ABSTRACT
A targeted sustainability strategy of taking constructionrelated greenhouse gas (GHG) emissions into account at an early stage in planning and realisation has brought expert approval to Hamburg’s Underground Line U5 project for its pioneering approach. The holistic CO2- reduction strategy includes specific measures in project management, planning and realisation.
In this major northern German city, the project is a core element of the mobility transition plan to shift traffic and also an important step in achieving the city’s climate targets.
At the same time, it presents major challenges for the structural realisation in the heart of the metropolis. With 22 new stations, the U5 line, which starts in the north-east of the city and extends over about 25km, will connect neighbourhoods to the public transport system for the first time. The project will be put into operation in stages by 2040 and is to transport around 315,000 passengers per day.
INTRODUCTION: PROJECT DESCRIPTION
The new underground Line 5 (U5) connects the east of Hamburg from the Bramfeld station via the main railway station to the Arenen Volkspark station in the west (see Figure 1). Over a route length of around 25km, 22 new stations are being built using the cut-and-cover method. The average depth of the platforms is approx. 15m. The majority of the line is being excavated by TBM.
Since the groundbreaking ceremony on 30 September 2022, the first section has been under construction – the approximately 5.8km long section between the Bramfeld and City Nord stations. The remaining section of the U5 to the east and west of the Alster, up to the arenas, is currently still in the design planning stage.
In future, the U5 will run as a fully automatic, driverless underground line with train services every 90 seconds, enabling the entire route to be travelled in around 40 minutes. The route was chosen so that as many parts of the city as possible will have a rapid transit connection that cannot yet be reached via a light rail (S-Bahn) or underground station.
In addition to connecting these important potential areas, which are currently still gaps in the network, the new U5 is also intended to relieve the heavily frequented MetroBus Lines 5 and 6. This will connect densely populated districts without rapid transit to the east and west of the Alster, as well as major educational and work locations such as Hamburg University and the Eppendorf University Hospital. At seven stations, transfers to other rapid transit lines or long-distance services will be possible in the future. The U5 will thus become a line that benefits far more people than just those living along its route.
The U5 project connects around 180,000 Hamburg residents to the rapid transit network for the first time or better.
1 CARBON REDUCTION STRATEGY FOR PLANNING AND CONSTRUCTION
Mega infrastructure projects such as U5 cause considerable CO2 emissions during the construction phase, which is why environmental and sustainability concerns play a much greater role than they did a few years ago. Since 2022, the developer of Line U5, Hochbahn U5 Projekt GmbH, has been focusing on a consistent CO2 reduction strategy for the planning and construction of its new underground line. It is placing climate responsibility at the heart of its planning.
The aim is to reduce CO2 emissions during the construction of the U5 by 70%, making the new line as environmentally friendly as possible. This means that instead of the 2.7 million tonnes of CO2 equivalent (CO2e) that would be produced by conventional construction methods, only 841,000 tonnes of CO2e will be produced. In order to achieve the 70% reduction target, significant optimisations have been made in the use of materials and construction methods, and sustainability has been included as an evaluation criterion in the ongoing planning of the U5.
The earlier sustainability aspects are considered in the planning process, the better they can be influenced. The reduction strategy for the construction of the U5 is the first overall strategy for reducing CO2 emissions during the construction of a transport infrastructure project in Germany.
A key aspect for the current and future planning steps is to intensively examine the possibilities for reducing greenhouse gas emissions during construction and to integrate them in accordance with the technical framework conditions. In this way, the CO2 impact of the overall project can be demonstrably reduced, and potential reductions can be identified, particularly at material and component level and in construction logistics (see Figure 2).
Other sustainability issues, such as resource conservation, production conditions, durability, ease of maintenance, usability, and an active soil management concept, are also considered.
2 INITIAL CONSTRUCTION WORK AND CURRENT PLANNING
The U5 consortium (Arge U5), consisting of Ed. Züblin AG (technical lead) and Wayss & Freytag Ingenieurbau AG is in charge of constructing Line 5. Construction work on the first section, Lot 1, of Line U5 began in December 2022 and in August 2023 works then started on Lot 2 (see Figure 3). Here, four stations and six emergency exits will be built using the cut-and-cover method and one existing stop will be converted.
A trial run between City Nord and Sengelmannstraße is planned for 2027 with passenger service starting in 2029. Completion of the entire first construction section of Line U5 is scheduled for 2033.
2.1 Construction Method for Lot 1
The first construction phase began at City Nord station with a cut-and-cover tunnel. This is followed to the north by the double-track reversing and stabling facility in the direction of the existing above-ground Sengelmannstraße subway station. This will be modernised and converted for a platform-level transfer between the existing U1 line and the new U5. The U5 line will continue in the direction of Bramfeld with two steel bridges over Sengelmannstraße to the so-called wye, the transition to construction lot 2. The wye includes the tunnel structure east of Sengelmannstraße and the launch shaft for the TBM.
2.1.1 Underpass Structure east of Sengelmannstraße
The Sengelmannstraße subway station is the only above-ground station on the U5 line. With a tunnel trough east of Sengelmannstraße, the line is lowered and leads into the tunnel. Here, the approximately 325m-long tunnel trough ends at the future launch shaft of the TBM shortly before passing under the S-Bahn line.
The trough will be constructed in sections using the cut-and-cover method. A diaphragm wall will be built and the floor will be concreted (see Figure 4). Work on creating the excavation pit began in October 2023. The U5 trough section is divided into five docks.
The individual docks will be built partially offset to each other. The shell of the tunnel trough is due to be completed by 2027.
2.2 Construction Lot 2 Lot 2
begins in the west with the launch shaft in the wye area and ends in the east at the target shaft, the later Heukoppel emergency exit. It includes the Barmbek Nord, Steilshoop and Bramfeld stations as well as five emergency exits. The underground stops, each approx. 200m-long, the starting shaft and the emergency exits will be built using the diaphragm wall/cut-and-cover or diaphragm wall construction method. The TBM bored tunnel will be excavated at a depths of up to 30m.
2.2.1 TBM Excavation
The approx. 3.4km-long tunnel will be excavated by a slurry shield TBM with an outer diameter of 11.70m. The tunnel will be bored in four standard drives (lengths of 636m; 1,011m; 1,402m, and 317m).
The TBM will drive through the four backfilled excavation pits of the emergency exit structures. When the TBM reaches one of the three later stations, it enters into the completed excavation pit, is pulled through and then exits to begin the next standard drive, for the stretch of tunnel beyond. In its final state, the target shaft is converted into the Heukoppel emergency exit.
To drive the TBM safely through the diaphragm walls of the launch shaft, the stations, the emergency exits and the target shaft, the construction solution is to install artificial soil blocks, as single-phase walls, in the transition area between the diaphragm wall and the subsoil.
Shield cradles and back-up cradles will be installed for the TBM’s approach and passage through the excavation pits of the stations.
The lining of the bored tunnel is steel fibre reinforced concrete (SFRC) rings (7+0) formed of 500mm-thick segments, 1.50m wide. Polypropylene (PP) fibres are used in the production of the segments to ensure fire protection requirements are met. The joints are sealed against groundwater by means of pressurised waterretaining sealing frames.
2.3 Implementation of Sustainability Approaches in Planning and Construction
Hochbahn U5 Projekt GmbH has developed a two-pillar reduction strategy for planning and construction in order to make Hamburg‘s mobility more environmentally friendly, and not just during the operation of the U5.
The first pillar comprises the optimisation of all planning services, and the second the minimisation of CO2 during construction of the U5. This has already led to successes in the use of materials and planning optimisation for the first section of U5, currently under construction.
These are primarily the result of the consistent implementation of the following measures:
- Contracts for construction are only awarded to companies that are committed to using environmentally-friendly building materials, in particular by using the most environmentally-friendly cements and steels available on the market;
- Saving material quantities through adapted construction methods and processes;
- Implementation of sustainable land management, including logistics; and,
- Exclusive use of green electricity.
For example, the mandatory use of environmentallyfriendly steel and cement has been included in the tenders and contract awards for the first section of the Hamburg Line 5 project.
As a result, only reinforcement steel with highly recycled content (maximum 500kg CO2e/t) has been used in the first two construction lots, since 2024.
In construction Lot 2 (launch shaft U5 to Bramfeld station), only reinforcement steel with a maximum of 400kg CO2e/t will be used – and this is one year earlier than initially planned.
By the end of January 2025, 32,000 tonnes of ‘green’ reinforcing steel and 180,000 m3 of clinkerreduced concrete had already been ordered and partially installed. The successes are the result of an intensive exchange and close cooperation with the steel and concrete industry. The U5 strategy is having an impact and sending a strong signal to the industry.
By saving on materials, a reduction in GHG potential of around 30% was achieved in the design planning for the emergency exits in the City Nord to Jarrestraße construction section (see Figure 5). At the same time, the width and/or depth of the platforms at the stations have been reduced in some cases, which should also result in less material being used. These initial optimisations will also be applied to the other planning sections of the U5.
In addition to these successes, which have already been achieved in the planning phase, Hochbahn U5 Projekt GmbH is initiating the first individual approval in Germany for SFRC segments to be used in construction. Thanks to their material composition, these contain around a third less steel and therefore have a significantly better CO2 balance.
The first pilot tests for the electrification of construction site logistics took place in 2025.
3 OUTLOOK FOR PLANNING, CONSTRUCTION AND SUSTAINABILITY
While construction of the first U5 section is underway, the design planning for the remaining 17km-long section is continuing. In addition to the structural planning of tunnels and stations, the planning also involves construction methods and processes, logistics, traffic concepts and scheduling, as well as the identification and implementation of measures to reduce CO2 emissions. Along with the optimisation potential that has been identified already, further measures are being identified and implemented to bring about an additional reduction in CO2.
Joint reduction targets were discussed with the industry. In the course of the project, the first interim targets have already been achieved earlier thanks to the partnership-based cooperation between industry, construction companies and clients (see Figure 6).
Lead markets for the decarbonisation of the cement, concrete and steel industries are being called for by industry partners and politicians. For U5, the requirement for sustainable products is being stipulated for the first time in tenders and implemented during construction.
In future, electrified trucks, excavators and wheel loaders will be used in test operation on the construction sites of the U5 project, thus gaining initial experience for their practical use. Hochbahn U5 Projekt GmbH assumes that there is great potential for savings here, too.
It is expected that even more potential can be leveraged at material level, which will become apparent in the further course of the project and would have a positive impact on achieving the target of 70%. This finding is based above all on the ongoing exchange with industry representatives and the contractors of the first construction phase.
The savings in construction and planning are documented in the annual balance sheet in the sustainability report and have now also been included in the risk management for the U5. This makes it possible to transparently measure the CO2 reduction measures for the remainder of the project. The target scenario drawn up two years ago is consistently compared with the annual balancing of the CO2 emissions actually occurring during construction and the expected CO2 emissions for the further planning stages. This ensures ongoing quality assurance and the achievement of the reduction target.
This means that all the parameters for reducing CO2 emissions during the construction of the U5 are set: sustainability has been integrated into the planning as an equally important criterion and is always taken into account. Thus, the maximum technically possible reduction measures can be considered at an early stage and ultimately implemented during construction.
In addition, solutions are still being developed together with the industry to consistently drive forward the development of sustainable building materials. It can be assumed that the level of CO2 emissions saved during the construction of the U5 can be further increased by the progress made in the coming years.
