Laying the Foundation – Challenges of Skyscraper Construction

Starting at the Bottom

Building construction begins at the base. In ancient times, pyramidal structures were often used to distribute the weight of tall buildings over a large area. In modern times, the advancement of technology and soaring land costs in cities have led engineers to consider the size of a building’s footprint while continuing to think vertically. Creating a large amount of real estate out of a relatively small amount of ground area is very appealing to developers in large cities.

Like the roots of a tree, a skyscraper’s foundation is laid below ground to create the most stability. To lay the most stable foundation possible, the bedrock (solid rock underneath the ground’s soil) must be reached. Accessing the bedrock, in most cases, requires significant excavation of soils (sand, clay, etc.) overlying the bedrock. However, building the foundation upon bedrock is necessary given the extremely large loads associated with skyscraper construction. For instance, a typical house might weigh 70 tons; but the Empire State Building in New York City weighs about 350,000 tons!

Challenges of Skyscraper Foundation Construction

In some areas, simply accessing the bedrock can be a real challenge. One of the more notable examples occurred during construction of the World Trade Center (WTC) towers in New York City (completed April 4, 1973). The bedrock in this area of New York is approximately 65 feet below the ground surface. However, the WTC construction site was located adjacent to the Hudson River, and just a few feet below the ground surface, the soil was saturated due to the shallow water table. This meant that simply excavating to the bedrock could not be accomplished, as the construction site would flood when the digging began.

A novel approach was developed to counter the intrusion of water into the foundation construction site. The construction crew used machinery to dig a 3 foot wide trench around the perimeter of the site. While digging, a mixture of water and bentonite (expansive clay) was piped into the trench and would expand along the sides of the trench, effectively blocking the groundwater. Once a section of the trench was completed, the crew would lower a steel framework into the hole and pump in concrete from the bottom of the hole. Pumping the concrete in from the bottom would displace the water/bentonite mixture, leaving a steel-reinforced concrete wall around the perimeter of the construction site; which measured four city blocks by two city blocks upon completion of the wall. This perimeter wall/dam and the construction site contained therein was jokingly referred to as the “bathtub” by the construction crew, and formed a water tight perimeter wall for the towers’ foundation structure to be built.

Once the wall was in place, the construction crew could begin digging down to bedrock to lay the buildings’ foundation system. However, as the soil within the construction site was removed, the weight of the soil and water outside the walls would push the walls inward. Thus a series of tiebacks (cables extending from the perimeter walls to the material surrounding the “bathtub” were installed to provide temporary support until the crew could finish a support structure inside the “bathtub”. Using this method, the foundation site could be excavated; culminating in the removal of more than 1 million cubic yards of soil! With the bedrock exposed, the massive foundation structure could be placed. The perimeter footing for the buildings consisted of 60 high strength, load bearing steel columns spaced closely together on each side of the buildings.

Looking Forward

While reinforced concrete and steel columns are mainstays of skyscraper construction; when combined with the modern techniques used to construct the World Trade Center towers, engineers have achieved heights never before imagined in our history. The Burj Khalifa skyscraper in Dubai (completed January 4, 2010) is currently the world’s tallest building; standing at a staggering height of more than half a mile! While the height of this building is impressive, what may be even more impressive are the weight-conscious advancements in the manufacturing and utilization of building materials used to construct the skyscraper. The Burj Khalifa is twice the height of the Empire State Building, but less than twice its weight!

It is the design and evolution of our building techniques, and specifically deep foundation construction, which has allowed for the erection of today’s mega-cities. Skyscrapers have allowed for tens of thousands of people to work in close proximity, allowing them to share ideas, and closely collaborate on tasks. In this way, skyscrapers have provided a foundation for modern society.

As we look to the future, the sky is quite literally the limit. While the decision to build dramatically tall buildings is usually not based on economics so much as prestige and the desire to attract attention; there is no denying that the construction of ever taller buildings enthralls the human imagination. So, while the buildings will only get taller, we must remember that a structure is only as strong as its foundation.

Sources

Lienhard, John H. Reinventing Foundations. 2010.

http://www.uh.edu/engines/epi2605.htm

Thomson Gale’s How Products Are Made. Ongoing.

http://www.madehow.com/Volume-6/Skyscraper.html

Tamaro, George J. National Academy of Engineering. World Trade Center “Bathtub”: From Genesis to Armageddon. Spring 2002. http://www.nae.edu/Publications/Bridge/EngineeringandHomelandSecurity/WorldTradeCenterBathtubFromGenesistoArmageddon.aspx