Bentley Incorporated : Under pressure

GEOTECHNICAL SOFTWARE

Under pressure

Digital analysis was applied to a complex excavation pit design for a Stuttgart skyscraper. Nia Kajastie reports.

Anew 66m tall skyscraper - the Turm am Mailänder Platz or the Tower on Milan Square - overlooks the European

quarter of Stuttgart in south west Germany.

Work to build the tower on a triangular, 1,800m2 site close to Stuttgart Central Station began in 2018.

Developed by Strabag Real Estate, the around E150M (£128M) high-rise is scheduled to be completed towards the end of this year.

The building covers a total floor area of 21,000m2 and will house two hotels, retail and dining space, and an underground parking garage.

Construction firm Ed Züblin was contracted to build the tower, while the ground engineering subcontractor for shoring and foundation works was Züblin Spezialtiefbau.

Both firms and the developer are part of the Strabag SE Group.

Züblin's technical office for geotechnical engineering was involved in the approval planning and the detailed excavation design. The main challenge was presented by two operational light rail underground tunnels that cross the construction site diagonally.

They run directly under the second

basement level of the building, while the third basement level has been built directly next to one of the tunnels.

EXCAVATION DESIGN

The retaining systems chosen for the excavation pit include an existing bored pile wall underneath a road tunnel to the north of the site; a king post wall to the east; a king post wall and the rail

tunnel to the south; and a king post wall to the west (see figure 1).

Some parts of the base slab for the second underground level are only 20mm above the light rail tunnel roof. To avoid unacceptable load conditions and deformations, void formers are used here to create a load-free separating layer through which a load decoupling of the tunnel roof is achieved.

Next to and between the two tunnels, loads from the base slab are transferred into the subsoil with bored piles to minimise entrainment settlements of the tunnels.

The contractor had to almost completely expose one side of the northern tunnel so that it could build the third basement level. This was a different method to the one initially required.

Originally, the plan was to excavate on both sides of the tunnel, in line with requirements of Stuttgarter Straßenbahnen - the city's transport operating company that owns the tunnels. It had said that excavation had to be carried out on both sides of the tunnel to avoid it being subjected to asymmetric horizontal earth pressure.

However, as the tunnel runs diagonally through the site, this would have complicated the alignment of the retaining wall at the interface with the tunnel and led to a significant increase in the excavation area outwards towards a neighbouring building.

Züblin geotechnical engineering office team leader Jörg Schreiber explains: "The public library is located next to the excavation pit, and if we had to excavate the tunnel on both sides, then the retaining wall would have had to change its direction into the road next to it. It

Footprint of the new building

would have meant, we would have needed to excavate into the road and towards the entrance of the library, which would have had a big impact on the construction process, on the library and the cost of the project."

To address these issues, the main contractor deployed digital analysis and calculations to figure out the most efficient excavation strategy and this led to the decision to excavate on only one side of the tunnel.

3D MODELLING

Working with Bentley Systems software, Züblin produced a 3D model to simulate every excavation phase of the project.

"Fortunately, we were involved in the first stage of the design - which is not the case on all projects. But this time, because all the parties involved in the project were from the Strabag group, we

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ding

Underground tunnels

Left: Overview of the project site from May 2019

schubert niels photo:

supporting berm could then be removed. The foundation piles of the new building were also included in the model.

have been involved from an early stage," Schreiber says.

"Because the first option would have meant excavating the whole road, the second option we looked at was to not avoid the asymmetric earth pressure.

"And we tried to prove that we can successfully excavate only one side, and that this is not dangerous for the tunnel

- that was the most important thing." Extensive 3D numerical investigations

were carried out with Plaxis 3D to determine whether asymmetrical loads could be permitted.

Based on these calculations, the expected tunnel movements were shown to be within safety limits and the stability of the tunnel with the one-sided load matched expectations.

The two tunnels at the site had been built using a cut and cover method and consist of 10m long segments connected

at the joints with mineral fibre boards. "As one of the tunnel blocks is

diagonally located in the 14m high retaining wall, this was really a spatial, three-dimensional problem.

"This is why you can't use simple analytical tools. In this case, software like Plaxis 3D can take into account that at least the edge of the tunnel block is loaded by a huge earth pressure from this side, because the road level is quite high compared to the excavation," Schreiber says.

As this tunnel segment was exposed to a high earth pressure load, two steel struts were installed under the protection of a temporary supporting berm. This connected the tunnel roof to the basement slab that had been constructed in advance in some areas next to the supporting berm. This connected the tunnel section to the basement slab, the

Top: The building should be finished by the end of this year

They act as lateral support for the tunnel to reduce potential horizontal displacements.

The finite element model generated with Plaxis 3D was used, among other things, to accurately assess the effect of the foundation piles as a deformation- reducing measure. For this purpose, the piles were defined as cylindrical solids and were generated with corresponding volume and interface elements.

The 14m high retaining wall was built as a soldier pile wall with four layers of prestressed grouted ground anchors.

Installing the anchors had its own challenge. Various utilities - including power lines, sewers and district heating pipework - the library, both tunnels and the required safety clearances had to be considered.

The 3D model of the pit simplified the planning of the shoring so that the anchors could avoid the structures and utilities. The complex layout of the site meant that some horizontal anchors Q

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GEOTECHNICAL SOFTWARE

• had to be installed as a steep angle was not possible.
  However, in some cases the required anchor load could not be reached, and the anchors had to be replaced.
  Züblin geotechnical office team leader Erik Linke worked on the anchors specifically. He explains: "It was a very difficult area for ground anchoring because of the narrow section between the shoring, library and the tunnel. Some anchors - I believe one or two - did not reach the required load and that means they had to be replaced. This was at a critical stage of the project towards the end of construction."
  The 3D model offered the required information on the new anchor inclination and attachment points for the replacement anchors.

GROUND CONDITIONS

The original site included a steep 7m high embankment leading up to the entrance of the public library.

The subsoil below the embankment includes artificial fill up to approximately 14m below the street level in front of the library. Below this level, there are gypsum Keuper layers, starting with the Bochinger Horizont, which consists of silty to gravelly silty claystones with a low to very low compressive strength. It is only occasionally overlain by the silty claystones of the dark red marl.

Underneath lie the silty mudstones of the so-called ground gypsum layers, and Lettenkeuper follows below the basement gypsum layers.

As the design water level during construction was below the excavation level, no measures for lowering the groundwater within the excavation pit or watertight shoring were required.

King post wall		City library
4 levels of anchors
Existing bored piles		14.2m

Existing road tunnel

2 Underground levels

3 Underground levels

	Underground tunnels
Footprint of the new building	(in operation)

District heating pipeline

Sewer

Excavation

pit

Basement of city library

Due to work already carried out before 2017 during the construction of the rail tunnels and because of the former use of the site, it was difficult to estimate areas where natural soil was still present and where backfilling had already taken place.

MONITORING MOVEMENTS

As there were different excavation depths along the tunnel and its inclination, the expected deformations and the stability under the one-sided load had to be determined for each tunnel segment.

In the end, it was calculated that due to the subsoil properties and the support

piles,theexpectedhorizontaldeformations were only about 4mm.

Predicted heave due to the stress relief during excavation and uncovering of the tunnels, causing the tunnels to move slightly upwards, was also expected to be 4mm.

During the project, a tunnel monitoring system was active. It found that the maximum heave of approximately 4mm corresponded with the prediction. For the horizontal tunnel movement, the measured values were a maximum of around 2mm.

As a result, neither the horizontal

Top: Figure 1: Excavation pit Bottom:

Boundary conditions next to the city library

trigger value of +/-5mm or the vertical trigger value of +/-10mm was reached during excavation and the movements were within the safety limits set for the tunnels.

Using its analysis, Züblin had been confident that excavating and exposing only one side of the tunnel was possible and safe. This decision simplified the whole construction process, and due to the accurate modelling and analysis, it was also able to gain the trust of the tunnel owner.

The last part of the foundation work was completed in March 2019.

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