How Many Soil Borings Do Development Sites Need?

One of the challenges that developers – both public and private – face from a geotechnical and environmental standpoint is the inherent uncertainty in what’s underground at the development site. Generally, we’d like to know the geologic layers, soil types, groundwater levels and potential environmental contaminants across a site. But trying to characterize a fairly large volume of soil with just a few pieces of information inevitably leaves knowledge gaps.

An illustration for this challenge comes from an unexpected source – a children’s book. “Sam & Dave Dig a Hole,” written by Mac Barnett and illustrated by Jon Klassen, is a funny, deadpan story about two boys (and a dog) who dig a hole, hoping to find “something spectacular” (website here). 

Sam and Dave Dig a Hole

Photos courtesy of Mac Barnett and Jon Klassen

As the boys dig through the ground, they come close to, but never discover, several spectacular gems.

Sam and Dave miss the gem

In fact, they seem to navigate around everything spectacular.

Sam and Dave digging around the gem

While the book is an admittedly whimsical analogy to geotechnical and environmental subsurface exploration, it actually serves to illustrate an important point – there may be more beneath the surface of a site than a couple of borings will indicate. Skimping on borings increases the chances that zones of contamination or soft soils, may be missed, only to be discovered during or after construction. More borings can help fill in gaps and increase confidence that the site has been well-characterized. In many cases, spending bit more money on site exploration may reduce overall project costs by reducing uncertainty about the site and what may be encountered during construction. And depending on project needs and site conditions, the use of less conventional site investigation methods (Cone Penetration Test, strataprobe) may be appropriate. These can often provide better spatial coverage at similar costs to traditional Standard Penetration Test borings, because they’re cheaper. 

Of course, there’s no one-size-fits-all approach to subsurface exploration. The best exploration program for a project will balance project needs, budget, and local experience with geologic conditions. But in order to minimize the chances of pulling a Sam and Dave, maximizing spatial coverage in the explorations program should be a consideration.