How Builders Discover What Lies Beneath
The Leaning Tower of Pisa is the most famous example of what happens when builders fail to do due diligence on the land beneath their buildings and the things that may lie beneath it.
Unfortunately, back in the 1100s, there wasn’t really anyone around who knew enough about such things to advise the tower’s builders on how to avoid potential problems.
Today, there are experts who can offer that advice. They’re known as “geotechnical engineers.”
Ernie Hanna is one. He’s the principal in the Philadelphia office of GZA, a Massachusetts-based engineering and construction management firm with two other offices in this area; the Center City Philadelphia office is its newest.
Geotechnical engineering is just one of the services GZA offers, but it’s the most unusual one. Geotechnical engineers help builders learn about issues that could cause problems with their buildings before they’re built. They also help builders figure out how to solve them if they arise afterwards.
Hanna gave this example of what his firm does on the “before” side. “If someone is looking to develop a brownfield site” — a lot that has been built upon before — “we do testing and drilling to find a suitable layer where they can put a foundation in.” The firm works with structural engineers to determine the best foundation solution and recommends alternatives that might allow for less expensive foundations where subsurface conditions are poor.
The “after” side is GZA’s forensic engineering work. “We determine the source of issues like settling, leaning or water intrusion and work with the owner or developer to determine how to remediate the problem,” Hanna said.
Before the shovels go into the ground, GZA’s work is valuable during the “value engineering” phase of a project. This is the part of the development process where the architects, developers and builders figure out how to trim costs from a project. For instance, Hanna said, “an architect or structural engineer might say that a building has to be put on a deep foundation with piles” because of poor soil conditions, “and that requirement causes a financial challenge to the project. The developer may then hire a firm like GZA to determine the subsurface issues. We may be able to reduce the number of piles or put part of the building on shallow foundations in order to reduce costs.”
The most common problems builders working with brownfield sites face, he said, arise from contaminated soil or groundwater. In those cases, GZA’s engineers recommend ways to clean up or otherwise deal with the contamination. The most common problems found on uncontaminated sites, Hanna said, have to do with soft soils, high groundwater tables and fill soils that may not be stable enough to support a foundation. In those cases, the geotechnical engineers work with the property owner and the developer to address those problems in a cost-effective manner.
Sometimes, the solutions are very simple. In one case Hanna described, a lot had contaminated soil, but moving the soil to a landfill would add to the project cost. So what GZA recommended was removing the contaminated soil from the surface and burying it beneath the slab foundation. The developer was then able to sell the clean clay soil that was removed in order to dump the contaminated stuff in the hole to a landfill owner who was looking for soil to cap his site. “The state was happy, for the contaminated soil remained on the site, and the developer came out ahead as well,” he said.
GZA works mainly with commercial and industrial clients, but it also does some work for developers of multifamily residential and lodging projects.
How much can developers save by hiring a geotechnical engineer to check out a site before construction begins? Hanna said, “If what we find in the subsurface is what everyone expected, there’s limited savings there. But we can save 10 percent on foundation costs just by having talented engineers who can put in a suitable bearing allowance in the soil. And if someone thinks something’s down there, but it turns out it’s just poor soil that can be fixed, we could save 15 to 20 percent.
“We tell our clients, It’s cheap to do a subsurface investigation relative to the total cost of the project. It’s better to do one beforehand than to find a surprise later on.”
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