Special Bedrock Conditions in Greater Bostonby John T. Humphrey, Haley & Aldrich Inc, Cambridge, United States,
Cetin Soydemir, Haley & Aldrich Inc, Cambridge, United States,
Abstract: The Cambridge Argillite, a weakly metamorphosed shale, forms the major portion of the Boston Basin, a structural as well as topographic depression. In the greater Boston area, the argillite bedrock and/or overlying glacial till (hardpan) provide suitable support for a wide range of foundation systems. The bedrock is generally hard and competent due to its poorly developed bedding planes and lack of fissility. However, locally and in an irregular manner, hydrothermal alteration of the bedrock has created zones of highly decomposed, kaolinitic materials of soft consistency and medium plasticity that vary in thickness from a few to hundreds of feet. Generally lying at depth, beneath high water table conditions, the altered argillite has been incorrectly indentified as the overlying till or, in some instances, the famous Boston Blue Clay. The zones of highly altered bedrock present major geotechnical concerns, and their unpredictable occurrence and orientation will have a considerable impact on foundation designs and construction in the Boston area.
Subject Headings: Bedrock | Soil properties | Foundation design | Foundation construction | Shale | Structural design | Clays | Boston | North America | Massachusetts | United States
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