A shopping mall in Berlin with a chequered history was the subject of a complex propping project involving the use of Groundforce’s largest propriety MP750 struts incorporating the latest hydraulic nut technology.
Groundforce supplied the propping solution for the basement construction of a prestigious €28 million new mixed-use development in the Mitte area of the German city.
The joint venture project, which started in 2017, is being delivered by Implenia, Stump and Keller, under the name of Arge Bau Grube Tetris, Keller undertook the commercial lead and Stump the technical, while Implenia took the lead on the project design at the tender stage.
Essentially the joint venture was to construct two basements with a total area of 21,000m2.
The site was previously occupied by a shopping mall which was built in 1915 and featured a central glass dome. Since then it has had various uses as a cinema, a nightclub and by artists following demolition of the Berlin Wall before being itself partly demolished in 1986. The new buildings will be built around four courtyards and will range from 10 to 12 storeys high, including basement levels with mixed residential, retail and office usage.
The site was irregular in shape and wrapped around some existing buildings. This, combined with pre-existing basements and underground obstructions, as well as its sheer size, presented multiple challenges.
The basement construction used diaphragm wall techniques being in the main, retained by a layer of strand anchors. There was one area however where the existing basements and the adjacent underground metro line meant that anchors could not be used. For this section of the site Groundforce supplied their 750-tonne capacity props - the largest to be used in mainland Europe to date - as flying shores, to span the 58m wide excavation.
The close proximity of listed buildings with basements on two sides plus a busy road with the underground metro line under on another side dictated the need for a “stiff” propping system to minimise deflection of the d-walls.
Berlin is built mainly on sand and this brings challenges when dealing with groundwater. Lowering of the groundwater was not an option as many of the surrounding buildings were supported on wooden 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 meant that uplift anchors were also constructed to overcome buoyancy.
Before work on the excavation could begin Stump performed a sealing test. The site was divided into six areas – none bigger than 4,000m2 – and a pump test was carried out to check the cut off. The aim was to limit the flow to not more than 1.5litres per second per 1,000m2 area.
The anchors meant an open excavation was possible over most of the basement pit but not the U5 subway which is below Freiderstrasse (behind the site) and the two-level basement in the adjacent building meant anchoring was not possible.
For the excavation in this area, Groundforce’s 750-tonne capacity MP750 props with twin intermediate supports were used.The supports provided a twofold benefit, primarily to increase resistance to buckling of the props over the 58m span and secondly to aid the removal prop process on completion. 12No. props were used in total, with four bracing across each corner and four stretching across the full 58m span.
One feature of MP750 struts is that they can be accurately pre-loaded to high levels using their in-built hydraulic locking nuts. This feature satisfied the geotechnical engineer’s demand for specific levels of deflection reducing preload to be induced into the props without having to resort to complex and expensive remote jacking systems. The recently developed dedicated hydraulic nuts were used to pre-load each prop in increments to approximately 3,000KN to minimise the risk of the diaphragm wall moving as the basement was excavated.
The props were installed in January 2018 and removed in January 2019.
Despite structural steel being the main excavation support solution in mainland Europe, proprietary props were considered to be the best approach for the Tetris scheme. Groundforce’s hydraulically operated props allowed for significant pre-loading which could not have been so easily achieved using a structural steel propping solution.
The stressing was done in winter, with temperatures of around -5°C dipping down to -12°C, therefore there was a potential issue with thermal expansion as the build programme moved into spring and then summer. The hydraulic nut system could theoretically allow the pre-stress load to be decreased which again could not have been achieved using a steel solution.
The on-board load monitoring system installed on the props was essential to the solution as the level of pre-loading required could only be verified with a monitoring system.
The loading was carried out in three stages over a 10-day period so the response of the diaphragm wall could be monitored.
This development was also the first site in Germany to benefit from Groundforce’s hydraulic nut technology which allows for higher pre-loading and creating the opportunity for the load to be altered, as well as making it easier to remove the props.
The project provided Groundforce’s technical team with some interesting data which has given them valuable insight into the relationship between pre-load and prop stiffness.