Determination of Unit Tip Resistance for Drilled Shafts in Fractured Rocks using the Global Rock Mass Strength

Miguel Truzman, David Corley, David Lipka

In the proceedings of: GEO2011: 64th Canadian Geotechnical Conference, 14th Pan-American Conference on Soil Mechanics and Geotechnical Engineering, 5th Pan-American Conference on Teaching and Learning of Geotechnical Engineering

Session: Deep Foundations

ABSTRACT: Over the last several years, we have been using the AASHTO LRFD Design Specifications to calculate factored resistances for drilled shafts with tips in fractured rocks around the Charlotte Area in North Carolina. These rocks typically consist of granites, diorites and metavolcanics. Most of these rocks are jointed with varying amounts of weathering. The current equation in the Design Specification gives overly conservative unit tip resistance values when the foundation rock has a Rock Mass Rating (RMR) less than 45. This paper evaluates the global rock mass strength equation proposed by Hoek, et. al. (2002) to determine more reasonable tip resistance values for poor to fair quality rock masses with RMR values less than 45. Additionally, we propose a correlation to determine the RMR and the Geological Strength Index (GSI) based on Rock Quality Designation (RQD) values.

RESUMEN: En los últimos años, los autores han utilizado las Especificaciones de Diseño de la AASHTO LRFD para calcular la resistencia factorizada de pilotes excavados con la punta en rocas fracturadas alrededor del área de Charlotte en Carolina del Norte. Estas rocas típicamente consisten en granitos, dioritas y metavolcánicas y la mayoría de ellas presentan un variado grado de meteorización. Con la actual ecuación recomendada en las Especificaciones de Diseño se obtienen valores unitarios conservadores de la resistencia por punta cuando la roca de fundación tiene una Valoración del Macizo Rocoso (RMR) menor a 45. Este artículo evalúa la ecuación de resistencia global de macizos rocosos propuesta por Hoek et. al. (2002) para determinar valores de resistencia más razonables para macizos con calidades de roca de pobre a mediana con valores del RMR menores a 45. Adicionalmente, se propone una correlación para determinar el RMR y el Indice de Resistencia Geológica (GSI) basado en valores de Designación de la Calidad de Roca (RQD).

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Cite this article:
Miguel Truzman; David Corley; David Lipka (2011) Determination of Unit Tip Resistance for Drilled Shafts in Fractured Rocks using the Global Rock Mass Strength in GEO2011. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GEO11Paper1036,author = Miguel Truzman; David Corley; David Lipka ,title = Determination of Unit Tip Resistance for Drilled Shafts in Fractured Rocks using the Global Rock Mass Strength,year = 2011}