New York City building settlement study

New York City has identified over 1 million buildings constructed on ground that is not a granite base, according to a study published in May 2023 in the journal Earth's Future. The study estimates that the ground beneath these buildings settles by between 0 and 600 mm, depending on the type of soil and foundation.

Building weight and distribution

The city has 1,084,954 buildings, with an estimated total weight of about 760 billion kilograms (1.68 trillion pounds) spread over 778 square kilometers in the five boroughs. This weight represents about 90 tons of buildings per resident, based on an order-of-magnitude calculation.

Research origins and methodology

The research was initiated by Tom Parsons, a geophysicist at the USGS (United States Geological Survey), during a visit to his wife's family in Belgium in 2019. Collaborating with Pei-Chin Wu, Meng Wei, and Steven D'Hondt from the University of Rhode Island, the team developed a model that calculates the cumulative mass of all New York buildings.

Soil type and settlement

The study found that soils rich in clay and artificial fill experience the highest settlement after construction, significantly more than more elastic soils. The 600 mm figure represents the upper limit of the range and is not an average. It depends on the physical parameters of the soil or rock and the foundation method.

Subsidence rates and causes

Radar satellite measurements (InSAR) and GPS indicate an average subsidence rate of 1 to 2 mm per year across the city, matching the regional post-glacial deformation. For lower Manhattan, GPS data suggest about 2.1 mm per year. In some areas, the subsidence rate is higher, which the study attributes to internal consolidation of fills and soft sediments.

Subsidence and sea level rise

The study emphasizes that each millimeter of subsidence equates to a year of sea level rise, as the sea level in New York is rising by about 1 to 2 mm per year. This combination increases flood risk for the city's coastal and riverfront areas. The researchers do not call for halting construction but suggest that mitigation measures may be necessary for new high-rise buildings.