Where Does Rock Begin Beneath Philadelphia?

by Edward F. Glynn, Villanova Univ, Villanova, United States,
William B. Fergusson, Villanova Univ, Villanova, United States,

Document Type: Proceeding Paper

Part of: Detection of and Construction at the Soil/Rock Interface


Philadelphia provides an excellent example of the engineering problems associated with the interpretation of saprolite/weathered rock/sound rock profiles. The bedrock beneath the city is the Wissahickon Formation of Cambro-Ordovician Age. This formation is a complex of strongly foliated schists intruded by felsic igneous rocks. It grades upward from sound unweathered schist, through a weathered rock zone up to 14 meters thick, into an uppermost saprolite layer as much as 18 meters thick. The rock/weathered rock and weathered rock/saprolite interfaces are transitional, difficult to identify, and are hidden beneath up to 18 meters of unconsolidated clastic sedimentary beds of Cretaceous, Tertiary and Holocene age. This paper presents a method for identifying the sound rock/weathered rock and weathered rock/saprolite interfaces within the Wissahickon Formation. Identification of these interfaces is accomplished through the assessment of boring log information using rock quality designation (RQD), percent core recovery and standard penetration testing criteria. The paper also discusses the physical characteristics of the sound rock, weathered rock and saprolite and the impact the properties of these materials has had on engineering practice in the Philadelphia area.

Subject Headings: Rocks | Rock properties | Weathering | Rock mechanics | Material properties | Aging (material) | Penetration tests | Subsurface investigation | Pennsylvania | United States | Philadelphia

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