Clearwater Construction Project Sinkhole

Photo of a retaining wall with a significant dip in the middle as a result of an underlying, previously undisclosed sinkhole affecting its integrity.

In Clearwater, FL, an ancient, large-diameter sinkhole was reactivated by overlying construction of an apartment complex, and its presence significantly impacted the property’s retaining walls and building foundations. This case study explores the effects of this initially undisclosed sinkhole on the construction project.

ECS’ role

The client engaged ECS to first characterize the sinkhole, then determine the remedial work needed to reconstruct a retaining wall and underpin one building’s foundations.

ECS was engaged for one and a half years to characterize and fix the problem, serving as the geotechnical engineer of record and also performing construction materials testing during construction. Our solution was expensive to implement, but the client was insured for adverse sinkhole activities and their impacts to construction.

Unique challenges and solutions

Supplemental, very deep borings in the vicinity of the most severe wall settlement led to the characterization of a previously undiscovered, large-diameter sinkhole whose deep throat was filled with low-strength, compressible organic soils.

Initially, the large-diameter sinkhole only appeared to significantly impact an overlying, 18-ft tall segmental retaining wall and its nearby underground utilities, including storm, sanitary and water. The wall settlement was as much as 5.5 ft, at which time the wall was declared severely damaged and likely to fail structurally.

Initial remediation efforts included de-constructing a 300-ft portion of a longer 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 the wall settlement was observed again and now measured 0.5 to 2 ft; less than initial wall construction but not expected with the net stress reduction. Also at this time, an adjacent building started showing adverse movement which did not occur during the initial wall remediation.

The engineers suspected 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 installation of inclinometers, 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.

Diagram of building 8 affected by underlying sinkhole. A color coded bar shows location and severity of the affected sections of the building.

Innovation and impact

The accumulation of lateral earthen movements from the inclinometers and wall settlement measured using conventional surveying methods allowed the geotechnical engineers to model future wall behavior using finite element methods, with the model calibrated by actual field wall measurements. With a confident finite element model, the engineers predicted the wall would eventually fail if not structurally reinforced.

Results and lessons

ECS collaborated with a geostructural contractor to design a deep sheet-pile wall system to intercept the failing soil mass and stop retaining wall and adjacent structure lateral movements. Ultimately, the segmental wall was stabilized from lateral movement using corroborating data from the inclinometers and survey measurements. Adjacent structures were jacked to restore finish floor elevation and were underpinned with helical piles and urethane grout as part of the remediation process. Ultimately the subject wall, underground utilities and adjacent building were either stabilized or repaired resulting in a completed project.