OasisLMS

Abstract: Sinkholes are a common karst-related concern affecting many projects in the state of Florida. While the state of geotechnical practice in Florida generally applies best-practices to identify and characterize sinkholes and their risks to overlying construction, occasionally a deep, dormant, undisclosed, large diameter sinkhole is activated by changes in overlying conditions such as large grade changes associated with building construction. Such a condition occurred on a multi-family apartment construction project in Clearwater, Florida between first recognition of the problem in 2022 and the ultimate remediation of the sinkhole conditions in 2024.

Presentation of this case study should help future generations of geo-practitioners when faced with similar challenges. Excellent field measurement data existed for the evaluation of the sinkhole’ s impacts on construction, including deep vertical inclinometers, robotic total station measurements of impacted adjacent construction, traditional survey monitoring of settlement and of course many deep soil borings in the affected area. All of these measurements will be presented in the paper.

Initially the large diameter sinkhole only appeared to significantly impact an overlying, 18-ft tall segmental retaining wall and its nearby underground utilities (storm, sanitary and water). Wall settlement was as much as 5.5 feet, at which time the wall was declared severely damaged and likely to fail structurally. Supplemental borings in vicinity of the most severe wall settlement led to the characterization of a previously undiscovered, large diameter sinkhole who’ s deep throat was filled with low-strength, compressible organic soils. Initial remediation efforts included de-constructing the segmental wall and reconstructing the wall using super lightweight glass-blown aggregate. After the wall was re-constructed, the net stress at the base of the wall was reduced by 1, 000 psf and this reduction was thought adequate to reconstruct the wall, repair the utilities, and get the project back on track.

Several months after the wall was reconstructed, top of wall settlement was observed again, and now measured ½ to 2 feet (less than initial wall construction but certainly not expected with the net stress reduction). The engineers began to believe below grade, lateral soil movement was a contributing factor to settlement and a third round of geotechnical exploration and installation of deep inclinometers began. Shortly after the inclinometers were installed, it was evident the larger soil mass underlying the wall and in the vicinity of the large diameter sinkhole was failing in a circular manner. The accumulation of earthen movements from the inclinometers and total stations allowed the geotechnical engineers to model future wall behavior using finite element methods, with the model calibrated by actual field measurements. With a confident finite element model, the engineers predicted the wall would eventually fail if not reinforced.

The second round of remediation was more robust than merely reconstructing the wall with lightweight aggregate and included very long, 110 ft sheet piles driven in front of the MSE wall, horizontally restrained with deadman anchors. The geotechnical engineers worked collaboratively on the design with a well known geotechnical contractor and eventually stabilized the wall, verified with months of post-construction movement data. Adjacent buildings were impacted, requiring their underpinning and jacking to restore finish floor elevations and these remediation measures will also be presented.

Mr. KARL A. HIGGINS

Annual Conference Proceedings