Assessment Of Potential Karst Conditions For A New Bridge In The Florida Keys

While the probability of locating a foundation over critical karst, and the probability of a collapse is usually<br>relatively low, problems have and do continue to occur. When a karst problem does occur, it usually has a<br>significant impact upon structures and or groundwater.<br>The purpose of this investigation was to evaluate the possible presence of significant karst (old sinkholes or<br>cavities) within the area of a proposed 7,500-foot bridge to be built over Lake Surprise and Jewfish Creek, just<br>north of Key Largo, Florida. Five independent sets of data indicate anomalous geologic conditions between Lake<br>Surprise and Jewfish Creek, and include:<br>. A large microgravity anomaly of 100 microGals;<br>. Three boreholes near the center of the gravity anomaly contain 45% of the total fluid loss noted among 34<br> boreholes;<br>. Subbottom profiling (seismic reflection) data indicates possible dissolution-enlarged joints;<br>. A major photo-lineament indicates the possible presence of joints and fractures; and<br>. Extensive road maintenance within the immediate area of concern.<br>In addition, the presence of a large 2,000 foot diameter paleocollapse sinkhole, located in the area by USGS,<br>indicates that larger deeper cave systems and sinkholes can and do exist in the Florida Keys.<br>The subbottom data, lineaments, borehole fluid loss, and the DOT maintenance records each by themselves could<br>be dismissed as irrelevant. However, the coincidence of these independent sets of data with a large gravity<br>anomaly clearly focus attention on the area as an area of unusual geologic conditions, with a strong possibility of<br>significant karst. This case-history outlines the approach and the resulting data used to identi& karst conditions<br>and assess its impact upon the proposed bridge design.