Last month, a study was released that found that the heavy weight of buildings, among other things, is causing New York City to sink at a rate of 1 to 2 millimeters per year. Researchers pointed to several factors that are causing the sinking, including sea level rise and groundwater mismanagement. But it was the weight of the city’s skyscrapers that has, understandably, gotten the most attention. Manhattan is an island absolutely brimming with high-rise office and residential buildings. According to the study, there are more than 1 million buildings in New York City. The majority of these are built with concrete and steel and the average building mass is 1.5 million pounds. That’s a lot of weight.
New York isn’t alone in experiencing sinking due to the heavy weight of its buildings. Jakarta, Indonesia, is thought of as the world’s fastest sinking city, while coastal cities like Charleston, South Carolina, and New Orleans are slowly sinking into the sea too. With the world’s biggest cities expected to grow disproportionately more over the next few decades than rural areas, there’s a growing urgency to prevent and ease these city’s slow descents.
In New York City, the threat of coastal flooding due to natural disasters and rising sea levels has been a concern for years, especially after Superstorm Sandy devastated waterfront areas in Lower Manhattan and Queens. The event prompted city officials to undertake resiliency measures to better protect coastlines in and around New York Harbor. The ongoing efforts include extending the shoreline between 50 and 500 feet into the East River in parts of Lower Manhattan, building dunes on beaches in the Rockaways, restoring wetlands in Coney Island, and building a levee in Staten Island. But the revelation in the new research out this month about the impact heavy buildings are having on the city, namely how the loads are causing the city to slowly sink, adds another hurdle to mitigation efforts.
In what researchers call “subsidence,” the weight of the city’s buildings is causing the land to sink down into the Earth. It’s a natural process that happens anywhere in the world where ground is compressed. But in a city with buildings covering nearly every available space of land, that process happens faster—especially in low-lying areas. Other factors that are causing the sinking include how groundwater and drainage systems are used. In New York City, development and the absence o tributaries has effectively cut off almost all of the sediment supply to both the East and Harlem Rivers, which has then stopped the deposit of sediment into New York Harbor, elevating the threat of flooding from major weather events like Nor’easters and hurricanes, according to the study.
The rate of compression differs between neighborhoods of the city and depends on the composition of the land, like whether buildings are built on bedrock or loose soil, and how many existing buildings there are. In Midtown, most of the skyscrapers are built on bedrock, which is more resilient to compression. However, in Lower Manhattan as well as outer borough areas of Brooklyn and Queens, the soil is looser and is sinking faster. While it’s happening very slowly, eventually, parts of the city will eventually be underwater. “It’s inevitable,” said Tom Parsons of the U.S. Geological Survey and the lead researcher of the study. “The ground is going down, and the water’s coming up. At some point, those two levels will meet.”
The new research will likely put pressure on NYC officials to develop more strategies on how to mitigate future flooding and update resiliency measures in the most at-risk neighborhoods of the city. Low-lying areas of the city are vulnerable to coastal surge flooding that happens during storms, which has been happening more frequently due to climate change. Sea levels have risen in New York City by 12 inches since 1900, and is predicted to continue to rise as much as 6.25 feet by 2100, according to the Mayor’s Office of Climate & Environmental Justice.
In Florida, where flooding from hurricanes and other natural disasters has caused billions of dollars in property damage over the last four decades, lawmakers recently introduced new legislation aimed at expanding and prioritizing resiliency projects. Existing law requires any publicly-funded projects within the coastal construction zone to undertake a Sea-Level Impact Projection study before breaking ground. A new bill introduced would expand that requirement to projects in all areas at risk of sea level rise, not just areas lying directly on the coast. Other steps being taken to mitigate the sea level rise are to better manage water in coastal areas and take caution to recharge water that’s drawn from underground aquifers.
But there are measures the real estate and construction industry can take to be part of those efforts too. A first step for development projects could be undertaking geological studies before construction to identify what local sediments may be more prone to subsidence. Another could be using alternative materials that are more resilient. In coastal areas of Florida, developers are using a stronger kind of soil composition to build skyscrapers in places where the threat of sinking looms large. The process is called deep soil mixing, and it entails mixing concrete into soil to create a kind of slurry called “soilcrete” that is three times stronger than regular soil. The technique has been around for some time, but was mainly used in industrial settings like oil refineries, where the soil needed to withstand heavy building loads.
In Miami, the development team behind the Waldorf Astoria Residences Miami is using deep soil mixing to strengthen the foundation for the forthcoming 100-story luxury condo and hotel. The problem the development faces is what makes up the land underneath it. The soil is made up of layers of limestone and sand, not ideal for a skyscraper with a planned height of 1,049 feet. By using the deep soil mixing technique, the ground becomes almost impervious to water and can help buildings withstand natural disasters like hurricanes that have high winds. It can also help make the building more profitable: with a stronger foundation, the building can add more high-floor units with high prices to match.
But there are risks in using the method. Some sand is too soft to be able to use the technique, and if water flows into the construction site during the deep soil mixing process, it could cause issues and pushback the construction timeline. But deep soil mixing is growing in popularity with developers, and is being used on projects around the country and the world.
Another way developers and builders can help lessen the load that tall buildings bring to the land could be through the use of lighter building materials. One of the most popular alternative materials at the moment is mass timber. From residential to office, mass and cross-laminated timber buildings are popping up across the U.S. Among the benefits of the more sustainable material is the fact that mass timber is five times lighter than concrete. While buildings constructed with mass timber have typically been limited in how tall they can rise, that’s beginning to change, paving the way for more developers to use the construction method.
New York and many other cities around the world that are slowly sinking are facing a serious threat in the not-too-distant future from sea level rise and flooding. Leaders will need to begin taking action in order to make their cities more resilient to future storms, whether it’s through infrastructure upgrades, restoration of beaches and wetlands, or building flood barriers. The real estate industry plays a role too. After all, the property industry is extremely vulnerable to flooding. Hurricane Sandy alone caused $19 billion in damages to the New York City area in just 48 hours. As new techniques being used in Florida and around the world have shown, innovative construction methods could play a key role in helping to mitigate the costly impact of storm surge and flooding that threatens life and property.