Anyone visiting the heart of the city of Hamburg, HafenCity (Harbour City), usually looks at the impressive new and old buildings – or at the wide water. But it is also worth taking a look at the ground. Only those who master the skill can build in a harbour – with sand, storm surges and an inrush of water.
Hamburg’s HafenCity is a place of superlatives: it is the largest development project in a European city centre. There are only a few similar projects worldwide. By 2025, mainly residential and office space will be built there on an area of 157 hectares. And there will also be space for shops, schools and even a university.
In the heart of the new quarter, right next to HafenCity University, three striking buildings were erected in 2017 – the IQ Towers. Particularly striking among them is the 70-metre-high office tower. It not only has 18 upper floors, but also 2 basement floors. All three buildings are connected via the two-storey underground car park – a special challenge for a harbour site. In addition, the U4 underground line crosses the ground directly below the underground car park. Close to the mouth of the Elbe River, the construction work was also exposed to high and low tides as well as storm surges.
So there were major challenges even before the first stone could be laid: How should the excavation pit be secured and kept dry?
Stefan Reich, HPC branch manager in Hamburg, took on this challenge with his team. The engineer has over 30 years of experience in building important infrastructure next to and partly in the water. With his team, he develops concepts for insular sewage treatment plants, flood protection, bridges or marine facilities.
Trained eyes on the bottom of the riverbank
On behalf of the client, ECE Projektmanagement GmbH & Co. KG, HPC AG carried out geotechnical, hydrogeological and waste engineering investigations and planning. The HPC staff found hardly any contaminants. This is mainly due to the fact that this area has a special history. Until the end of the 19th century, the old timber harbour was located here. When it was no longer in use and additional storage space was needed, the basin was filled with clean sand and enclosed with sea walls. Sheds and halls were then built on the newly reclaimed harbour area and used for handling and storing goods until the 1990s. Ultimately, contamination remained only near the surface. The results of the subsoil and groundwater investigations were to be expected: when analysing old boreholes and conducting pressure tests, the investigation team found that the building ground consists of loose sand up to a depth of ten metres, with organic material and silt layers from the old harbour bottom below.
Only at a depth of 13 metres do the denser, load-bearing sand layers begin. To determine how high and low tides affect the groundwater in the subsoil, the HPC field team then equipped observation wells with data collecting instruments. These recorded the changing water levels continuously over several months. Using this data, the HPC engineers were able to create hydrological models of the expected excavation pit water levels in relation to the water levels and tide of the Elbe.
With the analysis results and models, the HPC team was then able to advise the client on geotechnical engineering and cost estimates. During the subsequent implementation of the project, HPC was also commissioned by the general contractor, BAM Deutschland AG, to closely supervise all geotechnical work and to take on the role of flood protection officer.
The IQ Towers: built on sand – but safe
Since the first layers of soil can hardly support a building, pile foundations were necessary. These were drilled 28 metres deep into the load-bearing sand to provide the necessary support for the heavy buildings. Since only very low building loads can be carried above the underground train line, a pile construction bridging the train line was necessary. At the same time, the Elbe flows only six metres away from the excavation pit and is influenced by the tide.
The difference between high and low tide there is normally already over 3.6 metres. In addition, high water from storm surges can overlap the tidal flood levels several times a year. So the task was to secure the excavation pit and keep it dry in a very confined space, taking into account low tide, high tide and storm surge, in order to create the pile foundations for the buildings. To prevent groundwater from breaking through the bottom of the excavation pit, automatically controlled wells were used. Neither too much nor too little water could be pumped out. The amount of water to be pumped out was calculated by HPC beforehand.
November storm surge 2015: water ingress and rapid action
Only the tourists see the beauty of the harbour exclusively. Hamburgers also know the pitfalls of the river, which is regularly hit by storm surges. The so-called storm surge season is between 15 September and 31 March each year. The port of Hamburg is explicitly counted among the areas at risk.
Consequently, storm surge protection is required in Hamburg, which was therefore also designed and implemented for the IQ Towers. HPC advised BAM on the creation of an effective surge-protection system consisting of large encased watertight sand-filled bags, filters, filter mats, ballast loads and slope protections.
But reliable storm surge protection is not planned, built and then left to its own devices all at once. On the morning of 30 November, during a storm surge, the construction company noticed a small leak in the excavation pit behind a bank wall. This was initially deemed not to be of great concern at the construction site and was provisionally covered with a seal.
Stefan Reich went to the construction site on the same day and quickly realised: the weak spot was highly dangerous – especially during the storm surge season. During the second storm surge, which happened at the same time as the evening high tide, much more water was leaking out. The water from the Elbe threatened to erode the soil at the site and break through the bank wall.
November storm surge 2015: water ingress and rapid action
The HPC team reacted the same night: the leak was secured against erosion with a large-area ballast filter made of gravel on a geotextile base. Its weight additionally stabilised the water entry point. Over the next few days the construction work only had to be interrupted briefly to pump out water that had already entered. Afterwards, HPC designed a strategy for the weak spot and its long-term protection for the time after the completion of the buildings. Subsequent examinations revealed that the excavation protection in the run-up to the small water entry had no deficiencies whatsoever. However, it turned out that the pile foundation wall itself had a previously unknown leak, through which the high rising Elbe water could enter unchecked.
At the same time, it became clear once again that monitoring and experience are crucial to withstand the storm surges of the north of Germany. The buildings have long since been completed, and the underground continues its regular service beneath them unperturbed. From the upper floors of the IQ Towers you have a magnificent view over Hamburg’s harbour and the Elbphilharmonie concert hall. Few underground riders, residents or visitors know what know-how and how much experience were necessary to design this site. Those who know about the work of the Hamburg HPC team are sure to look at this imposing corner of Hafencity Hamburg with a different eye.