FEATURE ARTICLE The Challenges of Deep Excavation in Manhattan Bedrock Excavation of bedrock in Midtown Manhattan The increase in demand for commercial and residential space in major cities has elevated the importance of the geotechnical engineer’s role because of the need for deep excavations to maximize usable space on a site. In a dense urban environment, these excavations are often adjacent to existing structures, many very old. The geotechnical engineer is responsible for identifying subsurface conditions at the site and constraints of adjacent sites (i.e., buildings, tunnels, utilities, etc.). Many times the geotechnical engineer designs the temporary support system to facilitate the lot-line construction and deep, below-grade building levels. Often the engineer’s recommendations are developed in coordination with a specialty foundation contractor. The design and construction of deep levels has consid- erable impact on a project budget and construction schedule. Often in New York City, excavating and constructing a foundation may require as much time as constructing the superstructure. Excavation associated with lot-line construction is a complicated aspect of AUTHORS construction, especially considering the complex subsurface geology and the potential to damage adjacent buildings during underpinning, dewatering and other excavation-related activities. Each site presents its own constraints and involves careful attention to details by the engineer and contractor. Usually, the analyses related to such construction go beyond theoretical calculations of performance and rely heavily on the experience of the engineer and contractor. Obtaining comprehensive subsurface information during design and even construction, such as bedrock- structure data, is beneficial in developing cost-effective and suitable support of excavation systems. New York City Geology The subsurface conditions in Manhattan and its neighboring boroughs resulted primarily from repeated glaciation and ice advancement during the Pleistocene Era. In general, bedrock is typically less than 100 ft (32.8 m) below ground surface for most of Manhattan Island and the Bronx. However, the bedrock surface dips rapidly to several hundred feet below the ground surface as you move east toward Brooklyn and Queens. The overburden soils above the bedrock typically consist of glacial deposits commonly referred to as ground moraine, a widespread dense layer composed of a mixture of clay, silt, sand and boulders. Soils in the five boroughs of New York City were affected by several advances during Late Pleistocene glacia- tions about 25,000 years before the present, with glaciations receding between 12,000 and 14,000 years ago. The bedrock of Manhattan Island can be divided into the Manhattan Schist, Inwood Marble, Hartland Formation, Walloomsac Formation and Fordham Gneiss. A large sill of intrusive granite and pegmatite has been mapped from about West 35th to West 40th Streets; however, historical boring data indicate that these intrusions are more widespread across Manhattan. In general, most construction challenges associated with the types of bedrock in Manhattan result from the shallow rock formations. These formations Laurence E. Ford, P.E., Scott M. Garfield, P.E., Arthur J. Alzamora, Jr., P.E., and Andrew J. Ciancia, P.E., Langan Engineering, Environmental, Surveying and Landscape Architecture DEEP FOUNDATIONS • JULY/AUG 2015 • 75
