Good Vibrations

Few things prove more foundational to a new building than, well, its foundation. A job needs to start on solid footing before the first frame ever goes up. Yet many projects are more like building on sand than solid rock.

The land in major cities may contain fill soil from prior builds. Some job sites are full of soft silt and clay. Either way, contractors have to shore up sites ahead of any construction. That stabilization means swapping out the soft soil or installing a deep foundation. A century-old St. Louis business has bolstered its success with a new, more efficient process.

Subsurface Constructors established its business in 1906. Since then, the firm has helped pioneer recent usage of vibro stone columns. The process stabilizes new building foundations in the US. That innovative process has led to Subsurface’s success, with projects in more than 30 states.

Mast Appeal

Subsurface began with the union of two St. Louis companies: The Smith & Brennan Pile company and Wabash Drilling. Both companies built foundations for many of the city’s key buildings. The company made its name stabilizing soft soil via deep foundations.

Then, nearly a century after the company began, things shifted.

“The big change for us was about 2005 when we decided to get into ground improvement, specifically aggregate pier ground improvement,” says Lyle Simonton, director of business development. “We first partnered with an overseas company to acquire equipment, but that partnership ran its course and we needed to innovate in order to grow. That’s when we developed our mast attachment."

Subsurface places aggregate piers, known as vibro stone columns, using a mast-mounted vibratory probe to compact stone in columns. After learning about the equipment from a U.K. company, Subsurface became among the first stateside to work with vibratory aggregate piers. The company created its special mast now central to its process. As the rig crowds the vibratory into the ground, soft soil is pushed aside to create space for lifts of stone that are highly compacted with the probe.

At first, Subsurface planned to affix its masts to rental excavators across the country. But the unreliable availability of such machines caused the firm to seek out an equipment company “that would allow us to be able to be responsive enough, take this equipment around the country and do this work,” Lyle continues.

Subsurface is known as a full-service geotechnical contractor that’s become recognized for its ground improvement. With offices in St. Louis as well as Boston, Minneapolis, and Cleveland, the company tackles jobs nationwide. And all that requires a reliable fleet of machines.

Subsurface currently operates with 15 Doosan excavators, ranging from the DX170LC-5 to the DX350LC-5. The company’s mast, once installed to the machine, towers high over the working excavator.

Tight Spots in Old Cities

On a 2020 job in St. Louis, Doosan DX180LC-5 and DX350LC-5 excavators worked in tandem to install 410 vibro stone columns. The hundreds of columns, placed over 10 days in August, would support a residential building downtown.

The smaller excavator, affixed with an auger, served to pre-drill holes, says Subsurface Equipment Manager Ernie Dawson. It allowed the bigger DX350LC-5 excavator with the mast to install columns efficiently behind it. The columns ended up “real close to each other,” Dawson says, a result of the cramped urban site and soft soil conditions.

The trickiest sites Subsurface works on are bustling cities like St. Louis. There’s typically less space to maneuver in an urban area. And the older the city, the more likely multiple buildings have risen and fallen on that land, leaving remnants of their histories in the ever-softening soil.

So the biggest challenge for Subsurface’s five-man crew on the project? Just finding out what was in that ground in the first place, Dawson says.

“Urban fill can be anything: Rubble, old parking curbs,” he says. “You go out there and do a test hole to see what kind of material you’re going to be dealing with. You might drill into the perfect ideal location over here in Grid 1A. Well, when you get to D5, it’s a five-foot parameter of cement.”

Once the ground conditions are ascertained as well as possible, the column installation begins. Drill. Install. Drill. Install. Do that 408 more times. A few days and a few hundred holes later, the vibro stone columns were securely underground, all in a measured, methodical pattern that accounted for the site’s lack of space.

"You get out in a cornfield someplace and there’s nothing around to interfere with you and you can go to town,” Dawson says.

But in the city? In the city—whether on Subsurface jobs in St. Louis or Chicago, New York or Philadelphia — you’ve got to plan your moves.

“Every town, every city is like that. That’s the tricky part,” Dawson says. “How efficiently and how quickly can we do this? So we can move onto the next location.”