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

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A three-story apartment project in Clearwater, Florida, appeared to be a typical site after initial borings identified mostly sandy soils and no obvious sinkhole concerns. Shallow organic soils and debris were removed, and building locations were adjusted. During construction, however, substantial new fill was placed to raise site grades, requiring a tall MSE retaining wall near a wetland and an area where deeper organic soils had been encountered.<br /><br />In early 2022, part of the wall began settling dramatically—up to 5.5 feet. Further investigation revealed a very deep deposit of soft organic soil, interpreted as the infill of an ancient, cover-collapse sinkhole that earlier explorations had not identified. The settlement also damaged nearby stormwater and sewer lines. The team rebuilt about 300 feet of wall using lightweight foamed-glass aggregate to reduce loading, but movement resumed.<br /><br />Monitoring in 2023 showed both continued settlement and lateral ground movement extending to depths of about 60 feet. Modeling indicated that the wall could fail without further reinforcement. The selected solution was a continuous sheet-pile wall, up to 110 feet long, supported by ground anchors. Building 8 had also settled by as much as four inches; it was repaired by jacking its post-tensioned foundation with helical piles and filling gaps beneath the slab with polyurethane grout.<br /><br />By summer 2024, wall, building, utility, and hardscape repairs were complete. Monitoring showed the reinforced wall was largely stable, with minor lateral creep.<br /><br />The case highlights the difficulty of detecting deep, buried sinkholes; the importance of accounting for proposed grading and fill when planning subsurface investigations; and the value of ongoing survey and inclinometer monitoring, collaborative design, and staged remediation.

Keywords

Clearwater Florida

apartment project

sinkhole settlement

soft organic soil

MSE retaining wall

foamed-glass aggregate

sheet-pile wall

ground anchors

helical piles

inclinometer monitoring