A luxury condo tower in Miami will be home to the deepest underground parking garage ever built in the city. And at $25 million, it will also be one of the most expensive.

Construction of the 100,000-square-foot garage will rely on advanced technology, design and engineering. Expected to be finished by the end of next year, the 236-car garage buried three stories below ground will serve as a watertight foundation for the 47-story Una Residences. The building is located just feet away from the Biscayne Bay waterfront in the city's Brickell neighborhood. 

In order to ensure that the garage won't flood, the building’s general contractor, a joint venture between Civic Construction and Ant Yapi U.S., along with specialty subcontractor Keller International, set out to create a watertight concrete box underground. The project, which began earlier this year, required workers to drill 800 holes 50 feet deep into the ground and fill them with concrete and water. The interlocking pillars created a cement block that is hollowed out to build the garage, according to a statement from project developer OKO Group emailed to Construction Dive.

“A below-ground garage of this caliber costs triple the amount of a typical parking garage," said William Real, president of Civic Construction in an email. "This kind of major investment in underground construction has never been seen before in Miami." 

The construction team used a deep-soil-mixing construction process to create a waterproof bathtub-like structure that protects the building’s concrete mat above the tub from groundwater and forms the base of the garage. Engineers are now using a 10-foot high-torque drill to create the bathtub by digging into the site’s crushed limestone, while simultaneously injecting cement slurry into the ground and blending it with the limestone rock and sand, he said. 

This process changes the composition of the soil, creating a support system for the bathtub’s walls and floor while ultimately reducing waterflow and permeability to allow for excavation of the site, according to the OKO Group and co-developer Cain International.

Once the soil-mixing process and tub are complete early next year, the construction team will begin drilling piles 135 feet deep into the ground to support the tower and hold down the floor of the tub during construction. The loose-mix soil inside of the tub will then be mass excavated in a three-month process to reveal the new, waterproof underground form.

Subterranean construction

To create the underground structure, the development, design, and building teams worked closely together to determine the best method for subterranean construction in such close proximity to the bay, said Ahmet Oktay Cini, COO of OKO Group, which earned extensive experience with subterranean construction at its Capital City mixed-use development in Moscow. That project encompassed a six-level garage for over 2,000 vehicles built 72 feet deep underground adjacent to the Moscow River.

“When conceptualizing the design for Una Residences in Miami, we envisioned a similar urban-style, efficient tower that would maximize as much space as possible for the use of residents and showcase the site’s waterfront views," he said. "The result was a modern condo tower without a parking podium, meaning more square footage for luxury amenities and residences.”

Work during the coronavirus pandemic has required special safety protocols, according to the OKO Group. The contractor implemented daily symptom monitoring for all employees, mandatory face masks and handwashing stations.

After the site is excavated, the building’s steel mat foundation will then be constructed followed by a massive concrete foundation pour. Once the three levels of vertical columns are completed in the underground basement, ground-level vertical construction will then begin, estimated to take place in late 2021. From there, the tower’s rise will move at a fairly quick pace, accelerating approximately one level a week through topoff of the 47th floor in late 2022.

While typical condominium developments in South Florida tend to feature a pool deck situated atop several levels of parking, the Una project team hid the garage underground in order to realize an open waterfront concept, the statement said. 

When complete in 2023, the 579-foot-tall tower, designed by Adrian Smith + Gordon Gill (AS+GG) will be comprised of 135 condominium units boasting unobstructed views of the Atlantic Ocean, Biscayne Bay and the Miami skyline. Units are priced from $2 million to $7.4 million, with penthouses up to $21.6 million.