Digging Deep: Old processes and new products conquer subsurface challenges

In a region with expanses of high ground water and little undeveloped land, geotechnical challenges are bound to happen on some construction sites.

As contracting was about to begin on a 14,000-square-foot addition to the Bocek Recreation Center in Baltimore City, a geotechnical report threw a complication into bidders’ calculations. The site had groundwater just six feet below the surface and the project plan placed utilities and foundations well below that elevation.

Furthermore, the report found a layer of uncontrolled fill — construction debris and other rubble — 22 feet down.

“Anytime you’re doing a new building in Baltimore City, you’re going to run into some type of soil challenge,” said Cliff Milstead, Vice President of Plano-Coudon Construction. “But the last thing you want to do is turn your whole construction site into a muddy swamp. Then it becomes hard to get material and equipment in and out. It becomes hard to do anything and it’s very costly to remove all that mud and bring back stone or better soil.”

The Bocek site challenge convinced Plano-Coudon to add a subcontractor that specialized in dewatering operations to the project team.

“We leaned pretty heavily on them to come up with a design based on the soil classifications and the water level,” said Chris Moody, Project Manager at Plano-Coudon.

That design featured a wellpoint system of small-diameter wells installed around the perimeter of the addition and connected to a header piping system and pump which would discharge the water after passing through a baffled settlement tank. To operate the system, the project team had to bring in a large generator to power the pump and trucks to supply water for the installation of the wellpoints.

“It’s funny to think that you have to bring in water to do dewatering,” Moody said. “But the system used a track punch installation method where they utilize jetted water to assist with the installation of the wellpoints.”

The endeavor included some surprises and uncertainties, including the realization that the team would need to secure a Water Appropriation Permit (a six-month process). Also, the geotechnical report did not determine how quickly the groundwater would recharge so the team couldn’t be exactly sure how long it would take to lower the water level or how much it would cost.

After a week of 24-hour pumping, however, the groundwater level dropped from six feet to 13 feet and made the site stable enough to support the drill rigs coming in to install auger cast piles, pile caps and other foundation elements for the addition.

A convenience store project saddled ECS Mid-Atlantic with similar challenges.

“We had undocumented, existing fill materials and subsurface conditions included soft, organic, silt materials and a high groundwater condition,” said Chuck Shaw, ECS Mid-Atlantic Group Manager.

For that project, the solution was completing ground improvements through rigid inclusions due to the organic silt materials and a high groundwater condition.

Installing those cementitious columns in the ground “transfers the load of the building through the undocumented fills, through the soft organics and into a more stable layer, and can provide a small amount of densification of the surrounding soils,” Shaw said.

Solving other geotechnical challenges has proven more challenging.

The site of a car wash development in Delaware turned out to have “about 80 feet of dredge materials,” Shaw said. “Rigid inclusions or deep foundations are way too expensive for a car wash, so we are looking at a removal and replacement option. Theoretically, if we remove enough of the dredge material and replace it with lightweight material, so that the building loads and lightweight material are equivalent to the weight of the removed dredge material, this should prevent immediate settlement beyond the typical tolerances allowed for building construction.”

Shaw who is still working out details of a solution for that client, said geotechnical challenges are becoming a more common reality of construction projects.

“All the good sites have already been developed, so often what you are working with are sites where there is some wetland or high groundwater, or redevelopment sites that often have undocumented fill,” he said.

Processes for remedying challenging site conditions haven’t changed much.

“You have heard the term ‘as old as dirt,’ right? That’s what we are dealing with,” said Hasan Aboumatar, Principal Engineer at Froehling & Robertson.

Most solutions still rely on old processes and products, such as localized wells, undercutting, sumps, ground improvements, soil cement, French drains, and under-slab drains.

But some practices and products are evolving.

“Lime treatment to dry out wet soil is common in Virginia, and now I am starting to see it used more often in Maryland,” Aboumatar said.

The industry has also developed better options for dealing with unsuitable soils beneath road and bridge sites.

“If you have soft subgrade when you’re building roads or bridges, we tend to use a lot of Geogrid or fabric,” Aboumatar said. “We limit the undercut to maybe 18 inches or two feet, we utilize the Geogrid, lay stone over it, then sometimes one or two more layers of Geogrid. Now, we have much better Geogrid for these types of applications. Geogrid used to be square so you had strength in two directions. Now, it is hexagonal so you have strength in all directions.”

Technology advances in cone penetrometers and dilatometers are also producing more advanced analysis of subsurface conditions before construction begins, he said.