Hardly a “day at the beach”: Building an underground parking garage on the Atlantic shore
The night after New Year’s Day, Ron Choron stood on 3,200 tons of steel rebar crisscrossing a cavernous hole dug about 50 feet underground in Sunny Isles Beach, Fla. Only a sliver of beach stood between Choron and the Atlantic Ocean, but the vice president of construction at Fortune International Group calmly watched a Suffolk Construction crew symphonically conduct the opening stanza of one of the largest concrete pours in South Florida’s history.
Six pumping trucks and four giant red cranes at the lip of the 225-by-150-foot pit dangled hoses spewing the first of 10,000 cubic yards of concrete under what will eventually be the ultra-luxury condominium complex Jade Signature.
“We would need scuba tanks to stand here if we hadn’t put in the soil-mix garage,” Choron said as 116 cement trucks continuously unloaded concrete mixed at 11 nearby plants throughout the night. “We would be an atmosphere and a half below water. And it doesn’t even hurt our ears. This is the first time this has ever been done on the ocean.”
A first-of-its-kind subterranean parking garage that sits right on the ocean is the bedrock of the innovative Collins Avenue condo palazzo designed by Herzog & de Meuron — the famed architects behind such gems as the Olympics Stadium in Beijing, known as the Bird’s Nest, and the Tate Modern in London.
A 425-car parking garage under a sleek and glamorous tower will easily (and perhaps intentionally) feel like a monumental afterthought when Jade Signature opens in two years. In actuality, the $35 million garage is one of the greatest engineering marvels the area has known. It’s also one of the major reasons the 61-story tower sent shockwaves through the South Florida condo market when it broke ground in 2013 and approximately 85 percent of the condos are already sold. Being constructed on porous ocean-side limestone and sand, the two-and-a-half-story garage will sit below a water table so shallow that an ambitious 10-year-old could reach it with a plastic toy shovel. But the unthinkable task of burying a garage on the shores of the Atlantic Ocean will only result in Jade Signature’s signature feature: A magnificent beach-level pool area that naturally flows into the tower’s lobby and luxuriously unprecedented three stories of amenities. A resort-like atmosphere of this kind can’t be found anywhere else in the South Florida condo marketplace because most oceanfront pool decks are perched on above-ground parking garages.
Burying such a deep parking garage on the ocean — something that was previously unthinkable — started with engineers from Malcolm Drilling Company designing a giant waterproof “bathtub” that protects a concrete mat above the tub from groundwater. In order to form the bathtub crews used a method known as deep soil mixing. A nine-foot paddle bit specially designed for the Jade project has a hollow core and a hybrid mixing auger. The giant high-torque drill simultaneously crushed limestone while injecting cement binder slurry, or grout, into the ground. Crushed limestone and sand served as a natural aggregate that created the fresh concrete-like material that by the next day hardened into the bathtub’s concrete walls.
“The soil becomes like fresh concrete as you mix it,” said Charles Bartlett, the former Malcolm Drilling vice president who concocted the bathtub method and currently runs his own consulting firm called Foundation Solution LLC.
The limestone between the bathtub walls is crushed by the drill’s bullet-teeth laden auger. In order to allow for an easier excavation and to use less cement, water is used as the drilling fluid until the mixing tool reaches the top of the bathtub floor. Once the drill reaches the measured depth of the top of the bathtub floor, more cement slurry is injected and mixed with the limestone to install the bathtub floor. Then auger cast pilings are drilled 149 feet into the ground to support the 61-story tower. These piles also hold down the floor of the bathtub during construction of the tower. Finally, the loose soil-mix inside the tub is dug out to reveal the bathtub’s floor and walls.
“We don’t have any leaks,” Choron crowed as the crew poured the eight-foot concrete mat that will serve as the buildings foundation. “You could put a Kleenex on that wall and it wouldn’t stick to it.”
Traditionally concrete poured in or near bodies of water is pumped through a steel pipe called a tremie that helps to create a temporary barrier from the groundwater so the water inside the bathtub can be pumped out. One major benefit of the deep soil mixing technique for the bathtub over a tremie seal is that you can excavate and evacuate dry material immediately without waiting for it to dry.
Still, Sunny Isles Building Official Clayton L. Parker was skeptical about using a “bathtub” at Jade since the garage is the deepest in the city’s history by about 20 feet.
“I said ‘There’s no way they are going to get that dewatering done,’” recalled Parker, who has more than 30 years of experience in construction. “I said ‘There’s just no way’ but the first step right here is very successful.”
Choron himself didn’t believe the bathtub would work when Fortune and Suffolk started researching new ways to build an underground garage two years ago and first reached out to Bartlett at Malcolm Drilling — which first used the deep soil mixing technique to construct the bathtub for a 22-foot deep parking garage at a mixed-use mega complex near the Miami River and Biscayne Bay called Brickell City Centre.
“[The Jade] site really pushed the envelope of what had been done before,” Bartlett said of the deep soil mixing bathtub method. “It was much deeper than Brickell City Centre.”
Bartlett’s inspiration for developing the deep soil mixing method for bathtub construction came from his work on the recently opened Port of Miami Tunnel — where Malcolm’s team used deep soil mixing to build an airtight chamber 100 feet underwater for crews working on the tunnel boring machine on the $1 billion project. The air chamber was shaped like an upside down bathtub. While he was floating in his pool one day “daydreaming ideas” on dry excavation Mr. Bartlett thought, “Hey let’s flip this thing the other way around and make a bathtub.”
Choron, who chose Malcolm over the only other two firms in the world capable of doing such a project, said his initial misgivings were ultimately eased by a “gut feeling” and “a lot of research.” His research included a trip to Suffolk’s Boston headquarters with J. Rick Kolb, Suffolk’s senior vice president in South Florida. They toured several underground parking garages on the Charles River along with Joe Salvia of Boston-based McNamara-Salvia Structural Engineers.
“I’m just happy we finally reached it,” Choron said. “It took us a year and a half to put in the concrete bathtub. A year and a half in a construction schedule is a lot, so I’m glad we can actually come up with a building.”
For discerning developers, the deep soil mixing bathtub method will no doubt be the new norm, Bartlett and others agreed. In fact, Choron said, given the right project and market, he would not hesitate to do it again.
“I learned a lot,” he said. “I’ve never done this before on an underground garage. We all learned a lot, including the Suffolk team, but it’s something we’ll be prepared for on the next project.”
FOR FUN: Click here to check out the full mat pour video!