Thames Tideway Tunnel: design responsibilities, phases and system-wide design challenges Open Access

Millions of tonnes of untreated sewage mixed with rainwater (combined sewage) used to spill into the tidal River Thames every year from combined sewer overflows (CSOs), which were an essential part of Sir Joseph Bazalgette’s original sewerage system. In the early 2000s, the Environment Agency (EA) assessed and categorised the 57 CSOs discharging into the tidal Thames and identified the 36 most polluting CSOs that required attention.

The overall solution of intercepting and/or controlling the 36 most polluting CSOs is known as the London Tideway Tunnels, comprising

• the Lee Tunnel, which controls flows from what was the most polluting CSO at Abbey Mills Pumping Station on the River Lea

• the Thames Tideway Tunnel, which controls flows from 34 CSOs

• the remaining CSO, which was addressed individually.

The wider London Tideway Improvements scheme also included improvements at Mogden, Crossness, Longreach and Riverside Sewage Treatment Works and a capacity extension at Beckton Sewage Treatment Works to treat flows collected by the London Tideway Tunnels. The scheme has reduced the adverse environmental impacts of CSO spills on river ecosystems, reduced the unacceptable issues associated with detritus and litter carried by CSO spills and reduced the elevated health risks for recreational users of the river.

This paper considers design responsibilities, design assurance and some aspects of the design that are peculiar to the Thames Tideway Tunnel.

The Thames Tideway Tunnel project was principally delivered by a new regulated water company, Bazalgette Tunnel Ltd, operating as Tideway (responsible for Tideway’s Works), with some elements delivered by Thames Water (responsible for Thames Water’s Works). Tideway delivered their elements of the project through four NEC3 design and construct contracts. Option C was used for the three main works contracts (NEC, 2013a), while Option E was used for the system integrator contract (NEC, 2013b). NEC3 were the conditions of contracts used (earlier editions were known as New Engineering Contracts). Project Manager is capitalised in this paper as it is an NEC-defined term.

The scope split between Tideway and Thames Water is illustrated in Figure 1.

Tideway’s scope was divided into three main works contracts and a system integrator contract.

• The West Main Works Contract (C405) was awarded to BMB – a joint venture between Bam Nuttall, Morgan Sindall and Balfour Beatty, with designers Arup/Atkins (now AtkinsRéalis) and Morgan Sindall Engineering Services.

• The Central Main Works Contract (C410) was awarded to Flo – a joint venture between Ferrovial Construction and Laing O’ Rourke, with designers Aecom.

• The East Main Works Contract (C415) was awarded to CVB – a joint venture between Costain, Vinci Construction Grands Projets and Bachy Soletanche, with designers Mott MacDonald.

• The System Integrator Contract (C489) was awarded to Amey.

Figure 2 illustrates the typical components for interception of a CSO, including

• an interception chamber, intercepting flow from the existing sewer/CSO

• a valve chamber housing tunnel isolation penstocks, secondary isolation gates and tunnel flap valves, which provide the means to control intercepted flow into the tunnel system

• a connection culvert, passing intercepted flow on to the drop shaft

• a vortex intake and drop tube within a drop shaft, transferring flow from high level down to the main tunnel

• a connection tunnel, passing flow into the main tunnel at sites where the drop shaft is not on the line of the main tunnel

• a river outfall chamber housing river flap valves to allow flow to the river when the tunnel system is full

• ventilation and air treatment structures to manage and treat air flowing into and out of the tunnels and shafts.

The mechanical, electrical, instrumentation, control and automation (MEICA) elements of each interception site are consistent. Flow to the tunnel is usually controlled by hydraulically actuated tunnel isolation penstocks, which are normally open but close when the tunnels are full, although electrically actuated penstocks are provided at Thames Water operational sites. Flow from the tunnels back into the sewer network is prevented by tunnel flap valves, while river flow back into the tunnels/sewer network is prevented by river flap valves. Manually deployed secondary isolation gates will, in combination with the tunnel isolation penstocks, provide double isolation from the network if the tunnels need to be taken out of service for inspection. Automatic operation of the system is based on water levels in the tunnels and interception structures. Levels are measured by level monitors, which are either radar or ultrasonic, depending on the location. The air management system incorporates mainly three types of mechanical dampers for inlet, bypass and pressure relief, and the air treatment system has rechargeable carbon filters equipped with hydrogen sulphide, pressure and temperature monitoring. Electrical and control equipment and hydraulic power packs are located in kiosks at each site.

The principal parties with design responsibilities are shown in Figure 3.

The EA set out obligations that the design of the London Tideway Tunnels needed to satisfy in the form of environmental permits, and agreed with Thames Water the control rules for operating the system – known as the London Tideway Tunnel Operating Techniques (LTT OTs). For more information on the LTT OTs, see Lewis et al. (2025a) in this issue.

Thames Water’s design responsibilities included

• agreeing with the EA amendments to environmental permits and the LTT OTs

• management of the sewer network

• review of detailed designs prepared by Tideway’s contractors

• design assurance of the detail designs prepared by its contractors for their own scope (Thames Water’s Works).

Tideway’s design responsibilities included

• overall design coordination of the Thames Tideway Tunnel

• the design requirements and specifications for the four design and construct contracts

• review of detailed designs prepared by Thames Water’s contractors

• design assurance of the detailed designs prepared by BMB, Flo, CVB and Amey.

The majority of detailed design responsibilities for Tideway’s scope flowed down to its contractors, but some system-wide design responsibilities remained with Tideway. Tideway’s retained system-wide design responsibilities included system hydraulic performance and air management, integration of Thames Tideway Tunnel with the existing sewer network and the system control philosophy. The contractors designed parts of the system, so were not in a position to hold any system-wide design responsibility.

The NEC3 Project Managers for each contract were responsible for the review and acceptance of their contractor’s designs.

Outline designs for the project were prepared initially in sufficient detail to apply for development consent, then developed further for the procurement of design and construct contractors with detailed design undertaken by those contractors.

This design stage started with site selection to locate the CSO interception sites, confirm the tunnel alignment and identify the main tunnel drive sites. When a suitable location for each site’s CSO interception works was identified, system-wide hydraulic and ventilation designs were developed. The designs were sufficiently developed to show the scale of the below-ground works, to show indicative or illustrative designs of the above-ground works and to define the limits within which the works would need to fit.

Sewerage projects often have little showing above ground when completed, but this project presented the unique opportunity of creating new areas of public realm along the River Thames (see Donnelly et al. (2025) in this issue). The requirements for integration of the engineering structures into London’s urban realm were defined in a set of design principles (Thames Water, 2014) that were developed in parallel with the pre-contract design development through a combination of multi-disciplinary coordination and consultation with stakeholders. The integration of the above-ground elements into the urban realm was also subject to extensive consultation with stakeholders and independently reviewed by the Commission for Architecture and the Built Environment.

Varying levels of flexibility for the new areas of public realm were secured through the Development Consent Order (DCO) (HMG, 2014). During the consultation phases leading up to the application for development consent, most local authorities engaged with the design team. Where consensus could be reached, ‘indicative’ landscape plans showed what those sites would look like. Where local authorities did not engage, the ‘illustrative’ landscape plans only showed what those sites might look like, but the design principles provided flexibility for the contractors to develop detailed designs that met the aspiration to deliver distinctive placemaking design solutions at each site.

Use of the NEC3 Contracts required the preparation of Works Information (WI). The WI defines what the contractor is to build and to what standards. The design for tender phase was tasked with producing the WI in preparation for inviting tenders and then the detailed design and construction. The WI included specifications for the design and construct contractor’s scope of works. The WI also set out any constraints on how the contractor was to design and construct the works. The design for tender team prepared specifications for all elements of the works, including

• tunnels and shafts

• tunnel boring machines (TBMs)

• civil/structural/marine works

• MEICA

• hydraulics

• ventilation and odour control

• employer’s information requirements

• third-party interfaces

• durability

• architecture and landscape.

The WI also included design specifications setting out minimum standards for the design of key elements of the contractors’ scope.

On completion of the design for tender, the outline structural arrangement designs were approximately 30% complete, while the system-wide hydraulic, ventilation and air treatment elements were approximately 85% complete, as Tideway retained design responsibility for these. To ensure that the system-wide requirements were met by each contractor, the WI defined worksite hydraulic surfaces and worksite hydraulic structures to be delivered as a key aspect of each contractor’s scope.

Following on from a comprehensive site investigation campaign with over 200 overland and 100 overwater boreholes to depths of up to 90 m, the design for tender team also produced geotechnical baseline reports (GBRs) for each main works contract. The GBRs, and specifically the baseline statements therein, were solely related to the contractors’ means and methods of construction and were a key aspect of apportioning risk between each contractor and Tideway.

To fully understand the hydraulic performance of the CSO interceptions, computational fluid dynamics (CFD) and physical modelling were used extensively throughout the pre-contract design stages. CFD allowed a quick assessment of performance at different flow rates for each design evolution, while physical modelling enabled additional analysis of the preferred solution beyond the capabilities of CFD (e.g. estimation of entrained air).

Sewer network modelling and bespoke transient analysis of the tunnel during the design for planning stage were instrumental in predicting how the system will perform and for the development of the LTT OTs. The LTT OTs balance the need to reduce the volume and frequency of CSO discharges to the tidal Thames with protecting the tunnel system from adverse transient conditions and potential overfilling. More information on this is available elsewhere (Hon et al., 2017).

The next sections discuss three features of the system-wide hydraulic and pneumatic design that required bespoke solutions to be developed before design and construct tenders were invited for detailed design.

One major challenge for deep storage and conveyance tunnels for CSO control is the transfer of flows from a high-level existing sewer down to a deep tunnel level in a safe and controlled manner. This project featured 22 tangential vortex drops, whose principal objective was to meet that challenge (for more information see Plant and Crawford (2016)). Figure 4 shows twin vortex drop tubes under construction at Chelsea Embankment Foreshore.

Following accepted industry design guidance led to vortex intake arrangements with a large footprint, which did not suit the tightly constrained area available at most of the sites. Design flows ranged from 2 m³/s to 46 m³/s, so intake arrangements with a compact footprint and high flow capacity were achieved by minimising the length of the intakes using steeper slope angles and shorter tapers. The vortex intake attached to the main vortex tube is clearly visible in Figure 5, which shows the installation of the vortex drop liner at Hammersmith Pumping Station.

This departure from accepted design practice needed to be tested, so CFD modelling was undertaken for all conceptual designs to investigate the three-dimensional (3D) hydraulic behaviour of the designs. Designs were then validated by physical modelling of the vortex drops and associated hydraulic structures, which was undertaken by Iowa Institute of Hydraulic Research (IIHR) (Iowa, USA), Hydrotec (Leeds, UK) and the British Hydromechanics Research Association, now known as Framatome BHR (Cranfield, UK).

The combination of CFD and physical modelling demonstrated the acceptable hydraulic performance of the project specific vortex intake layouts.

Vortex drops facilitate the controlled transfer of flow from an existing high-level sewer to a deep tunnel, but there is potential for the entrapment of air within the flows entering the tunnels, which can lead to uncontrolled air/water release at shafts resulting, in extreme circumstances, in geysering. Therefore, another challenge of the system-wide design was to limit, as far as practicable, the transport of entrained air into the tunnels (for more information see Plant et al. (2017)).

Conventionally, deaeration employs an arrangement where the aerated flow from the vortex drop tube enters a horizontal chamber that has a restriction at its downstream end (Figure 6(b)). The restriction reduces the velocity of flow in the chamber, which allows entrained air to rise and collect at the chamber crown where it is ventilated away from the chamber by way of a vent pipe system.

Horizontal deaeration was used at six sites where ground conditions were considered suitable for the construction of the enlarged horizontal chamber. These locations were typically in the London Clay Formation.

For sites with more challenging ground conditions, non-cohesive ground and/or permeable strata below the water table, two in-shaft vertical deaeration designs were developed: one for an on-line shaft (where the tunnel passes through the shaft) and one for an off-line shaft (where the shaft is connected to the main tunnel by a connection tunnel).

Hydrotec and IIHR undertook physical modelling to validate the performance of the vertical in-shaft deaeration design. Physical modelling was constructed at scales between 1:10 and 1:15 to provide a balance between minimising scale effects while safely constructing, operating and observing performance in the larger models (e.g. one shaft was 24 m in dia. with a 50 m drop). Physical modelling was undertaken because the limitations of CFD predictions of air entrainment and air transport were recognised.

The vertical in-shaft deaeration arrangement consists of a weir wall downstream of the drop tube, which creates a stilling pool that dissipates energy from the drop shaft discharge and slows down the flow, allowing air to escape prior to the flow passing over the weir wall and entering the tunnel. Flow is further restricted with a baffle wall between the drop tube and the weir wall.

Figure 7 shows the vertical deaeration arrangement at the drop shaft at King Edward Memorial Park Foreshore – the vortex drop tube can be seen behind the baffle wall with the weir wall in the foreground to the right. Figure 8 shows Blackfriars Bridge Foreshore drop shaft, with double vortex drop tubes encased in concrete, each with a baffle wall and weir wall either side of the main tunnel. The baffle walls and weir walls can be seen in the base of the drop shaft, with the main tunnel passing through the centre. Figure 9 shows the physical model for Blackfriars Bridge Foreshore.

A further challenge with deep storage and conveyance tunnels for CSO control is the ventilation and air treatment systems that control the air in and out of the tunnels. Further information on the project’s air management system is available elsewhere (Georgaki et al., 2017).

The London Tideway Tunnel’s air management system uses active (fan-assisted) and passive air treatment. There are six active mechanical air extraction and treatment plants (active sites) situated at the extremities of the tunnel system (see Figure 6 in the paper by Lewis et al. (2025a) in this issue). Three were provided as part of the Lee Tunnel, with three additional active sites provided as part of the Thames Tideway Tunnel. Figure 10 shows the above-ground ventilation structures at Acton Storm Tanks – an active ventilation site). Passive treatment plants consisting of passive carbon filters and no fans are located at the other 17 sites.

There are four modes of system operation – tunnel empty, tunnel filling, tunnel static and tunnel emptying.

When the tunnel is empty, the active plants at Carnwath Road Riverside and Abbey Mills Pumping Station provide one exchange of the air within the tunnel system every 24 h. Extraction of air at these two active sites also establishes a negative pressure within the system, which generates an air flow into the tunnel through the 17 passive sites.

When the tunnel is filling, the air displaced from the tunnel is treated and vented by the six active sites operating at full capacity. When there is headspace in the tunnel and no shafts are drowned, all passive sites act as air inlets. As the water level in the tunnel rises and the shafts progressively become drowned, the space within the shaft becomes cut off from the tunnel by the rising water level. When an active shaft is drowned, the fan output at that shaft is reduced. When a passive shaft is drowned, air is pushed out of the shaft through the passive carbon filters by the rising water level in the shaft.

The air management system is protected against over pressurisation by provision of mechanical bypass and pressure-relief dampers at each site.

Air is released at active sites through 15 m high ventilation columns (an example is shown in Figure 10) and through 4–6 m high ventilation columns at passive sites. Ventilation columns up to 6 m high in public realm areas have a signature shape (see Donnelly et al. (2025) in this issue).

The design of the ventilation structures and air treatment units involved numerical modelling to analyse the movement of air, air release rates and the amount of odour generated. Dispersion modelling was also undertaken to assess the impact of air releases on local receptors. Analysis of a 1 in 15 years, 120 min duration design storm was also evaluated to determine the maximum air release rates associated with rapid filling of the tunnel system to use for the design of pressure-relief facilities.

To support the modelling, extensive samples were taken to determine the dissolved to total sulphide ratio in the combined sewage, and septicity trials were carried out to assess levels of sulphide generation and potential odour release during the storage phase of tunnel operation.

Combined sewage samples and dry weather flow sewage samples were obtained from the catchment. Given the potential advantages of using hydrogen sulphide as an indicator of odour, correlations were performed between hydrogen sulphide and odour concentration by comparing the hydrogen sulphide concentration in the gaseous phase (mg/m³) with laboratory olfactometry evaluation of the odour concentration (ouE/m³). A strong relationship was found between hydrogen sulphide and odour units for hydrogen sulphide concentrations less than 0.5 mg/m³ while, for concentrations greater than 0.5 mg/m³, the relationship was weak. Hydrogen sulphide concentrations in London’s combined sewage are typically less than 0.5 mg/m³. The strong relationship finding was an important factor in deciding to monitor background odour emissions using hydrogen sulphide as the surrogate for odours.

Using a septicity rig, designed and built by Thames Water at Abbey Mills Pumping Station, trials were undertaken to assess (a) the impact of sulphide generation on system performance, (b) the potential for, and concentration of, hydrogen sulphide generation and (c) the risks of adverse odour release during filling, storing and emptying.

The results of the septicity trials showed that dissolved oxygen levels reduced over time, indicating that there is potential for the development of anaerobic conditions when combined sewage is stored within the tunnel system. Anaerobic or septic conditions encourage dissolved sulphide to be released as hydrogen sulphide, hence odours will be generated when combined sewage is stored in the tunnel system for extended periods of time. These trials allowed the determination of 48 h as the maximum allowable storage time before septic conditions would develop.

As required by the NEC3 Contracts, the four contractors progressively submitted their designs to their Project Manager for acceptance using a gate process. Tideway’s design submission gate stages were based on elements of two industry-standard progressive design assurance processes, namely

• Networks Rail’s Governance for Railway Investment Projects, which had eight stages with early stages focused on feasibility and option selection (since replaced by the Investment Decision Framework and Programme Delivery Lifecycle (Network Rail, 2019)) and

• the Royal Institute of British Architects Plan of Work 2010, with seven stages (now the Plan of Work 2020 Overview) (RIBA, 2020).

Both were considered to be overly complex and not aligned with Tideway’s design requirements, so Tideway developed a bespoke six-gate design submission process that better reflected the approach to design assurance and accounted for the maturity of the hydraulic structure and system-wide design. Tideway’s design gates were

• gate 1: preparation

• gate 2: concept design

• gate 3: developed design

• gate 4: detailed design

• gate 5: for construction and manufacture

• gate 6: testing, commissioning, operation and maintenance.

Tideway’s design gates gave the contractors flexibility on the scope of each design gate submission, allowing submissions for worksites to be broken down to each element (Figure 2) or down to the principal components of a worksite element (e.g. drop shaft primary lining, drop shaft base slab etc.). This allowed the contractors to better manage their design programme to ensure that timing of design submissions always supported the progress of construction.

Very early in the contract, each contractor provided their design management plan for acceptance by the Project Manager. The design management plan typically included

• a schedule of design deliverables, setting out the contractor’s proposed design packaging, including the typical components of design submissions in line with the design submission gates

• an organisation chart with key roles, including the lead design organisation and their sub-consultants

• management of sustainability, environment, risk and health and safety within the design process

• design coordination and communication between disciplines

• processes for design validation and verification

• workshops and other review processes, such as hazard and operability studies, operability and maintainability workshops and interdisciplinary reviews.

The design management plan was a key element of Tideway’s overall design assurance strategy as it defined – early on – each contractor’s approach to delivering the detail design for both temporary and permanent works.

Early in the detailed design process, the contractors established project-wide working groups to ensure consistency of approach for discrete aspects of their scope. The ‘design’ working group and the ‘MEICA’ working group were extremely successful in promoting consistency and sharing lessons learned and were attended by all contractors, representatives from the Project Manager’s area engineering teams (AETs), Tideway’s Design Authority (DA) and Thames Water. These two working groups were maintained right through to commissioning – a period of over 9 years.

Design assurance of the contractors’ designs on behalf of the client was undertaken by an AET embedded in each of the Project Manager teams, administering the west, central, east and system integrator contracts, and by the DA (Figure 11). The contractors were required to submit all their designs to the Project Manager for acceptance. The DA, operating independently of the Project Manager teams, supported the AETs by providing a wide range of discipline specialist advice and guidance. The DA also safeguarded the performance requirements of the permanent work assets on behalf of the client through ownership of the WI and the design responsibilities retained by Tideway (see Section 3) and also by managing acceptance of any contractor or client requested change to the scope or to the specifications. The DA also managed the design assurance interface with Thames Water, who were required to review and accept any change related to components of the works that they would eventually own, operate and maintain. Over the course of design, construction and commissioning phases, by mid-2025 there were approaching 560 WI changes, of which about 60% required review and were accepted by Thames Water.

The DSOG was formed to independently assess the continued suitability of the design as it evolved from the pre-contract design phase through detailed design by the contractors and onwards to commissioning. The DSOG did not have direct contact with contractors but liaised with project teams largely through the DA. The DSOG operated by undertaking periodic focused reviews and reporting directly to the client.

The DSOG consisted of five Jacobs subject matter experts, including a chair, who were external to the project with expertise in odour and air management, programme and risk management, supervisory control and data acquisition, network integration and modelling, hydraulic structures and advanced hydraulic modelling (including CFD and transients). The DSOG provided independent design oversight for Tideway’s retained design responsibilities.

Tideway’s contractors were responsible for design coordination and integration with each other. However, mindful that there would be differences between the designs by the three geographical contractors, a range of project-wide design requirements were imposed where consistency was important. The principal ones are briefly described below.

The design life for all civil engineering works – including tunnels, shafts, sub-surface chambers, culverts, river walls and foreshore structures – was 120 years. Mechanical items like penstocks and flap valves had a design life of 60 years and other mechanical and electrical plants such as ventilation fans, hydraulic cylinders and power packs had a 20 year design life. Tideway specified the required design life for every element of the works. All the Tideway-specified design lives were considered to be minimum requirements.

The contractors were required to define the serviceability limit state for the durability for each element of the works to meet the design life requirements. By way of example, the serviceability limit state for the durability for all reinforced concrete elements was defined as the initiation of reinforcement corrosion. The contractors presented their element-by-element assessment of the durability requirements to meet the minimum design life in the contractor’s durability assessment report.

Like the Lee Tunnel, the tunnels and shafts on the Thames Tideway Tunnel have a primary and secondary lining, which provides an asset that is extremely robust and will deliver the 120 years minimum design life with limited inspection and no anticipated maintenance. If any part of the tunnels were to fail or require significant maintenance, then the whole system – including the Lee Tunnel – would be taken out of service, with the attendant severe environmental consequences.

As noted in Section 4.2, Tideway had developed the system-wide hydraulic design to around 85% complete in the pre-contract design phases. Therefore, the components making up each interception were defined (see Figure 2) and the required internal dimensions of all hydraulic structures had been confirmed. As each contractor was only responsible for the design and construction of a part of the system, Tideway had to ensure that when the three parts were completed, combined and operated as one, the system would perform as designed. Tideway specified worksite hydraulic structures and worksite hydraulic surfaces in the WI for each site. Worksite hydraulic structures defined the basic components of each site, such as a valve chamber, interception chamber, connection culvert, vortex drop, overflow chamber and the like. Worksite hydraulic surfaces defined the dimensions of the wetted internal perimeter of each structure, including the number and sizing of penstocks and flap valves. The contractors were required to provide the structures and the surfaces as defined, but how those structures and surfaces were provided was entirely up to them.

The works at each interception site largely consisted of the same civil engineering components and MEICA elements (see Section 2). To assist in the future operation and maintenance of these sites, the contractors were required to standardise on each element of the MEICA design. Standardisation across the three main works contracts required the contractors to agree on common suppliers or use suppliers nominated by the client. Standardisation also extended to the means of access and egress to the underground chambers and culverts and, where possible, similar layouts were provided. Standardisation of these key elements across the four contracts improved operation and maintenance safety, streamlined operation and maintenance training, significantly reduced the cost and inventory of spares/consumables and reduced the cost of specialist tools and plant.

While the DCO set out requirements for a project-wide interpretation strategy – included at the request of Historic England – further detail was needed on how this would be realised within the landscape design of new areas of public realm by the three main works contractors. Tideway commissioned a Heritage Interpretation Strategy (HIS) (Tideway, 2017a) to define, in consultation with Historic England, the historic and cultural themes that would be interpreted through the architectural and landscape designs at each site. The final HIS report was titled River of Liberty.

Given the often abstract nature of historic themes and the project’s aspiration to provide high-quality, site-specific public realm design, Tideway identified an opportunity to engage with artists to foster exceptional placemaking. The Public Art Strategy (Tideway, 2017b) articulates Tideway’s commitment to public art as a means of engaging with local communities and broader audiences to deliver the interpretation of the HIS. Tideway was responsible for briefing, selecting and commissioning the artists. The contractors were responsible for working with the artists to agree the final details with the local authorities and develop maintenance costs and for the fabrication and installation of the artworks.

The interpretive artworks were developed as part of the design assurance process in parallel with the overall architecture and landscape proposals for each site. More information on the public art programme is provided by Donnelly et al. (2025) in this issue.

Tideway provided the contractors with the employer’s (Tideway) information requirements, which set out minimum requirements for key aspects of BIM delivery, including BIM execution plans, information exchange, asset data standards, asset and project information models (3D models) and computer-aided design standards. Critically, the employer’s information requirements did not prescribe what tools or software the contractor should use in delivery of their BIM obligations, thus giving the contractors freedom to use their own best practices to deliver what was required. The contractors detailed all aspects of their BIM delivery in their BIM execution plans, which were updated from time to time to reflect the changing needs of different stages of the project.

In the latter stages of construction, Tideway provided guidance to the contractors for updating their as-built models to reflect out-of-tolerance construction and other changes that may have occurred after gate 5.

Tideway maintained a project-wide GIS from the pre-contract design phase through construction, commissioning and into operation. Tideway’s GIS was made available to the contractors and was the repository for information such as instrumentation and monitoring results, bathymetry, third-party asset information, site investigation data, property information, DCO boundaries, limits and zones and so on.

Tideway used the 3D as-built models provided by the contractors at gate 6 to be the means of access to all the necessary information pertinent to the operational phase of the tunnels and shafts. Information accessible through the graphical interface provided by the 3D models included design reports and calculations, setting out details, quality records, and operation and maintenance manuals. These 3D asset information models were made accessible through Tideway’s GIS.

The Thames Tideway Tunnel was made a reality through the detailed designs and construction undertaken by the design and construct contractors for the three main works contracts. BMB, Flo and CVB had many notable successes. Some of the most notable achievements are as follows.

• Collaboration between BMB, Flo and CVB to develop an acceptable and more practical alternative to the Water Research Centre’s ‘drop test’ (WRC, 2023) for the watertightness of structures.

• Collaboration between BMB, Flo and CVB to develop standardisation for MEICA equipment across the project and standard access arrangements.

• Design, fabrication and successful deployment of state-of-the-art TBMs by BMB and Herrenknecht for the west section (earth pressure balance TBM), Flo and NFM for the central section (two earth pressure balance TBMs) and CVB and Herrenknecht for the east section (slurry TBM) (see Lewis et al. (2025b) and Newman et al. (2025) in this issue).

• Design, fabrication and successful deployment of state-of-the-art secondary lining shutters by BMB and Kern for the west section, Flo and Tecozam for the central section and CVB and Kern for the east section (see Eccles et al. (2025) and Lewis et al. (2025b) in this issue).

• BMB’s (with Morgan Sindall Engineering Solutions) successful use of a boltless steel-fibre-reinforced concrete primary lining for the 7 km Main Tunnel A in the west section.

• BMB’s subcontractor Ward and Burke’s use of a caisson shaft and caisson interception chamber and ‘double lined’ pipe jack pipes at Putney Embankment Foreshore.

• BMB’s (with Hewson) design of shaft headworks at Carnwath Road Riverside using integral concrete-encased steel truss supports to avoid internal falsework.

• BMB’s interlocking pre-cast unit design for the 15 m high ventilation columns at Carnwath Road Riverside (Figure 12) and Acton Storm Tanks (Figure 10).

• BMB’s flume and inlet channel diversion works at Hammersmith Pumping Station (see Eccles et al. (2025) in this issue).

• Flo’s deployment of Keller to stabilise shaft excavation by jet grouting at record depths for the UK at Blackfriars Bridge Foreshore.

• Flo’s floated-in Fleet Main CSO interception culvert at Blackfriars Bridge Foreshore (Figure 13) (see Chittenden et al. (2025) in this issue).

• CVB’s implementation of deep soil mixing and an off-line shaft design to overcome the challenging ground conditions at King Edward Memorial Park Foreshore (see Gonzalez et al. (2025) in this issue).

• CVB’s design of a one-piece cover slab for a single lift installation over the Main Tunnel D reception shaft at Abbey Mills Pumping Station (Figure 14).

The resounding success of the Thames Tideway Tunnel can be attributed, in part, to the continuity of Tideway’s engineering resources, which allowed the engineering intent and detailed project knowledge to be maintained from concept through to commissioning. Most of the engineers who developed the ‘design for planning’, in support of the application for development consent, moved on to produce the tender documents for the main works design and construct contracts. Many of those same resources then joined the AETs or Tideway’s DA, providing design assurance and oversight throughout the detailed design by the contractors, with some even working for the contractors’ designers.

The successful outcome can also be attributed to a client with engineering expertise at the highest level that had a willingness to support and encourage the innovation and creativity of the contractors and provided steadfast support to the assurance activities of the AETs and the DA.

Tideway also relied upon the innovation, skill, expertise and professionalism of the three main works contractors and the system integrator contractor, whose detailed designs turned the concept into a reality.

The pre-contract development of the system-wide hydraulic design to 85% proved to be a success with only one material change to the worksite hydraulic surfaces that was not the result of ground conditions or programme imperatives.

The performance-based specifications prepared in the design for tender provided an ideal balance between the need for a high-quality durable asset that met all the functional requirements of the system while giving the contractors flexibility to apply their unique expertise and skills.

The assurance model of a DA independent from the Project Managers’ teams owning and managing any change to the specifications and scope proved to be very successful in giving stakeholders confidence in the quality and functionality of the completed asset. The confidence was enhanced by the scrutiny of the DA’s performance by the independent DSOG reporting directly to the client.

Special thanks go to members of Tideway’s Design Authority – Andy Eccles (Civil/Structural Engineering Lead) and Clare Donnelly (Legacy Design Advisor) – for their contributions to this paper. Recognition is given to Thames Water, the client prior to September 2015, when the site investigation and design development work was undertaken. An extra special mention is given to civil engineers Richard Evins, David Dolan, Dave Cullen, Marc Bush and Derek Arnold whose contributions to the design effort over many years was immense. Very sadly, they died before seeing the project completed.