Foundation Design In Florida Karst  (Page 1 of 7)

by John E. Garlanger  
Ardaman & Associates, Inc.  
8008 South Orange Avenue, Orlando, Florida, USA 32809

John E. Garlanger

The entire Florida peninsula is underlain by solution-weathered limestone, with cavities in some areas that are known to exceed 100 ft (30 in) in height and width. The lengths of these natural conduits are measured in miles. The sinkhole-dotted surface of the limestone is typically buried beneath significant thicknesses of overlying sediment, and the foundation hazards associated with building on the limestone generally are not visible from ground level.

Fig. I is a generalized cross section through a hypothetical site in the sinkhole-prone area of central Florida. The limestone is overlain by consolidated clays of Miocene age and unconsolidated sands of Pleistocene age. The groundwater table in the sand is typically within 5 to 10 ft (1.5 to 3 in) of the ground surface and the piezometric level in the underlying limestone is typically 40 to 80 ft (12 to 24 in) below ground surface. Of primary importance from a foundation engineering perspective is the sand-filled breach in the clay layer. Groundwater from the surficial aquifer flows through this breach and recharges the much more productive limestone aquifer.

From a hydrological perspective, this ability to recharge the lower aquifer, which is the principal source of potable, agricultural, and industrial water in central Florida, with water from the rain-recharged surficial aquifer is quite beneficial. However, as the water supply demand in the deeper aquifer causes increases in the hydraulic head difference between the two aquifers, the potential for "piping" sand through the breach in the clay layer into an underlying cavity increases.

Piping occurs when a subterranean conduit or tunnel is eroded backward from a location where groundwater is discharging from an unconsolidated soil deposit, such as at a spring.' Once erosion begins, it proceeds backward along the line of maximum hydraulic gradient toward the source of seepage. The end result can be catastrophic for a building foundation.

In Fig. 2, sufficient sand has piped into the cavity system in the limestone to create a cavity in the overburden sands. When the cavity enlarges to a size at which the beam action of the overlying soil can no longer support itself, the roof collapses, resulting in a sinkhole at the ground surface (Fig. 3). Fig. 4 illustrates this phenomenon at a site in the Orlando metropolitan area.

In Fig. 5, the groundwater table is higher, the soil above the water table is cohesionless, and the available void space in the limestone is small. In this case, the depression at ground surface is more saucershaped and the subsidence is more gradual. However, because of the resulting settlement, a building foundation constructed above the depression would still have failed. 

Fig. 6 illustrates this phenomenon at a site near Brooksville, Florida.