Located on tight sites in highly populated areas inside city limits, the projects are challenging and require the implementation of both traditional and unique construction methods to get the job done safe, on time, on budget and to the highest degree of quality.

The new wastewater lift stations utilize sunken caisson wet wells as the primary structure and construction method. The cast-in-place concrete construction of the wet walls was completed above grade in sections, ranging from 16 to 18 feet in diameter, through multiple cylindrical concrete lifts. In addition, the wet wells are designed with a cutting profile cast into the concrete bottom. Two primary mechanics ensured the structures could sink in the Floridian soils with a high water table. First, the sheer weight of the structure enabled settling, with each section lift weighing upwards of 300,000 pounds. Second, excavating the soil within the interior of the structure initiated the cutting edge of the structure’s bottom, allowing the wet well to sink to its final location. Once the structure was sunk to its elevation, a plug slab was placed using an underwater tremie method, creating a watertight wet well structure.

In order to excavate the material from within the structure, the team used an innovative telescopic dipper arm, a teledipper. This specialized excavator attachment is designed for deep excavation applications and features a clamshell-style bucket mounted to a telescoping arm. The equipment has the ability to reach distances of 10 to 50 feet in depth at steep angles (including approaching 90 degrees), making it ideal for this type of work.

PC self-performed concrete construction along with site maintenance. We were fortunate to work with our trade partner, Danus Utilities, who expertly managed the specialty excavation via teledipper during this process.

This innovative sunken caisson method, and the use of the teledipper, reduced the area of impact by 80% when compared to conventional open-cut excavation and cast-in-place methods. The overall process also minimized hazards to existing utilities, public roads and pedestrian and vehicular traffic.

Photos Top to Bottom: 1) The teledipper excavating soil from within the interior of the structure; 2) Excavation at a near 90-degree angle; 3) The sunken wet well.

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