Geology Under Cities
The nine papers in this volume cover the geology beneath Washington, D.C., Boston, Chicago, Edmonton, Kansas City, New Orleans, New York City, Toronto, and St. Paul/Minneapolis, and present methods of data gathering that could be used in most cities.
Most cities in the United States are within one or two physiographic units, and only a few major rock and soil types must be dealt with in civil works in different parts of a city. New York City's five counties, however, cover three physiographic units (the Coastal Plain, New England Upland, and Triassic Lowland), which contain nine different foundation rock types and dozens of soils. The foundation types range from high-strength gneissoid granite through soluble marble to soft-sensitive, low shear strength, and high moisture content, organic silty clay having very limited bearing capacity. Each rock and soil type has its own engineering characteristics, and, in addition, local problems exist within each major type. Serious problems of foundation support in loosely consolidated materials of the Coastal Plain are solved by use of spread footings, whereas a similar structure in the New England Upland may require the use of caissons where bedrock underlies thick till.
The New York City crystalline rocks are separated into the New York City Group and the Hutchinson River Group by extension of Cameron’s Line, a regional north-northeast-trending thrust fault. The formations of these groups are cut by several sets of fault systems, and seismic activity has been noted along the northeast-striking fault system.
Slope stability can be a problem in valleys at the north end of the city or in deep cuts within the moraine areas. The use of reinforced concrete walls and line drilling have helped, where rights-of-way are tight, to control slopes.
Ground water, once used in the city for water supply, is now an engineering problem. As population expanded, salt-water encroachment due to drawdown by pumping and contamination from human activity made the ground water unfit for human consumption. The impotability of the ground water fostered the construction of reservoirs and hundreds of kilometers of rock tunnels and distribution mains, which make up one of the greatest water supply systems in the world. A negative result of the cessation of groundwater use has been the rise in the water table, which has flooded structures, such as subway tunnels, that were built when groundwater pumping had greatly lowered the water level. Sewer tunnels and treatment plants have been built to reduce the pollution problems in the waters surrounding the city.
Erosion is a serious problem on New York City’s ocean beaches. Building up the beaches by placement of dredged material and the wiser planning of protective structures are important measures in attempting to preserve these recreational facili-ties from natural erosion.
The complexity of the geology and the resulting foundation problems in the city have resulted in the writing of a comprehensive building code for New York City. This 1970 code requires a scientific approach to foundation exploration and generally allows the use of modern material and technology not allowed under the pre-World War II codes that were in use through the 1960’s.
In addition to the vast number of engineering projects, such as tunnels and bridges for transportation and large buildings in which people live and work, hundreds of hectares of land have been added to the city’s perimeter through landfill projects that were started during the time of the English settlers and that continue to date. The latest perimeter landfill is the site of the future Battery Park City in Lower Manhattan in the Hudson River; the landfill consists largely of the spoil from excavation for the Twin Towers of the World Trade Center.