Underground parking construction in waterfront urban environments presents some of the most complex engineering challenges in the building industry. The Una Residences condominium project in Miami’s Brickell neighborhood demonstrates what is possible when developers commit to subterranean parking as a design strategy rather than an afterthought. This 47-story luxury tower required its developers to construct a three-level, 100,000-square-foot underground parking garage nearly 50 feet below sea level, at a cost of $25 million. The scale and complexity of this Parking Space Types and Multi Level Car Parking project sets new benchmarks for deep excavation in South Florida’s high-water-table conditions. For construction professionals, the methods used on this site offer valuable insights into deep-soil mixing, waterproofing strategies, and phased excavation in sensitive waterfront locations.
The $25 Million Underground Vision
The decision to build a fully subterranean parking structure for Una Residences was driven by design ambition rather than necessity. Developers OKO Group and Cain International wanted to preserve the site’s Biscayne Bay waterfront views for residents, which meant eliminating the parking podium that typically occupies the first several floors of South Florida condominium towers. Instead of stacking parking above ground, the team chose to hide the entire 236-car garage beneath the tower.
Why Subterranean Parking Cost Triple the Standard Rate
According to William Real, President of Civic Construction Company, a below-ground garage of this caliber costs triple the amount of a typical parking garage. Several factors drive this cost premium:
- Deep excavation requires specialized shoring and groundwater control systems not needed in above-grade structures
- Waterproofing measures must address hydrostatic pressure from groundwater that rises and falls with tidal cycles
- Material transportation and concrete placement at depth add logistical complexity and labor hours
- Structural design must resist lateral earth pressure in addition to vertical loads from the tower above
- Safety protocols for deep excavation in urban waterfront settings require continuous monitoring
Ahmet Oktay Cini, Chief Operating Officer of OKO Group, noted that the firm’s experience with subterranean construction in Moscow, where a six-level garage for over 2,000 vehicles was built 72 feet underground adjacent to the Moscow River, directly informed the approach taken in Miami. The goal was to create a modern condo tower without a parking podium, freeing up square footage for luxury amenities and residences with unobstructed waterfront views.
Project Team and Design Coordination
The project brought together a world-class team of specialists. Adrian Smith + Gordon Gill Architecture (AS+GG), known for designing some of the world’s tallest towers, designed the 579-foot-tall tower. The general contractor was a joint venture between Civic Construction and Ant Yapi U.S., with Keller International serving as the specialty subcontractor for the deep foundation and soil mixing work. This collaboration between design, development, and construction teams was essential for determining the best method for subterranean construction in such close proximity to Biscayne Bay.
Deep Soil Mixing and the Watertight Bathtub
The central engineering challenge was building a parking garage that would not flood despite being constructed nearly 50 feet below sea level on the edge of Biscayne Bay. The solution was a massive watertight concrete box deep underground, constructed using a deep-soil-mixing technique that creates a waterproof bathtub structure. This approach has parallels in other challenging parking structure projects where groundwater management is critical, as discussed in the article on Designing Greener Parking Structures How Mobility Changes Are.
The Deep Soil Mixing Process
The deep-soil-mixing process fundamentally changes the composition of the ground to create an impermeable barrier. Engineers used a state-of-the-art, 10-foot high-torque drill to dig into the site’s crushed limestone while simultaneously injecting cement slurry into the ground and blending it with the native limestone rock and sand. This changes the soil’s composition, creating a support system for the bathtub’s walls and floor while reducing water flow and permeability to allow for excavation.
The construction sequence followed these steps:
- Workers drilled 800 holes 50 feet deep into the ground in an interlocking pattern
- Each hole was filled with concrete and water to create interlocking cement pillars
- The interlocking pillars formed a continuous cement block wall around the excavation perimeter
- The interior of this cement block was later hollowed out to build the garage
This method creates a barrier that protects the building’s concrete mat above the tub from groundwater while also forming the base of the garage itself.
Waterproofing Strategies for Below-Sea-Level Construction
Waterproofing a structure built below sea level in a coastal environment requires multiple lines of defense. The deep-soil-mixed bathtub wall serves as the primary barrier, but the design incorporates several additional features:
- A concrete mat foundation constructed after excavation provides a permanent floor seal
- Steel reinforcement within the mat addresses tensile stresses from hydrostatic uplift pressure
- The interlocking soil-mix columns create a monolithic barrier with minimal joints or seams that could leak
- Continuous grout injection during drilling ensures complete filling of void spaces in the limestone
Construction Equipment and Sequential Phasing
The bathtub and plug operation required six different types of specialized equipment working in sequence. Each piece of equipment performed a specific function in the soil mixing and foundation installation process, and the coordination of these machines on a constrained urban waterfront site demanded careful planning.
Equipment Types Used in the Bathtub Construction
| Equipment Type | Function | Application |
|---|---|---|
| Soil-mix predrilling rig | Pre-drills pilot holes through limestone | Creates initial penetration paths for mixing tools |
| Soil-mix bottom plug rig | Mixes soil and cement at base level | Forms the impermeable floor plug of the bathtub |
| Twin soil mix (TSM) 20-inch wall cased CFA rig | Constructs secant pile wall sections | Builds the primary wall barrier at specified thickness |
| TSM 36-inch wall rig | Constructs deeper wall sections | Handles the deeper perimeter where greater wall thickness is required |
| 18-inch augercast piles rig | Installs smaller-diameter foundation piles | Supports lighter structural loads within the tub |
| 36-inch augercast piles rig | Installs large-diameter foundation piles | Transfers tower loads through the tub to bearing strata |
The use of twin soil mix technology was particularly significant. The TSM rigs could simultaneously drill and mix, creating continuous overlapping columns that formed a monolithic wall system. This eliminated the need for conventional sheet piling or diaphragm wall construction, which would have been more disruptive in the sensitive waterfront environment.
Phased Construction Timeline
The construction sequence followed a carefully orchestrated timeline with distinct phases:
- Phase 1 – Soil mixing and tub creation (2020-early 2021): The deep-soil-mixing process created the waterproof bathtub walls and floor plug. This phase required all six equipment types working in coordinated sequence.
- Phase 2 – Deep pile installation (early 2021): Once the tub was complete, crews drilled piles 135 feet deep into the ground to support the tower and hold down the tub floor during excavation. The deep piles resist the buoyancy forces that would otherwise lift the empty bathtub structure.
- Phase 3 – Mass excavation (three months): The loose-mix soil inside the tub was excavated to reveal the waterproof underground form. This three-month process required careful monitoring of groundwater levels and structural stability.
- Phase 4 – Foundation construction: After excavation, the steel mat foundation was installed followed by a massive concrete foundation pour that created the permanent garage floor slab.
- Phase 5 – Vertical construction (late 2021-2022): Once the three levels of vertical columns were completed in the underground basement, ground-level vertical construction commenced, accelerating to approximately one floor per week through the top-off of the 47th floor in late 2022.
Engineering Lessons for Urban Parking Structures
The Una Residences underground parking garage offers several lessons for construction professionals working on deep excavation projects in challenging urban environments. The integration of Metal Panels for Parking Garage Facades Material Performance and other enclosure systems must be coordinated with the below-grade waterproofing strategy to ensure continuity of the building envelope.
Key Takeaways for Similar Projects
- Deep-soil-mixing technology makes below-sea-level parking garages feasible even in high-water-table environments, eliminating the need for dewatering systems that could impact adjacent properties
- The triple cost premium for underground versus above-grade parking must be weighed against the value of preserved waterfront views and increased amenity space on lower floors
- Joint venture contracting structures, as used between Civic Construction and Ant Yapi U.S., allow firms to combine local knowledge with specialized deep-foundation expertise
- Sequencing that completes the waterproof tub before excavation begins reduces risk and allows for dry construction conditions below the water table
- Deep tension piles are essential for resisting hydrostatic uplift forces in below-sea-level structures
Cost-Benefit Analysis of Underground Parking
The $25 million investment in underground parking at Una Residences represents a significant premium over conventional above-grade parking podiums. However, the preserved waterfront views, which allowed the tower to offer unobstructed vistas of the Atlantic Ocean, Biscayne Bay, and the Miami skyline, contributed to unit prices ranging from $2 million to $7.4 million, with penthouses reaching $21.6 million. The relationship between parking configuration and property value is a critical consideration for developers weighing similar subterranean strategies.
| Parking Approach | Typical Cost per Space | Impact on Buildable Square Footage | View Preservation |
|---|---|---|---|
| Above-grade podium (standard) | $15,000 – $25,000 | Reduces available residential floors by 3-4 levels | Obstructed lower-floor views |
| Underground (Una Residences) | $75,000 – $106,000 | Full site available for amenities and tower | Unobstructed from all floors |
| Off-site leased parking | $5,000 – $10,000 annual lease | Full site available for tower only | Unobstructed but inconvenient |
For existing structures requiring basement waterproofing or parking garage restoration, the techniques demonstrated on this project offer relevant reference points. The article on Hydrodemolition for Active Parking Garage Restoration Managing Noise covers complementary approaches for managing concrete structures in occupied urban settings.
Broader Implications for Waterfront Construction
As sea levels rise and coastal development densities increase, the techniques pioneered at Una Residences will become more relevant to the broader construction industry. Deep-soil-mixing technology provides a method for building below-grade structures without dewatering the surrounding aquifer, which is increasingly regulated in coastal zones. The ability to construct watertight bathtub structures in porous limestone geology opens possibilities for underground parking, storage, and utility spaces in cities where high water tables have historically limited basement construction. Completed in 2023, Una Residences stands as a demonstration that deep underground parking is technically and economically feasible in South Florida, provided the project team brings the right combination of specialized equipment, engineering expertise, and commitment to quality construction.