Variability of weathered limestone properties in light of drilled shaft design in Florida

Infrastructure assets supported by drilled shafts socketed in highly variable limestone are very common in Florida. The Florida Department of Transportation's methodology for computing side shear resistance of rock sockets is a function of unconfined compressive strength and split tensile strength. The variability of these strengths was assessed at three different scales using limestone specimen data collected from a geotechnical test site. At the site scale, the average split tensile strength was 0.64 MPa with a coefficient of variation of 89% and the average unconfined compression strength was 2.08 MPa with a coefficient of variation of 64%. Moving from the site scale to the borehole scale decreased the coefficient of variation in five out of seven borings. It was noted that limestone properties do not show the expected increase in properties with depth, which is attributed to numerous weathering cycles from sea level lowering and rising. Variability assessment at the core run scale was difficult because 48% of the core runs did not have either tensile or unconfined compression specimens. To assess variability at this level, specimens must be obtained from every run. This may require a change in drilling procedures or testing methods.