Strength Envelopes of Florida Carbonate Rocks near Ground Surface

Abstract

Florida carbonate rocks are typically weak, with a median unconfined compressive strength value of approximately 3 MPa. Due to the infeasibility of attaching strain gauges to their vuggy and shelly surfaces, an innovative volume change device was added to a Hoek cell triaxial system, enabling the measurement of volumetric responses during triaxial tests. In this study, 223 triaxial tests on five different Florida rock formations were performed, with confining pressures ranging from 0.35 to 20.7 MPa. The tests indicate that (1) only a small percentage of Florida carbonate rocks exhibit brittle rupture; and (2) most Florida carbonate rocks exhibit ductile stress-strain behavior, even at the lowest confining pressure. A ductile response is typically associated with contractive volumetric behavior due to the rock’s porous structure. The study established a ductile threshold σd/σ3 ratio, which was found to not be a constant, but to vary with confining pressure. It was confirmed through the triaxial test results that (1) the Hoek-Brown criterion works well for the dense specimens, which are not commonly encountered in Florida; and (2) for the typical porous Florida rocks, the stress-strain response is ductile and the strength envelopes in Lambe’s p-q diagram have a linear increasing (constant slope) zone followed by a decreasing slope zone, which is a function of the Florida formations as well as the specimens’ bulk dry unit weights.