Case Study- Effects of Undisclosed Large Diameter Sinkhole on Construction, Clearwater, Florida-Paper

Paper

Pdf Summary

A previously undisclosed, dormant sinkhole caused severe settlement during construction of a multifamily apartment complex in Clearwater, Florida. Although extensive preconstruction borings identified generally suitable soils and the sinkhole risk was judged low, a deep, organic-filled karst feature beneath an 18-foot segmental retaining wall went undetected. Site grading and the wall’s added load likely contributed to its activation.<br /><br />In 2022, roughly 230 feet of wall settled by 3–4 feet, eventually reaching about 5.5 feet. Supplemental borings revealed very soft organic soils extending 30–60 feet deep, while drilling-fluid losses indicated possible hydraulic connections between aquifers. The initial repair replaced the wall using lightweight recycled-glass aggregate to reduce loading. However, renewed settlement followed, and deep inclinometers showed that soil beneath the wall was moving laterally in a rotational, slope-failure pattern, with movement extending to depths of 58 feet.<br /><br />Engineers used survey, inclinometer, and robotic total-station data to calibrate finite-element models and assess failure risk. A more robust solution installed long steel sheet piles in front of the wall, restrained by deadman anchors. Monitoring showed the repaired wall stabilized, with movements generally below the one-inch design threshold. Nearby Building 8 was underpinned with helical anchors, jacked close to its original elevation, and grouted beneath its post-tensioned slab; damaged utilities were also repaired.<br /><br />The case highlights the need to compare final civil and structural plans with geotechnical assumptions, especially when major grading or retaining walls are introduced. In Florida karst terrain, closely spaced, sufficiently deep borings or cone penetration tests remain important for detecting sinkhole conditions; geophysical methods may have mixed results. The project’s post-tensioned foundations and sinkhole insurance also proved valuable.

Keywords

sinkhole

karst

retaining wall

ground settlement

Clearwater Florida

geotechnical engineering

sheet piles

helical anchors

inclinometer monitoring

foundation repair