Standard Penetration Test (SPT) Borings
Standard penetration test (SPT) borings are completed in general accordance with ASTM standard D-1586 using the mud-rotary drilling method. In general, the size of the drill platform utilized depends on the accessibility of the site and the anticipated depth of completion. Samples are collected using a 1.4-inch I.D. split-spoon sampler driven with a 140-pound slide hammer falling a distance of 30 inches. Samples are collected continuously in the upper 10 feet and generally at 5 foot intervals thereafter. The number of blows required to advance the sampler a distance of 18 total inches is recorded. The first 6 inch interval is referred to as the ‘seating blow’ and ensures the sampler is seated in previously undisturbed soils and below any soils that may have settled into the borehole. The blows required advancing the sampler the next two, six inch intervals are combined and are referred to as the SPT ‘N’ value of the soils.
The reported ‘N’ values indicate the relative density or consistency of different soil types, 
depending on the percentage of the soil that is finer than the No. 200 mesh sieve. Sandy soils are considered “very loose” with N values between 0 and 4 blows per foot, “loose” between 4 and 10 blows per foot, “medium dense” between 10 and 30 blows per foot and “dense” to “very dense” above 30 blows per foot and 50 blows per foot, respectively. Clayey and silty soils are considered “very soft” with N values between 0 and 2 blows per foot, “soft” between 2 and 4 blows per foot, “firm” between 4 and 8 blows per foot, “stiff” between 8 and 15 blows per foot and “very stiff” to “hard” above 15 and 30 blows per foot, respectively.
The mud rotary method is generally preferred to complete SPT borings in Florida and utilizes a bentonite slurry to remove cuttings from the borehole. As the boring is advanced into the soils, the slurry is pumped through the drill rods and drill bit and is collected into a tub at the ground surface, allowing soil particles to settle out. Commonly, borings completed using the mud rotary method experience a loss of circulation of the drilling fluid, whereby the drill fluid no longer comes back to the surface. This occurs most often near the soil/limestone interface. In some instances, this loss of circulation may represent highly permeable soils, raveled soils or potential voids above the soil/limestone interface.