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What Lies Below

by Jan Cermak

Most owners and developers that have developed sites in New York City realize the importance of understanding the potential impact of subsurface conditions on the site potentials. Subsurface conditions not only impact design but also construction schedule and its cost. This understanding is not only limited to the subsurface soil and rock profile, but also includes groundwater conditions, previous development/structures present at the site, adjacent buildings, and infrastructure that all might have a significant impact on construction planned for the site. Possible infrastructure in the City includes rail tunnels (subway, Amtrak, etc.), roadway tunnels, water tunnels and large utilities (e.g., sewers). All of the above will impact what can be reasonably constructed at the site as it will dictate the type and cost of foundations and cellar spaces. Difficult subsurface conditions and presence of infrastructure will may drive the cost of below grade construction up and can make some development types not feasible. Let’s review briefly two of the important factor affecting any building development in New York City: its geology and development history.

New York City’s unique and variable geologic conditions are the result of numerous glacial activities taking place over the last one and a half million years. The City lies at a junction of three physiographic provinces containing a variety of different foundation rock and soil types. New York City's subsurface conditions were greatly influenced by a succession of glacial ice advances that swept across southeastern New York State. Engineering characteristics of the area’s soils and rock can be best understood by recognizing the causes and circumstances of the local geologic processes. The main rock types found in the City are crystalline rocks (e.g., gneiss, schist, and granodiorite). However, younger serpentinite and sedimentary rocks can also be found. In some locations, the bedrock outcrops well above sea level, while at other locations it is several hundred feet deep. There are also several geological faults within the City, some of the major faults are expressed as surface features such as river channels. The soil stratigraphy includes natural soils ranging from stiff glacial deposits to very soft organic fine grained soils of recent origins and surficial man-made fills.

Recent human activities further changed the City’s landscape. The many swamps, creeks, ponds and valleys present at the time of discovery (see above 1865 map) were gradually filled in to create more land for human development. Moreover, the shorelines, particularly of Manhattan Island, expanded well beyond their original locations into the rivers and bays. The fill materials mostly originated from nearby large excavations. As the city continues to expand, marginal land with difficult subsurface conditions, including filled-in land is being developed. Additionally, site with other challenges, for instance, over subway tunnels, are also being considered for development.

To provide appropriate foundation recommendations for structures with complex below grade challenges, engineers must be knowledgeable of the site history and geologic origins of the soils present. Soil exploratory programs and laboratory tests, if necessary, have to be planned carefully to address all possible foundation issues present. Many projects adjacent to City infrastructure also require experience in dealing with the various City agencies when their approval is required.

The subsurface profiles and City infrastructure have major impact on the cost and type of possible foundation systems that can range from shallow foundation where bedrock shallow to very costly deep foundations extending into the deep bedrock.

Not all developers are aware that a review of geotechnical and foundation challenges for site development prior to any investigations is possible by experienced local foundation engineers. For instance, our firm's in-house files include subsurface information from thousands of projects we completed in our City in our 110 year history. This information in combination with other available information often provides sufficient data for initial reviews of below grade issues on mosts site in the City.