A tall order

by Andrew Lees, on 13-Aug-2020 04:53:56

The advent of the ‘supertall’ building has set new challenges for geotechnical engineers, requiring innovative foundation design to cope with huge loads from buildings and lateral wind forces, as well as to protect against earthquake damage.

As I discussed in my recent blog on construction materials, the UN predicts that about two thirds of the world’s population will be living in urban areas by 2050. Inevitably, this will lead to more people living and working in tall buildings, as demand grows and available space becomes more scarce.

There has been a dramatic increase in the number of tall buildings being built in recent years. According to the Council on Tall Buildings and Urban Habitats, a record 26 ‘supertall’ buildings (more than 300m high) were completed in 2019. It was also the sixth year running that one building taller than 500m was completed.

As buildings have become taller, so have the challenges for geotechnical engineers; the foundations for these mega-structures go beyond conventional approaches, requiring cutting-edge design and analysis to ensure they can support the enormous building loads, withstand lateral wind forces and protect buildings from earthquake damage.

Piled rafts are commonly used for supertall buildings as they spread the load across a large volume of soil, increasing bearing capacity and controlling total settlement. They are also effective at controlling differential settlement, which can occur between the building and lighter podium structures built around the base.

The Burj Khalifa

At 829m, the Burj Khalifa in Dubai is still the world’s tallest. Completed in 2009, it has 163 floors, with a three-spoked triangular footprint shaped like a local desert flower, the Hymenocallis.

When it came to designing the foundations, the ground conditions proved particularly challenging – the site was underlain by weak marine silty sands and interbedded weak and very weak sandstone, siltstone and conglomerates. The solution was to build a 3.7m thick reinforced concrete raft, supported by 192, 1.5m diameter cast in situ bored piles, each 47.5m long, for the tower. A further 734, 30m long piles supported a thinner piled raft beneath the podium structures.

Ground Coffee Episode 22 – Andrew Lees’ pre-lockdown visit to the Burj was the ideal chance to talk about the challenges of designing foundations for very tall buildings.

Key to success of the project was the accurate prediction of the soil-structure interaction, the interaction between the piles themselves and also between the piles and the raft. This required extensive finite element analysis, along with static and dynamic pile load testing on site, to confirm the design and to accurately predict the settlements – estimated to be between 45mm and 65mm.

Professor Harry Poulos of Coffey led the independent geotechnical peer review of the foundation system, which was designed by Hyder Consulting (UK). If you want to find out more, I can recommend two of his papers:

Geotechnical foundation design for some of the world’s tallest buildings, co-authored with Frances Badelow, the 15th Asian Regional Conference on Soil Mechanics and Geotechnical Engineering which can be downloaded here

Foundation design for the Burj Dubai – the world’s tallest building, co-authored with Grahame Bunce, 6th International Conference on case histories in geotechnical engineering, 2008, available for download here