Most built structures are seated in or on the earth. But what lies beneath? How do you know your investment won't settle and twist? What are the chances of a basement being under water in spring? What soil layers are reusable for structural fill? These questions rarely have absolute answers, but geotechnical data helps produce design and construction recommendations with more certainty.

I was recently on a site where a Kinsley Construction worker was also working nearby. We exchanged traditional greetings and asked, “What are you up to?” followed by “Can't say.” He inquired whether we were taking core samples. It turns out, he’s a foreman who is not only familiar with geotechnical data but also uses geotechnical reports to plan his approach to site work.

Data is collected at discrete points during an investigation. When drilling is involved, the total hole diameter is about 6 inches, the drill stem is a 3.25" hollow pipe through which 2" split spoons are sent down to collect a sample. So, our window is quite small, often no more than 2 inches. A test pit gives a larger view, although shallower, which can be entered to examine in situ conditions. Still, everywhere in between these points, the data are assumptions.

Civil designers can use the data to optimize earthwork activities. With large sites, the variability between data points can become costly. I have seen a deep foundation job where the client/tenant paid for a driller to auger probe to the top of rock at every column location (we're talking $20-30k for supplemental data! They already had top of rock data across the site!) Because unknowns may cost unknown dollars when you’re in the middle of construction.

All the steel and concrete you can buy won't fix bad ground conditions. Or maybe you gamble that the site is fine. Well, part of Geotechnical work is forensics. We would be happy to squeeze into your new building and discover why that crack is growing, and the floor is slowly dropping!

As with many construction functions, experienced hands will advise you as to when more testing will be helpful or when it may not be necessary. As for planning, designing and executing a project, forget x-ray glasses, use geotechnical data to understand what supports your site.