Mega-basements usually mean deep excavations, but for one new development in Berlin it is the sub surface work that justifies its claim to be a mega-project
Final excavation of a 21,000m2 basement for a new mixed-use development designed by architect Herzog and de Meuron in the Mitte area of Berlin in Germany is currently underway.
While much of the mainstream media focus has been on the above ground structure, it has taken two years of carefully planned ground engineering to get the project ready to come out of the ground.
The below ground elements of the project have an estimated value of €28M (£25M) and are being delivered by a joint venture of Implenia, Stump and Keller under the name of Arge Bau Grube Tetris.
The joint venture, which started work on site in 2017, is focused on construction of the basement pit for the new development using diaphragm wall techniques. In the main, the diaphragm wall is being retained by a layer of strand anchors but there is one area where the presence of existing basements and underground metro line meant this was impossible.
For this section of the site Groundforce deployed the longest prestressed proprietary props used so far in mainland Europe to span the 58m excavation.
Stump project manager Jörn Lohse describes Berlin’s ground conditions as sand, sand and more sand. This brings challenges when dealing with groundwater.
Lowering of the groundwater was not an option as many of the surrounding buildings are supported on timber piles making them sensitive to groundwater level changes. Instead, a high pressure injection (HPI) grout blanket was constructed across the site to seal the excavation pit sufficiently to allow dry excavation and casting of the base slab.
The groundwater levels mean that uplift anchors are also being installed to overcome the natural buoyancy.
The site was previously occupied by a shopping mall – Tacheles – that was completed in 1915. When it was completed, Mitte was not prosperous, and the mall went bankrupt in 1918.
It has had various uses in the years since including a showroom for AEG.
It was heavily damaged during World War II and was partly demolished in 1986 and was used as a cinema, a nightclub and by artists following demolition of the Berlin Wall.
According to project architect, Herzog and de Meuron, the buildings are planned to be positioned around four courtyards, and will range from 10 to 12 storeys, including basement levels with residential, retail and offices. The site is irregular in shape and wraps around some existing buildings.
“It is a huge basement and the differential settlement behaviour is complex due to the size and shape,” says Lohse.
The shape combined with pre-existing basements and underground obstructions, as well as the sheer size of the site, has presented some challenges.
“The old foundations were up to 6m below ground level and extended below groundwater level too,” says Lohse. “We also found a number of old wells on the site – one was 3m in diameter and very deep.”
Groundwater is at 3m below ground level but the basement will extend to 12m.
The diaphragm wall, which was constructed by Implenia and Stump, is 15m deep but the top of the wall is 3m below ground level.
“We used a king post wall to excavate the site to 3m below ground level in order to remove the existing basement to above groundwater level,” says Lohse.
“We had to core through the line of the diaphragm wall too.”
The diaphragm wall panels are 0.8m thick and cover an 11,200m² area with 850t of steel reinforcement and 8,600m3 of concrete.
“The panels were between 3.4m and 9m in width depending on the loading from surrounding buildings,” says Lohse.
Work on the diaphragm wall started in March 2017 and was completed by October of that year.
The HPI blanket was constructed by Keller using mixed in place columns to create a 1m to 1.3m thick blanket starting at 18m below ground level with 4m thick sections below the diaphragm wall. In total, Keller constructed 3,800 columns to create a cut off.
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Stump also installed the anchors. There are 406 in total and these are up to 30m long and installed at 30° to 35° with an 8m fixed length to give a loading capacity of 1482KN.
“The total length of anchors installed at the site is 10.1km,” says Lohse. “We also installed 29.6km of uplift anchors, which are formed from Dywidag Gewi 050 micropiles.”
The uplift anchors were installed to depths of up to 30m from ground level ahead of excavation.
With the ground engineering complete, work turned to the excavation phase.
“We had to carry out a sealing test before we started the excavation,” says Lohse. “We divided the site into six areas – none bigger than 4,000m² – and undertook a pump test to check the cut off. The aim was for the flow to not be more than 1.5litres per second per 1,000m² area, but we achieved a figure far lower than that.”
The anchors meant that an open excavation was possible over most of the basement pit but not everywhere.
“The U5 subway below Friedrichstrasse behind the site, and the two level basement in the adjacent building meant that anchoring was not possible,” explains Lohse.
The joint venture worked with ground support specialist Groundforce to develop a solution for the area by Friedrichstrasse.
Groundforce major projects Europe director Peter Richardson says: “The excavation in this area called for Groundforce’s 500t capacity MP500 props with twin intermediate supports – one to stop buckling and a second was added to reduce the weight of the segments to aid the removal process.
“At 58m, they are the longest proprietary props ever used in continental Europe.”
The are 12 props in total with four bracing each corner and four stretching across the 58m span.
“The props were pre-loaded in increments to around 3,000KN to minimise the risk of the diaphragm wall moving as excavation progresses,” says Richardson.
The props were installed in January 2018 and are expected to be removed this month.
Despite structural steel being the main excavation support solution in continental Europe, proprietary props were seen as the best approach for the Tetris scheme.
“The props allowed for significant pre-loading which could not be done with structural steel,” explains Richardson.
Lohse adds: “The stressing was done in winter with temperatures of around -5°C, which dipped down to -12°C, so there was an issue with thermal expansion as we moved to spring then summer and the props allowed the prestress load to be decreased which we could not have done with steel.”
According to Richardson, the monitoring equipment installed on the props was essential to the solution.
“You can only do that level of pre-loading with monitoring,” he says. “It is too dangerous without.
“It is really important to understand the movement when working between existing, occupied buildings.”
The loading was carried out in three stages over 10 days so that the response of the diaphragm wall could be monitored.
The site was also the first big site to benefit from Groundforce’s hydraulic nut technology which Groundforce technical director Tony Gould says allows for higher pre-loading, as well as creating the opportunity for the load to be altered.
“They also make it easier to remove the props,” he says.
Lohse says that one of the other drivers for the use of the solution from Groundforce is the speed at which the props can be removed compared to structural steel.
When GE visited, the project team were planning the removal phase.
“The maximum weight of any section is 4t, so that they can be lifted out by the tower crane that has now been installed on site,” says Richardson.
While the joint venture still has work to do on the excavation, Lohse says that everything so far has gone according to plan. Work on the basement is expected to be completed soon, ready for the above ground work to get underway.