Construction of a luxury skyscraper on a constrained site in the heart of London called for precision engineering and a suite of digital tools.
The addition of yet another high rise development to London's skyline is not particularly uncommon. But the construction of the 50 storey Principal Tower, which includes two basement levels, required an innovative engineering approach, as it faced numerous site constraints and the challenge of protecting neighbouring rail assets.
Commissioned by a joint venture partnership of developers Brookfield, Concord Pacific and W1 Developments, the building's layout features an unusual design, comprising three shapes arranged to make a cruciform measuring less than 25m by 25m.
Within the mixed-use building, amenities include a pool, spa, gym, cinema and lounge. The tower is part of the wider Principal Place development, which includes smaller mixed-use buildings and Amazon's new London headquarters.
Above the tracks
The tower is above the six rail lines feeding into Liverpool Street station, so its design had to mitigate any impact on the railway and a protected corridor for the addition of two more tracks. Securing Network Rail's permission was vital to ensuring the Principal Tower could proceed. In addition, the structure is very close to a masonry tunnel housing Transport for London's (TfL's) Central line.
While conventional approaches might call for arches or A-frames to bridge the rail lines, architect Foster & Partners' vision required a solution that hides the railway lines which run below the 163m high tower, adjacent to its basement levels.
The developers appointed consultant WSP to produce the structural design for the £200M project and to provide a guide for construction. The design involved forming the sides of the protected rail corridor from 1.5m diameter piles and 6m deep reinforced concrete capping beams 6m deep across the railway, effectively creating a 9.1m wide tunnel.
The support piles had to be constructed as close as 50mm from the Central line masonry tunnel which dates back to the Victorian era.
WSP also had to consider the frequent vibrations caused by railway operations 24 hours a day, seven days a week, as well as reducing soil displacement.
Due to the orientation of the building, eight 50m deep piles were needed on the eastern side of the tower. As there was insufficient depth to carry loads from the eastern columns of the tower directly onto the eight track tunnel structure, WSP instead created a maze of reinforced concrete walls to transfer loads from level 7 of the building down to piles.
WSP associate director Nello Petrioli explains that one of the conditions of getting approval from Network Rail was an assurance that the railway would remain unobstructed during the project. This added to the construction challenge.
"There was no opportunity to close the railway lines and undertake strengthening work, for example. Working closely with Network Rail, understanding their operational constraints and then trying to adjust our design… to work with their operational constraints and the physical obstacles was absolutely key," he says.
As well as the subterranean challenges, the team had to consider planning constraints which protect views of key London landmarks, including Westminster Abbey and St Paul's Cathedral. This had an effect on the tower's exact location.
"Positioning a tall building on the constrained site was the biggest challenge. It was also important to maximise the footprint because that determined how many apartments we could have and how slender the tower would be," says Petrioli.
There was no opportunity to close the railway lines and undertake strengthening work, for example
To tackle the project's construction constraints, the team chose to undertake a top-down construction method where the internal structure (ground floor slab?) is used as temporary propping to support the basement? excavation below. This approach was also favoured by the client, as it allowed some of the above ground sections of the building to open sooner while work was underway.
"As we were building right next to existing Victorian tunnels and removing soil from [around] those tunnels, there were concerns about instability [caused by heave?]," says Petrioli. "So, the ground floor slab was key in helping to resist that movement. It acted as a temporary prop at the same time as being the ground floor slab."
Petrioli explains that a detailed ground movement assessment had to be undertaken to ensure that Network Rail and TfL infrastructure assets would not be impacted by the project.
"Quite early on, we had to do that analysis to demonstrate to Network Rail and agree with them the loading criteria and the movement criteria that we were working towards," Petrioli says.
"So, as we load the soil next to those assets, what would the impact be? What would the movement be and would there be any adverse effects structurally to those assets?"
WSP selected Bentley applications Ram Concept and Plaxis to complete the design and analysis of the complex design elements.
"Most traditional design software comes from the world of academia, but Bentley software focuses on the real work of engineers on the design side," explains Bentley product marketing manager Jana Miller. "The software focuses on code checks, member selection and design, and the process of producing design documents for construction."
Miller adds that one key benefit of the software is that it considers the design requirements for building codes. "Ram automates those tedious and time consuming tasks for users, so they can work more productively and profitably," she says.
"Plaxis works across a large breadth of geotechnical engineering projects. It provides analysis of excavations, foundations, embankments, dams, tunnels and slopes. The calculations and automation tools help users perform any kind of geotechnical analysis. And it provides seamless integration of analysis methods - both limit equilibrium method and the finite element method - so users can collaborate on all sorts of infrastructure projects."
The tower is 50 storeys high
Using Plaxis, WSP was able to analyse multiple potential designs and model soil movement during excavation, construction and the building's lifecycle.
"By undertaking both 2D and 3D analysis, the team confirmed that constructing the ground floor slab first would create a horizontal prop and provide lateral support to protect the masonry tunnels during underground construction.
According to Petrioli, the quality of information gathered through Plaxis also made it easier to demonstrate the ground movement assessment "visually as well as through numbers" to Network Rail and other key stakeholders.
Armed with the information gained through the Plaxis analysis, WSP used Ram Concept for all stages of the foundation and basement design, right from early stage concept through to final detailing.
Positioning a tall building on the constrained site was the biggest challenge
Enabling works started in 2008, but delays to the project due to the financial crisis meant that construction on site did not ramp up until 2015.
Looking back now the work is completed, Petrioli says that while the project was immensely challenging, it was "fantastic" to be involved with the process. "Not just from the technical design side but also the individuals involved. It was a very good team and the relationship between the client, the architect, the structural engineer and contractor made a very challenging project happen," he says.
"The key lesson I have taken from the project is about collaboration and bringing different disciplines to the table to come up with holistic solutions."
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