Construction Progresses on $25M Subterranean Parking Garage Reaching 3 Stories Below Sea Level

Construction is progressing on what’s said to become Miami, Florida’s deepest, most expensive underground parking garage. Reaching nearly 50 feet and three stories below sea level, the 100,000-square-foot, 236-car garage for Miami’s Una Residences condominium will cost the developers $25 million to build. The first-of-its-kind subterranean parking garage, set on the edge of the Biscayne Bay waterfront, will become a key element of the 47-story luxury residential tower upon its completion in 2023.

Developers OKO Group and Cain International have enlisted architects Adrian Smith + Gordon Gill to design Una Residences. The 579-foot-tall tower was envisioned to embody the spirit of the city and the aesthetics of the Brickell neighborhood’s urban, waterfront environment. The building’s grand, double-height lobby was designed with 20-foot-tall glass windows that frame the sparkling views of the bay, with a resort-style amenity pool deck set directly above facing the water surrounded by shady trees and lush foliage.

This open waterfront concept could only be realized by “hiding” the parking garage underground – a challenging and costly approach that contrasts sharply with typical condominium developments in South Florida, which tend to feature a pool deck situated high atop several levels of parking. To create the underground structure, the development, design and building teams worked together to determine the best method for subterranean construction in such close proximity to the bay.

In order to build a garage that doesn’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 massive watertight concrete box deep underground. Using advanced technology and equipment, the complicated undertaking requires workers to drill 800 holes 50 feet deep into the ground and fill them with concrete and water. The interlocking pillars create a cement block that is hollowed out to build the garage.

Following the building’s groundbreaking earlier this year, the construction team embarked on the deep-soil-mixing construction process to create a giant waterproof “bathtub” that protects the building’s concrete mat above the tub from the 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. 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.

Once the soil mixing process and tub is complete in early 2021, the construction team will begin drilling piles 135 feet deep into the ground to support the tower, and to hold down the floor of the tub during construction. The loose-mix soil inside of the tub will then be mass excavated – a three-month-long process – to reveal the new, waterproof underground form. Once 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 commence, 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 top-off of the 47th floor in late 2022.

Photos: OKO Group/Cain International.