Ground engineering provides the foundations for the built environment and is therefore an important and essential part of civil engineering.
The Pinnacle Tower in the City of London will be the tallest building in the UK when complete - 62 storeys high (290m), with a deep three-level basement. Consequently the tower will pass very large forces to its foundations, which will comprise the deepest, largest foundations of their kind in the UK.
The Pinnacle Tower (Architects: KPF. Rendering: Cityscape)
Foundations for The Pinnacle Tower
The Pinnacle Tower is being constructed on a complex site. It had three existing buildings on the site, leaving the ground riddled with existing piled foundations and basement structures which were obstructing any new works. If an obstruction were to be hit unexpectedly during new constructions works, there could be long delays and additional constructions costs that would threaten the entire job. Quantifying these obstructions at an early stage was therefore critical to keeping the project on time and budget for the client, Arab Investments Ltd.
To achieve this, the engineer designers, Arup, carried out extensive research (searching archives, talking with old contractors) as well as undertaking intrusive investigation (coring and probing of existing foundations, geology and basement structures) on the site to find out more details about the old building.
It was not possible to support the weight of such a huge tower using normal London foundation design and construction methods. The designers had to think out of the box and come up with an innovative solution to the problem. The result was a decision to build the deepest and largest diameter piles in the UK. The designers had to make use of the high capacity of the geology at deeper levels beneath the ground surface. Even by making the foundation so big, there were concerns that they might settle too much and cause the impressive tower above to crack.
To avoid this, the foundations were 'base grouted', which means that grout was injected under pressure, through pipes, to the base of the pile in order to increase stiffness of soil beneath the base of the pile and reduce settlement.
The design and construction team had to think carefully about how this could be achieved successfully, and how quality could be controlled on site when the grout was pumped out under a column of concrete in excess of the length of an Olympic swimming pool.
This was achieved by development of a standardised scoring system to rate construction quality and by a close working relationship between the foundation designers Arup, who were present monitoring works on site throughout construction, and the foundation contractors, Bachy Soletanche.
Piling through 3 levels of basement presented technical and health and safety management challenges.