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How Far Underground Are Skyscraper Foundations?

January 4, 2021

Most people marvel at the height of a skyscraper. They count the stories of the buildings that dominate skylines, and they measure the spires of the "supertalls" that achieve impressive heights. 

However, the true engineering marvels of these buildings often happen below ground and out of sight. 

Below each skyscraper needs to be a foundation that's strong enough to support it. Before ever building up, construction teams must build down. 

Found out how deep the foundations are for some of the world's tallest buildings.

How Far Underground Are Skyscraper Foundations

Why Are Skyscraper Foundation Depths so Different?

The design of the foundation needs to suit the building and its site. 

Foundation depth is determined by:

  • Skyscraper design
  • Soil composition
  • Depth to bedrock
  • Environmental risk (water, wind, earthquake)

How Do Skyscraper Foundations Compare?

 

Building Height (Ft.)

Depth of Foundation and Supports (Ft.)

Number of Basement Floors

Foundation Details

Wilshire Grand Center (Los Angeles)

1,100

90

5

  • 18-foot-thick concrete mat that set a Guinness World Record for the largest continuous concrete pour in history

Willis Tower

(Chicago)

1,451

100

3

  • 114 reinforced concrete caissons set into bedrock
  • Five-foot thick concrete mat

Taipei 101

(Taiwan)

1,667

262

5

  • Four-foot-thick wall extending down 154 feet cuts off groundwater that starts about six feet below grade
  • 380 foundation piers, each with a five-foot diameter

One World Trade Center (NYC)

1,776

150

5

  • Rock anchors extend 80 feet into bedrock
  • Underground supports built around the pre-existing commuter trains

Shanghai Tower

(China)

2,073

282

5

  • 980 foundation piles, each with a three-foot diameter
  • 20-foot-thick reinforced concrete mat

Burj Khalifa

(Dubai)

2,723

164

1

  • 192 foundation piles, each with a five-foot diameter
  • 12-foot concrete mat

 

How Does Building Design Affect A Skyscraper Foundation?

The general rule of thumb is the taller the building is, the stronger the foundation needs to be. For example, a skyscraper foundation is much deeper than the foundation of a residential home. 

However, in order for skyscrapers to achieve the heights that we know today, building design and foundation design had to evolve hand in hand. 

A key development in skyscraper design occurred when buildings evolved from using load-bearing walls to steel frames to support the vertical load. With the conventional system of load-bearing walls, the wall depth of the lower stories needed to be extremely thick to anchor the height of the tower. By using the steel frame design, the inner skeleton of the skyscraper was anchored directly into the foundation. 

Building skyscrapers has also evolved to be more efficient in how it manages live load-bearing capacity. For example, the Willis Tower in Chicago is eight stories taller than the Empire State Building, but its tubular design uses one-third less steel. Achieving more height with less steel means that the structural system of the foundation has less building weight to support. 

How Does Site Location Affect A Skyscraper Foundation?

The environmental factors of a site location have a significant impact on foundation construction style and depth. The foundation needs will vary from city to city, and the soil type in Dubai will influence tall structures differently than the swampy soils of Chicago. 

Foundation needs can even change within a relatively small area. For example, the depth to bedrock in Midtown Manhattan is only about 35 feet, but in lower Manhattan, it can be as much as 150 feet to bedrock. This can mean foundation piers need to be drilled deeper to support tall buildings. 

When designing a foundation system, structural engineers will ask questions such as:

  • Is there sandy soil that's prone to shifting? 
  • How far down is the bedrock?
  • Are there flooding risks that could create foundation fissures? 
  • What type of lateral strength is needed against wind loads?
  • How far down does the foundation have to reach in order to get the vertical strength of bedrock?
  • Does the foundation need to withstand environmental threats such as the earthquakes of California or the lateral winds of Chicago? 

The skyscraper site assessment will then factor into structural calculations. By including a wide set of structural design considerations, the foundation can be constructed to support the skyscraper's height and weight.  

Skyscrapers Aren't the Only Buildings Depending on Foundations

Your home isn't as tall as a skyscraper, but it still relies on its foundation. 

An average house weighs about 50 tons, and a good foundation keeps it from sinking, tilting, or shifting. Movement isn't the only concern. A good foundation will also keep out moisture, improving the thermal efficiency of your home. 

Plus, a home's foundation is integral to its value. Foundation damage can result in a 30 percent loss in market value. This means your building foundation is critical to both structural stability and financial equity. 

Just like skyscrapers, residential foundation depth and design vary significantly. Deep foundations are common in the Northeast where construction must reach below the frost line. This is why full basements are common in the colder regions. 

Other home foundations have a narrow crawl space below the home. The shallowest foundation is a concrete slab that's poured directly at ground level. It's usually about a four or six-inch deep concrete mat with gravel underneath for drainage. 

This concrete depth provides a stark contrast between a residential foundation and a skyscraper foundation. In a home, the concrete mat may only be a half of a foot deep, but the Burj Khalifa in Dubai has a 12-foot deep concrete mat. 

How Is Your Home's Foundation Performing?

Your home foundation may be underground, but there are telltale structural signs of foundation damage so you can know how your foundation is performing even though you can't see it. 

The first thing to look for is cracks in the interior walls or exterior concrete block. If your foundation isn't effectively supporting the weight of the home, the building could start to shear apart, causing cracks. You may also start to notice that floors are no longer level, indicating your home has started to tilt. 

One of the worst recent examples of this is the Millennium Tower, a luxury high-rise in San Francisco. Just seven years after construction, the building had sunk 16 inches and was leaning 14 inches. 

Other signs of foundation damage include sagging floors, stuck doors or windows, a cracked chimney, and water issues. 

Interested to learn more about your home foundation? A free inspection from the country's leading foundation repair experts can help you understand what's happening underground so you can protect your home.