1 | The location of the construction site in the middle of Munich‘s city centre is a logistic and organisational challenge. The photo was taken during the concreting of the last deck section
Credit/Quelle: DB/ARGE Marienhof
Around 6 million people live in the Munich region, 1.5 million of them in the city itself. With an increase in employment of around 23 % over the last ten years, the southern German metropolis is one of the fastest growing regions and commuter capital of Germany. Commuters extensively use the well-developed public transport network. One particularly busy section is the 11.3 km long 1st S-Bahn main line, which was opened in 1972 and runs through 4.3 km of the Munich city centre in tunnels.
It was originally designed for 250 000 passengers/day, but by now 850 000 people a day use the S-Bahn, which runs every two minutes – making it Germany‘s most frequented double-track railway line during rush hour. A new west-east express link to the main hubs is intended to provide a substantial improvement in capacity: the 2nd S-Bahn main line. The 10 km long new line runs parallel to the existing main line from Laim station in the west, descends into a 7 km long tunnel before Donnersbergerbrücke, stops at three new stations and resurfaces before Leuchtenbergring station in the east (Fig. 2).
2 | Alignment of the 1st main line (green) and the new 2nd main line (red) with tunnel sections (dashed) as well as the three new stations and two stations to be converted (red)
After a multi-stage tendering process, the client Deutsche Bahn awarded the contract for this construction lot to the JV Marienhof at the end of December 2018. The consortium consists of Implenia Construction GmbH, Munich division (37.5%, technical management), Hochtief Infrastructure GmbH (50%, commercial management) and Implenia Spezialtiefbau GmbH (12.5%). Lot VE 41 comprises the construction of the Marienhof Station using the cut-and-cover method, the station tunnels and the connecting tunnel to the existing underground stations of the U3 and U6 lines applying conventional tunnel construction methods.
2 Marienhof S-Bahn Station – A Large-Scale Project
“The construction site, which is roughly the size of a football field, is more or less sandwiched – above ground by prominent neighbouring buildings, a pedestrian zone and narrow streets, as well as underground by the two existing metro tubes of lines 3 and 6,” says Jens Classen, technical project manager of the Consortium Marienhof, as he explains the exceptional project (Fig. 3).
3 | Appearing like a toy when viewed from above, the boom of the left crane actually has a length of 55 m
Credit/Quelle: DB/Jürgen Stresius
The central access structure of the Marienhof, some 100 m long and 60 m wide, is being constructed below the groundwater level using the diaphragm wall/cut-and-cover construction method. Extending to the west and east, compressed-air excavation will be applied for the 60 m long platform tunnels of the S-Bahn, each of which will run directly underneath the existing metro lines (Fig. 4). In total, the S-Bahn platform will reach a length of 220 m. The connection to the metro station will be made in the southern direction towards the town hall, also using conventional construction methods.
4 | Model of the access structure with the 60 m long platform tunnels of the S-Bahn, extending to the west and east under the existing metro lines and surface buildings
Starting in 2023, the compressed-air drive will run underneath the mezzanine level of the metro and connect to the two former shafts that were built in the early 2000s for the ground freezing measures required for the platform extension under the town hall.
Marienhof Station is the deepest construction measure in the tertiary ground formations that has been carried out in Munich so far. A filling layer is located in the upper area. The further geology is characterised by predominantly fine to medium sandy, water-bearing layers partially containing silty, slightly clayey material. This is followed by an aquiclude consisting of silt and clay, partly with calcareous concretions and friable clay (clay with slickensides); further down, claystone and sandstone are encountered. The entire construction project is located in groundwater, with alternating water-bearing and impermeable layers.
The layout of the construction site installation area is of particular importance. The construction site traffic is organised by access authorisations as part of an online transport notification system in order to avoid conflicts with other transports. Due to the confined space of the construction site, relocations of materials, machines and working areas have to be announced and discussed almost weekly. The main access route is in the east (up to 90 trucks/day), and the route for long and heavy transports in the northwest leads through a pedestrian zone (max. 8 trucks/day).
In order to avoid truck traffic with ready-mixed concrete trucks and to guarantee a reliable supply, a concrete mixing plant with a capacity of approx. 650 m3 of concrete was additionally installed in the south-east of the construction site.
Two up to 75 m high tower cranes with 55 and 65 m long jibs are used to service the entire construction site on the surface as well as all logistic openings in the concrete deck. A duty cycle crane with a 7.5 m³ grab is used for the vertical disposal of the excavated material from the various levels.
5 Geotechnical Monitoring
Numerous measuring points were installed in the metro, on buildings, in the streets around the construction site and in the excavation pit itself. All measurement data that is generated is displayed together and automatically visualised in real time. If threshold values are exceeded, an automated alarm is triggered. A total of 1.5 km of inclinometer pipes, 2 km of boreholes and 4 km of cables have been installed for geotechnical monitoring.
Until 2021, almost 130 wells and water gauges have been installed to a depth of approx. 60 m for dewatering. At the time of the site visit, the first stage of dewatering was underway with the drainage of the two uppermost groundwater horizons for the first excavation works.
Inside the diaphragm wall, 15 wells and gauges are being used to lower the groundwater for atmospheric construction operations for excavation and civil engineering. Outside the diaphragm wall, the groundwater is reduced to relieve the diaphragm wall excavation pit statically during the construction phase.
7 Construction Progress
The 54 m deep diaphragm wall to secure the excavation pit and the subsequent 68 m deep primary supports, which will carry the decks during the construction period, have been completed. The deck of the excavation pit, the so-called level 0, has also been completed, level –1 has been excavated and the concrete paving of the deck for level –2 started at the end of May 2022 (Fig. 5).
5 | Construction progress at the end of May 2022: Level –1 under the deck has been excavated. The exposed temporary primary supports (rectangular cross-section), the wells and gauges (thin, round cross-section) as well as the collision protection in the surface area can be seen
Credit/Quelle: DB/Jürgen Stresius
7.1 Diaphragm Wall
Before the diaphragm wall could be constructed, 56 boreholes had to be drilled in the southwest of the construction site in the area of the containers at depths of 14 to 31 m in order to remove obsolete existing anchors from the metro construction. The boreholes were backfilled with artificial soil or gravel.
For the construction of the 1.50 m wide diaphragm wall, it was necessary to build a temporary guide wall to steer the diaphragm wall cutter. The 2.50 m high reinforced concrete wall surrounded the excavation pit over a length of around 320 m.
It ensured the minimum height of the suspension level in the upper area and partly served as bracing for the preliminary shoring. A diaphragm wall cutter and a diaphragm wall grab were used to construct the diaphragm wall. The diaphragm wall was designed as a statically highly loaded, watertight excavation pit shoring. It protects the excavation pit, which is the size of a football field, from groundwater down to a depth of 54 m and consists of 110 slabs that are 3.20 m long and 1.50 m thick. A total of 15 800 m2 of diaphragm wall was built. Each slab is highly reinforced with 41.4 t of steel, which was delivered in 3 to 4 individual cages up to 18 m long and assembled on site (Fig. 6). The concrete placement for one slab with approx. 260 m3 concrete of grade C20/25 took about six hours.
6 | Lifting an approx. 18 m long reinforcement cage into a diaphragm wall slab with the duty cycle crane. In the front the diaphragm wall cutter can be seen
Credit/Quelle: DB/Jürgen Stresius
7.2 Primary Supports
A total of 50 primary supports bear the loads of the deck slabs during the construction period. Originally planned as double T-girders, they were designed as box girders for structural reasons. Due to their heavy total weight of 31.2 t, they were delivered in parts, assembled on site and then lowered from the ground level to depths of 61 to 68 m (Fig. 7). Each foundation consists of an approx. 18 m long reinforced pile base, which supports the 54 m long support. Some of the primary supports have been fitted with extensometers to detect possible ground heave effects during excavation in the highly overconsolidated Munich subsoil.
7 | Bolting of a primary support, which was sunk to a depth of up to 68 m, including the approx. 18 m long pile base
Credit/Quelle: DB/Jürgen Stresius
7.3 Compensation Grouting
Since the beginning of 2022, the consortium is planning the compensation grouting to the east and west of the excavation pit from the upper and lower areas of the building to counteract possible settlements. Here, about 15 000 running metres of guided drillings from levels –1 and –2 are to secure the existing buildings. In addition, approximately 3000 running metres of unguided drillings from level –4 will secure the tubes of the metro lines U3 and U6.
7.4 Concrete Deck and Bracing Levels
The construction of the 1.20 m thick concrete deck slab and the various levels, including the temporary bracing levels, have been combined technically and for organisational purposes in the structural engineering section (KIB) 1.
After completion of the diaphragm wall, the entire deck (level 0) was constructed in the second half of 2021, as well as concreting and venting openings for the concreting of the walls underneath. Numerous fixtures and empty pipes have already been planned for each level and will be installed directly. The ceiling construction generally consists of an approx. 10 cm thick blinding layer of concrete, followed by linoleum in the exposed concrete areas. The concreting will be carried out in eight concreting sections. Additionally a protection layer has been cast on top of the deck, which contains pipes and cables from wells and measuring equipment that are led to the outside of the pit. As of the end of May 2022, level –1 has been excavated and the blinding layer of bracing level –2 is being concreted. In the western section of the construction site, a large space remains open in bracing levels –1 and –2 for a three-level golden cube as the architectural centrepiece of the structure. With half of the bracing level -1 completed, the excavation of the next lower level –2 is already underway.
The machines used underground, such as the tunnel excavator and crawler loaders, are fitted with a short tail to maximise space savings and avoid collisions with exposed primary supports, wells and measuring installations. The supply/disposal of water, air, electricity, spoil, equipment and access for personnel is provided through four temporary supply openings in the cover and in the levels below. The sandy soil is quite stable as it is over-consolidated and drained, and must be removed by machine (Fig. 8).
8 | Below the concrete deck, the soil is removed with a short-tail tunnel excavator and a crawler loader. The spoil is lifted out with the duty cycle crane through a deck opening
Credit/Quelle: DB/Jürgen Stresius
8 Compressed Air Tunnelling
The construction lot of the JV Marienhof includes the excavation of the platform tunnels with connection to the TBM drives approaching from the west and the east. The plan is for the two main drives to arrive at the Marienhof station after completion of the structure. Therefore, large, 10 m long light concrete blocks will be installed, into which the TBMs can enter and be dismantled.
8.1 Connecting Adit U3/U6
The platform level of the U3/U6 metro station will be connected by conventional tunnelling under compressed air up to 1.0 bar. The tunnel will be excavated from the temporary pressure chamber in level –3.
8.2 Platform Tunnels
Excavation of the western and eastern platform tunnels underneath the existing structures is planned to start from a temporary pressure chamber in level –5 using conventional tunnelling methods under compressed air up to 1.0 bar. Excavation and securing of the five-part vault cross-section with 430 m2 excavation area will be carried out in several partial headings. The lateral escape tunnels will be excavated first, followed by the central tunnel with its four partial cross-sections. Finally, the platform tunnels will be excavated successively with two partial cross-sections.
9 The Cube
A highlight in structural engineering is the cube with its gold-coloured X-columns, which extends over levels –1 to –3 (see Fig. 4). During the excavation work down to the invert, a highly complex temporary bracing structure made of prefabricated concrete elements is installed in this area, which will later be disassembled again.