Foundation Design In Florida Karst  (Page 3 of 7)

Data from historic sinkhole mapping can also be used to locate areas on a proposed building site where a breach in the confining layer is more likely to exist. Because sinkholes in the limestone surface are most likely to have occurred at the intersection of two joints, it is possible to determine the potential location of breaches in the confining layer by mapping the joint systems.

Unfortunately, the joint system in the limestone typically is buried under overlying soil deposits; consequently, the joint system can only be inferred from such linear surface features as stream segments, alignment of lakes, and ponds, etc. These linear or near linear surface features are referred to as lineaments. Fig. I I is a partial lineament map of south Orlando.

Note that there are two sets of lineaments, one which strikes N47 E and one which strikes N43 W.
Fig. 12 shows a proposed building site and the approximate location of all of the historic and prehistoric sinkholes that are known to have occurred in the immediate site vicinity. Also shown in Fig. 12 are bands representing the inferred location of joints in the limestone surface. These bands were constructed using the strike of the lineaments presented in Fig. 11. All of the site area crossed by the bands and particularly those areas containing the intersection of two bands is suspected of containing a breach in the confining layer.

If the building can be positioned to avoid all of the suspect areas and is outside the zone of influence of a potential sinkhole, the foundation exploration and design proceeds normally. However, if the building must be located near or over a suspected breach, a much more extensive subsurface exploration and testing program is required.

In many cases it is possible to begin the subsurface investigation using such geophysical techniques as ground?penetrating radar, EM conductivity, or seismic surveys.` All of these techniques can detect anomalies in the subsurface profile. Fig. 13 is an example of a suspected breach located using ground?penetrating radar.

After all of the geophysical data has been reviewed and anomalous areas located on the site map, a drilling and testing program is planned for the site. Within the areas of the site not located above a potential breach in the confining layer or paleosink in the limestone surface, the test borings or soundings are spaced conventionally. However, at least one of the test holes in this area is advanced into bedrock, i.e., into the confining layer, so that the unaffected site stratigraphy can be documented.