The use of the Geological Strength Index (GSI) has not been commonly used to characterize rocks like Miami Limestone. Since the limestone is jointless, its appearance is like an intact or massive rock and estimate the GSI values could be wrong. This paper proposes a modified GSI table to characterize the Miami Limestone. Based on the Hoek-Brown Criterion, the GSI values are used to determine the global shear strength of the rock mass to evaluate potential punching failure of shallow foundation for bridges through the Miami Limestone. Additionally, statistical information about the geotechnical property indices, such as unit weight, unconfined compressive strength, modulus of elasticity and tension, as well as percentage of recovery and Rock Quality Designation (RQD) are included in this paper to have a better idea of the conditions of this limestone.


The Miami Limestone (formerly the Miami Oolite), named by Sanford (1909), occurs along the Atlantic Coastal Ridge in portions of Dade, Monroe, Broward and Palm Beach Counties.

The use of the Geological Strength Index (GSI) introduced by Hoek et. al. (1994) has been limited in the Miami Limestone and minimum information has been previously reported.

In recent years, the Florida Department of Transportation (FDOT) has been improving existing roadways and developing new projects where some bridges have been designed bearing on Miami Limestone. FDOT requires the use of the FDOT Soil and Foundation Handbook (SFH) and American Association of State Highway and Transportation Officials (AASHTO) Load and Resistance Factor Design (LRFD) Bridge Design Specification to design bridge foundation.

Based on AASHTO LRFD 2014 to define the elastic modulus of the rock mass (Es) and the global rock mass strength (scm), the GSI should be determined using Figure 1. These two parameters are the basis for calculating the total settlement and punching failure of shallow foundations.

Determination of the type of the structure and the surface conditions of the discontinuities in the Miami Limestone is a challenge because this lithology is jointless. This limestone has been subjected to minimum tectonism and it looks like an intact or massive rock mass that can be misinterpreted in the GSI application with subdesigned results with severe consequences to the projects.

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