face of the main tunnel and the potentially high compressedair pressures required means it is unlikely to be appropriate. The 7.2minternal diameter main tunnel has an approximate external diameter of8.8m based on a 350mm primary lining thickness, a 300mmsecondary lining thickness and an assumed 150mm overcut.

d. dewatering to control the inflow of groundwater into shafts and tunnelexcavations, thus ensuring excavation stability. This can take the form

of either regional (widespread) or localised dewatering methods,depending on the purpose and the extent of pressure reductionrequired. These methods would include deep borehole wells orlocalised drains, well points and injector wells.

Main tunnel site requirements

Main tunnel sites

3.5.26 Three types of main tunnel site may be needed to construct the maintunnel: Drive sites, reception sites and intermediate sites.


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3.5.27 The main tunnel would be driven from main tunnel drive shafts, whichwould be equipped to enable the efficient operation of the tunnellingexcavation and construction.

3.5.28 Reception shafts would be used to remove the TBM from the tunnel at theend of a drive. Given a sufficient size of site, a shaft could be used forboth drive and reception purposes.

3.5.29 Intermediate shafts can be used to gain access to the main tunnel boreduring construction, either to inspect and/or maintain the TBM or toprovide access for secondary lining construction (should a secondarylining be required).

Location of sites

3.5.30 The required number and distribution of sites for tunnel construction wouldbe informed by the following key considerations:

a. The Thames Tunnel Project construction period.b. The TBM types must be appropriate to the geological conditions

expected.

c. The risk of TBM breakdowns/servicing requirements, and their severityand frequency, increases with the length of the drive.

d. The emergency egress of the construction workforce would becomemore difficult the longer the length of the drive.

3.5.31 The final decision on the number of TBMs, and hence the number ofassociated drive sites, would be based on a balance between the type of

TBM appropriate to the ground, the available locations of main drive sites,geology, programme, environment, amenity, health and safety, risk, costand procurement considerations.

3.5.32 Construction of CSO connection tunnels would be constructed from maintunnel sites to reduce the space required for CSO sites where possible.Where CSO connection tunnels are driven from main tunnel sites, theCSO sites would comprise smaller reception sites. Excavated materialfrom the CSO connection tunnel could also be handled at the main tunnelsites.

Main tunnel site requirements

3.5.33 The Site selection background technical paperprovides information onconstruction activities at main tunnel sites and their size requirements.The sizes are summarised as follows:

a. Main tunnel drive sites from which slurry TBMs would be driven needto be approximately 20,000m2, whereas sites hosting an EPB TBMneed approximately 18,000m2. If there are site space constraints, itmay be possible for an EPB TBM to be driven from a 15,000m2 site,but this may reduce the efficiency of the tunnel operations andincrease the risk of delays.


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b. Main tunnel reception or intermediate sites would range from 5,000m2for sites with shafts constructed into the London Clay to 7,500m2

3.5.34 The construction activities that follow tunnel excavation are less onerouswith respect to site spatial requirements. These would include tunnelsecondary lining (if required), shaft lining, buildings and surface works,and mechanical and electrical fit-out works.

ifdeep diaphragm walling is proposed.

Construction logistics

3.5.35 For the purposes of this Engineering options report Abbey Mills route,the following logistical needs have been considered:

a. The ability to provide efficient site layouts

b. Logistics hubs

c. Critical services power

d. Transport of materials and equipment

e. Main tunnel segment fabrication and supply.

Site layouts for logistics

3.5.36 The layouts of individual sites for the logistics purposes would depend onthe specific site use and local constraints. The Site selection backgroundtechnical paperindicates typical layouts for the different types of sites.

Logistics hubs

3.5.37 The supply and servicing of the smaller CSO sites could be carried out assatellites to the main tunnel sites. These main tunnel sites may thereforerequire an allowance for a logistics hub area for facilities to service thesatellite sites. This has not been taken into account at this stage of theproject and is likely to be contractors responsibility.

Critical services power

3.5.38 The temporary power supply requirements for construction sites typicallyvaries from 1.25MVA to 3.5MVA for the smaller CSO sites, and up to12.5MVA to 17.5MVA for the large main tunnel drive sites serving a single

TBM and 25MVA for a double drive site.3.5.39 The number and potential spread of sites for main tunnel drives is such

that for the majority of areas, it is likely that insufficient capacity exists, orwould be available from existing UK Power Networks at the timeconstruction commences. Therefore, power supply improvement workswould be required for main tunnel drive sites and should therefore beplanned to accommodate new substation installations

3.5.40 Discussions with UK Power Networks have established that it would beprudent to plan for the early procurement of power supplies for the maintunnel drive sites to ensure there is sufficient supply available for the

TBMs to meet the project programme.


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Transport of materials and equipment

3.5.41 Construction of the shafts and tunnel works would require a wide variety ofmaterials and equipment to be transported to and from the working sites.

3.5.42 Tunnel excavated material would need to be taken away from the drive

sites and a variety of materials would need to be delivered, particularly theconcrete segments for the main tunnel lining. Other logistical activitieswould include workforce arrival/departure, equipment deliveries/return,consumables and, for the drive sites, the delivery of the large TBMcomponents.

3.5.43 Due to the large volume of materials to be transported, the objectives areto use the river to transport main tunnel excavated material by barge andto enable construction contractors to move other materials by river wherepracticable and cost-effective.

3.5.44 The practicality of rail freight transportation would depend on both the

proximity of the main tunnel sites to suitable rail sidings and the localnetworks capacity for freight movements.

3.5.45 It is expected that some deliveries would also need to be transported byroad even if barge and/or rail transport facilities were available. Anynecessary highway routes would need to be identified as part of theproject development. Major deliveries/removals would be subject tospecific movement restrictions and conditions imposed by police andtraffic authorities.

Main tunnel segment fabrication and supply

3.5.46 The supply of tunnel lining segments to the individual drive site locationswould depend on their final location and the location of the potentialfabrication facility or facilities. This has not been taken into account at thisstage of the project and is likely to be the contractors responsibility.

Excavated material handling and disposal

Material type and handling

3.5.47 The main excavated material types would be London Clay, LambethGroup, Thanet Sand Formation and Chalk.

3.5.48 The type of material and TBM choice would dictate the material handlingand treatment requirements; the excavated material consistency wouldvary from relatively dry London Clay to Chalk slurry.

3.5.49 For the purposes of site planning, an allowance has been made for onsitestorage of excavated material equating to five days production. Thisallows for issues relating to maintenance, plant breakdown and risks tobarge operations on the River Thames. If there are site space constraintsbut good transport links, it may be possible to reduce the allowance tothree days storage at the risk of delays should this prove insufficient.


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Quantities and programme requirements

3.5.50 The total quantity of excavated material for all tunnels and shafts isanticipated to be in the region of 1.7million m3

3.5.51 The in situ volume of excavated material arising (unbulked) per drive atmain tunnel drive sites would be approximately 300,000m

(in situ quantity). Thiswould vary, depending on the tunnel alignment and connections.

3 to 500,000m3

3.5.52 Where two drives are carried out from the same site location, this wouldincrease the storage capacity required if these are to be carried outsimultaneously.

,assuming a tunnel length of between 5km to 8km.

3.5.53 The tunnelling advance rates dictate the requirements for materialremoval. For the purposes of preliminary planning, a rate of approximately1,000m3 to 2,000m3

Marine transport

per day from a site is assumed, depending on TBMtype and ground conditions.

3.5.54 The feasibility and use of marine transport for the removal of excavatedmaterial from potential main tunnel drive sites along the river is dependenton location.

3.5.55 Operations in the upper reaches of the River Thames beyondHammersmith Bridge are considered to be impractical due to therestrictions of bridge height, tidal range, and width of the navigablechannel. These would impose constraints on barge movements thatwould reduce substantially the quantity and rate of material that can be

removed, making the viability of solely marine transport in these areasunacceptable.

3.5.56 The operations between Putney Bridge and Hammersmith Bridge areconsidered to be challenging, especially when servicing the peaktunnelling rates. However, sites along this length of the Thames could beaccessed and serviced but would require careful planning to mitigate theproblems associated with navigational constraints.

3.5.57 Downstream of Putney Bridge, there are fewer navigational constraintsand, as such, it is possible to reduce the number of barge movements byusing larger size barges on the lower reaches of the Thames to the east.

Hence, only 350t barges can be used around Putney Bridge, 1,000tbarges can be used in the vicinity of Battersea Power Station and 1,500tbarges or larger can be used downstream of Tower Bridge.

3.5.58 The Abbey Mills Pumping Station site is located on the River Lee, adjacentto the Three Mills Lock. At this location, the river is tidal and onlynavigable for about four hours on each tide. Downstream from the site,the river is narrow and constrained by physical features, including lowbridges. Although not impossible, using the river to transport the materialsrequired to service a main tunnel drive would introduce financial andprogramme risks that would need to be carefully investigated before a final

decision on using the site to drive a main tunnel is made. For the purposeof this options report, driving from Abbey Mills is included as a feasible


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option to be evaluated against other options during subsequent stages ofsite selection.

In-river facilities

3.5.59 Jetty/wharf structures and their location with respect to the navigational

channel, together with associated dredging of the river for accesspurposes, are site specific. Each main tunnel drive site not havingsubstantial jetty or deep water wharf facilities is likely to require a bespokesolution with specific consents from the Port of London Authority (PLA)and the EA.

3.5.60 The issues above, with respect to in-river facilities, are more onerous onthe upper reaches of the river. Thus, beyond Hammersmith Bridge andto a lesser extent beyond Putney Bridge the scale of facilities for bargesis likely to impinge greatly on the existing river and its users, leading todifficulties in obtaining the required consents, feasibility and unacceptable

risks to other river users.3.5.61 Particular risks to in-river facilities and barge movements relate to other

river users and the need to obtain a marine risk assessment foroperations. As such, it is noted that in the upper reaches of the riverbeyond Putney Bridge, the presence of recreational users, such as rowersand small boats, presents a major hazard and risk to be considered whenevaluating sites.

Disposal of material

3.5.62 The methods of treatment, transport and disposal are dependent upon the

nature and consistency of the excavated material and requirements forfinal disposal.

3.5.63 The overall policy is to favour marine transport of main tunnel excavatedmaterial along the River Thames, where practicable and cost-effective.

3.5.64 The details of potential disposal site options are not discussed orconsidered in this report. These would be covered by the project wastemanagement strategy, forming part of the future Environmental ImpactAssessment.

CSO connection to the main tunnel

3.5.65 Where the CSO connection tunnels are directly connected to the 7.2minternal diameter main tunnel, it has been assumed that the internaldiameter of the connection tunnel would be no greater than 4.5m. Thejunctions would be axis-to-axis, and have a horizontal angle of about 70degrees to the main tunnel where practical. Where the internal diameterof the main tunnel is smaller than 7.2m, the connection tunnel diameterwould need to be appropriately sized. The limitation on diameter of theconnection tunnel is due to both constructability and the designrequirement for achieving a stable structural opening.

3.5.66 The CSO connections to the main tunnel are to be grouped into five

generic options/types. These are outlined in greater detail in Section 4.


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Connection with Lee Tunnel

3.5.67 The Thames Tunnel projects main tunnel would connect to the LeeTunnel at Abbey Mills PS. The proposed arrangement is for the maintunnel to connect at the Lee Tunnel Shaft F (proposed Lee Tunnel shaftto be located at Abbey Mills PS). The connection would need to provide asmooth hydraulic confluence to combine the flows from both the AbbeyMills CSO and the main tunnel towards Beckton. The design of theconnection would need to minimise disruption of the Lee Tunneloperations.

Third-party infrastructure impact

3.5.68 The nature of operations involved in construction of the main tunnel andassociated shafts has the potential to cause ground movements that couldaffect existing third-party infrastructure and buildings. The horizontal andvertical alignment of the main tunnel shaft locations and construction

methodologies should be selected so that the impact on third-partyinfrastructure due to ground movement would be avoided or minimised, asfar as is reasonably practicable.

3.5.69 Searches of historical and other records have revealed groundwaterabstraction wells located within the alignment corridor, some of which areoperational abstraction wells. The tunnel alignment would, whereverpossible, avoid any adverse affect on these wells.

3.5.70 Searches have revealed, in addition to road and underground rail transporttunnels, a number of existing deep level service tunnels, including NationalGrids Richmond to Fulham high pressure pipeline tunnel and a number of

BT Openreach tunnels. In addition to these, the planned National GridWimbledon to Kensal Green tunnel is also noted along with the proposedUK Power Networks New Cross to Wellclose Square cable tunnel. Thealignment of the main tunnel and connection tunnels would avoid theseassets, with acceptable clearances.

3.5.71 Liaison with third parties has commenced with the objective of obtainingApproval in Principle agreements to cross major assets where possible.This includes an assessment of all significant assets, the development ofpreliminary instrumentation and monitoring plans, and identification ofmitigation works where necessary. The scope includes tunnels, bridges,

river walls, utilities and existing buildings.

3.6 CSO engineering and construct ion requirements

General considerations

3.6.1 The design requirements for CSOs are outlined in Section 3.3 with a list ofthe controls required for all 34 CSOs, as well as indicating the 18 CSOsrequiring interception and three connections to the existing northern LowLevel Sewer No.1 (see Table 3.1).

3.6.2 The CSO interceptions identified comprise a combination of direct gravity

overflows and pumping stations. In each case, the location of the CSO


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interception works would be constrained by the layout of the existingsewer system.

3.6.3 In general, interception of gravity CSOs would be downstream of the lastincoming connection to the overflow before the overflow sewer reachesthe river, to ensure that the CSO interception is not bypassed during astorm event.

3.6.4 For the interception of flows from pumping stations, there are advantagesand disadvantages associated with interception pre- and post-pumping.For example, intercepting the flows pre-pumping allows direct gravityinterception without reliance on the pumps and therefore provides energysavings, whereas post-pumping interception allows the pumps to be usedregularly and therefore reduces the need for special maintenance facilities.If pumps are not used regularly, maintenance procedures are required toperiodically start pumps manually to ensure they do not seize up due toinfrequent use. In practice, the criterion governing whether pumping

station flows are intercepted pre- or post-pumping is likely to be theavailability of suitable CSO sites.

CSO interception design and construct ion

3.6.5 The CSO interceptions typically consist of the following elements:

a. CSO interception chamber

b. CSO connection culvert

c. CSO drop shaft

d. CSO connection tunnel.

3.6.6 Details of each of these elements are outlined below.

CSO interception chambers

3.6.7 The CSO interception chamber would typically be a box-shaped structureand would be positioned on the line of the existing sewer pipe. Thepurpose of this structure is to intercept the CSO flow and direct it into theconnection culvert leading to the drop structure.

3.6.8 The size of the interception chamber would be determined to suit theexisting sewer and to accommodate the maximum flow requirements for

interception. This would be done using a combination of calculations andphysical modelling.

3.6.9 The depth of the interception chamber would be determined by the depthof the existing sewer.

3.6.10 It is envisaged that the interception chambers would be constructed as areinforced concrete structure. However, the construction methodology forthe chamber would be dependent on the depth, ground conditions andother site specific criteria. Generally, steel or sheet piling may be used toprovide the excavation for the construction of the chamber. Where thedepth of the chamber precludes the use of sheet piling, an alternative

method, such as secant piling, may be required.


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3.6.11 The existing line of the CSO overflow is to be retained for use as anoverflow for the system in the permanent case. An overflow is alsorequired to be maintained during the construction of the interception worksto allow construction of the interception chamber and maintain thefunctionality of the existing sewerage system during a storm event for the

duration of the construction period, prior to commissioning of the project.3.6.12 The overflow to the river would be protected by double isolation in the form

of two lines of flap gates. These flap gates would either utilise the existingflap gate arrangement (where acceptable) or, in some cases, a newstructure and flap gate arrangement.

3.6.13 The interception chamber would also be protected against reversesurcharge flows from the drop shaft by means of two lines of flap gateslocated on the line of the proposed connection culvert. An actuated,motorised penstock would also be positioned within the interceptionchamber at the junction of the connection culvert. This penstock would

remain open during normal operative procedures, but would be closed toprevent flows being diverted through the connection culvert during tunnelmaintenance activities.

3.6.14 It is envisaged that a control kiosk would be required at each CSOinterception site to operate the motorised penstock. This kiosk may alsobe used to accommodate other control and monitoring equipment andwould be sized accordingly.

3.6.15 An opening would be required in the roof of the interception chamber tofacilitate maintenance access and to allow for repair or replacement of theflap gates and penstock in the future. These openings would be fitted with

suitable lockable covers. It is envisaged that the roof of the chamberwould be at or below ground level, with the covers to the openingspositioned at ground level.

CSO connection culverts

3.6.16 The CSO connection culvert would join the interception chamber to thedrop shaft. It is the intention to minimise the length of the CSO connectionculvert by positioning the chamber and shaft as close together as possible,although this is dependent on the individual constraints at each site.

3.6.17 The depth of the connection culvert would typically be determined by the

depth of the existing sewer, which in turn sets the depth of the interceptionchamber. In some cases, it may be required to increase the depth of theconnection culvert to minimise impact on third-party assets, particularly ifthe culvert has to pass under existing structures or utilities.

3.6.18 The connection culvert would be sized to accommodate the requiredcontrolled or maximum design flow rate.

3.6.19 The form of construction of each CSO connection culvert would becontrolled by the constraints at each site. Typical forms of constructioncould include open cut supported by sheet piling or an opencut trenchsupport system, microtunnelling/pipejacking (utilising precast concrete

pipe units) and sprayed concrete lining tunnelling connections. Therefore,the connection culvert may be either circular or box-shaped in cross


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section and could comprise precast concrete pipes, precast concreteculvert units, or sprayed or in situ concrete.

3.6.20 There may also be a series of access manholes along the length of theculvert to facilitate the installation, removal, inspection and maintenance ofthe required flap gates and penstocks.

3.6.21 For foreshore interception of CSOs, the interception chamber may beaccommodated within the top of the drop shaft and no connection culvertwould be required.

CSO drop shafts

3.6.22 The purpose of the drop shaft is to allow the intercepted flows from theCSO to be dropped to the level of the main tunnel or, in some cases, tothe level of the connection tunnel, with an acceptable amount of airentrainment. Three forms of mechanism have been considered to dropthe flows within the drop shaft. These are summarised as follows:

3.6.23 Straight drop: Due to energy dissipation, the use of a straight drop isonly considered appropriate where the drop in height is less than 10m.The direct drop approach would maintain the flow within a pipe rather thanbeing a waterfall. For the majority of CSOs, the drop in height is greaterthan 10m and therefore a straight drop would not be used.

3.6.24 Cascade drop: Cascade platforms within shafts are used to dissipateenergy for drops greater than 10m. The cascade typically includesalternating platforms at approximately 3-6m intervals over the full depth ofthe shaft, causing the energy to be dissipated in stages as the flows dropto the required level. Due to the regular inspection and maintenanceregime required for cascade type drops, and the associated health andsafety issues, cascade type drop shafts are not preferred.

3.6.25 Vortex drop: Vortex drop tubes can be used for drops greater than 10m.In order to generate the vortex at the top of the drop tube, vortex tubes areenvisaged to be in the range of 0.9m to 3m diameter. A vortex drop is asystem that accelerates and spins the flow so it adheres to the wall of atube, which is a proven and robust means of transferring flows from ashallow structure to a deep tunnel.

3.6.26 Drop shafts would be sized to accommodate maximum flows, havingregard to the mechanism used to drop the flow to tunnel level.

Connection tunnel

3.6.27 Connection tunnels take flows either between two drop or from one dropshaft to the main tunnel/main tunnel shaft.

3.6.28 Details of the types and methods of CSO connection to the main tunnelare outlined in sections 4.3 and 5.1.

Air management

3.6.29 When the system fills with CSO discharges, the air in the tunnels will be

displaced, and when the flow is removed from the tunnels, air will need toreturn. When the tunnels are empty, the design also includes means of


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refreshing the air within the tunnels. Therefore, the interaction ofcombined sewage inflow and management of air requirements areconsidered and addressed.

3.6.30 The air management system would involve a combination of air extractionand intake structures, and buildings to house air treatment equipment.The size and configuration of the structures would depend primarily onhow air moves through the system and the amount of air to be moved.

Construction sites and logistics

Site requirements

3.6.31 CSO site requirements would depend on the size of the connectiontunnels, diameter depth and type of drop shaft, space requirements forconstruction activities, access constraints, and whether the drop shaft is tobe used as a drive or reception shaft for the connection tunnels.

Considerations for in-river si tes

3.6.32 In-river (foreshore) sites are considered for a number of locations. Ingeneral, these locations are not favoured as an engineering solution dueto the added complications of both working in the river and access to sites.Nevertheless, in certain areas, the complication of the connection to themain tunnel and availability of suitable sites means that such sites areconsidered as the only feasible sites available.

Transport of materials and equipment

3.6.33 Construction of the CSO works would require a wide variety of materialsand equipment to be transported to and from the working sites. Thesesmaller sites could also be managed as satellites to main tunnel drive sitelocations, negating the need for facilities such as offices, stores and othersite facilities.

3.6.34 For the purposes of this report, it is assumed that all transport to and fromthe CSO sites would be by road.

Power supply and s ite services

3.6.35 The temporary service requirements for the CSO sites are less demanding

than those required for the main tunnel drive sites.

Third-party infrastructure impact

3.6.36 The works at CSO sites have the potential to affect third-partyinfrastructure and buildings, specifically near surface services and the riverwalls forming the River Thames flood defences. Near surface serviceswould be present at all sites, but the complexity of the existing layouts andthe possibility of diversionary routes would vary. The construction workswould be designed to avoid or minimise their impact on third-partyinfrastructure and buildings as far as is practicable.


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4 Main tunnel drive options


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4.1 Introduction

4.1.1 Section 4 establishes main tunnel drive options and factors that can be

used to assess the engineering advantages and disadvantages of theseoptions. It also identifies the five different ways to connect the CSOinterceptions to the main tunnel.

4.2 Main tunnel engineering opt ions preparation

4.2.1 The Abbey Mills main tunnel route is shown in Figure 3.1. This sectionfurther develops and establishes feasible drive options and presents themin a format to take forward to the next site selection stage to determine apreferred drive option and an associated list of preferred sites for theAbbey Mills route.

4.2.2 This report considers issues that affect the selection of main tunnel sitesboth for construction of the main tunnel drive and the CSO connections tothe main tunnel.

Shortlisted sites

4.2.3 Potential sites have been identified using the short list, established via theprocess set out in the Site selection methodology paper. The shortlistedsites fall into two categories:

a. Sites potentially suitable as main tunnel sites

b. Sites potentially suitable as CSO sites.4.2.4 The site selection process searched for three types of sites to construct

the main tunnel:

a. Main tunnel drive sites

b. Main tunnel reception sites

c. Main tunnel intermediate sites.

4.2.5 The requirements and site area of a main tunnel reception site are similarto those of an intermediate site but not those of a main tunnel drive site.For site selection purposes, a main tunnel reception site was considered in

the same way as an intermediate site. Therefore, only two categories ofsites needed to be identified in the site selection process, which were:

a. drive sites the term used for main tunnel (single or double) drivesites (at the beginning of site selection this category of site wasreferred to as main shaft sites)

b. intermediate sites the term used for both main tunnel intermediatesites and main tunnel reception sites (at the beginning of site selectionthis was referred to as intermediate shaft sites).

4.2.6 There are a total of 53 main tunnel sites (all potentially suitable as

reception or intermediate sites, but not all are suitable as drive sites) and63 CSO sites identified on the final short list for phase two consultation.


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There were 52 main tunnel sites and 71 CSO sites on the short list forphase one consultation. Main tunnel sites may either be used individuallyor combined with an adjoining site to provide the required site area.

Main tunnel sites types

4.2.7 The number of locations from which the individual drives are launched andreceived would vary according to directions in which the drives areconstructed. There are potential benefits in reducing the number of drivesites by having double drive sites where two TBMs are driven in oppositedirections, enabling efficiencies to be gained

Documents

110 RG PNC 00000 000557 EOR AbbeyMillsRoute