A CME-75 drill rig sets up on a tight lot near Centennial Park. The crew positions the 140-pound safety hammer over the borehole. The auto trip releases. The hammer drops 30 inches. This is the SPT drilling operation that Nashville engineers rely on to get N-values from the Ordovician limestone residuum and alluvial clay layers that define the Central Basin. The Standard Penetration Test remains the most practical way to sample soil when you cannot see through the chert gravel and stiff red clay that sit above Nashville's bedrock. The data goes straight into bearing capacity calculations for shallow foundations—a necessity in a city where the subsurface can shift from dense gravel to weathered shale in less than 10 vertical feet.
In Nashville's Central Basin, an SPT boring that stops at 15 feet can miss a buried karst void at 22 feet—always go deeper than the initial site plan suggests.
