A warehouse, roadway and sign foundation may all sit on the same type of soil, but they do not affect that soil in the same way. The size, weight and construction method change what engineers need to know about the ground below.

That’s why each geotechnical investigation should start with three basic questions: What are you building, where are you building it, and how do you plan to construct it?

The answers determine where soil borings should be placed, how deep they should go and what laboratory testing may be needed.

Start With What, Where and How

Every project changes the conditions at a site. Buildings and pavements add weight, increasing stress within the soil. Excavation removes material and reduces that stress. Both can change how the subgrade is affected by the project.

Understanding what will be built helps the geotechnical engineer focus the investigation on the soil conditions most likely to affect the project.

The location is just as important. Soil, rock and groundwater conditions can vary considerably, even between nearby properties. A foundation or pavement design that performs well at one site may not work at another.

Engineers also need a general understanding of how the work will be built. Site access, excavation methods, equipment and construction sequencing can affect whether a recommendation is practical in the field. What looks straightforward on a drawing may be much harder to execute on the jobsite.

Deciding Where to Drill

The size of the project usually drives the number of borings. A warehouse with a large footprint and surrounding pavement needs more investigation points than a roadway sign or other small improvement.

To select the locations, engineers review the proposed construction along with the site’s topography, available soil and geologic information, and historical aerial photography. This helps identify areas where conditions may change across the property.

“A boring provides the most detailed information at the specific location where it is drilled,” Ficenec said.

Borings are generally placed where the proposed construction will have the greatest effect on the ground. For a building, that usually means drilling within the footprint rather than in an area the project will not disturb.

The locations also have to be accessible. Drill rigs vary in size and capability. Steep terrain, overhead restrictions, existing structures and other obstacles may limit where the equipment can safely operate, and deeper borings require bigger rigs.

Depth depends on how far the project is expected to influence the ground. If crews will excavate, engineers need information about the materials they may encounter. If construction will add weight, the investigation must extend far enough to evaluate how that load could affect the soil below.

“Soil failure and soil settlement do not occur only at the surface,” Ficenec said.

What Happens in the Laboratory

Once the soil samples are collected, they are sealed and transported to the laboratory. An engineer reviews the field logs, examines the samples and selects the tests that will provide useful information for the project.

Depending on the samples and investigation method, testing may measure moisture content, density, shear strength, grain size or compressibility.

“The boring data allow us to model the subgrade to the depth explored, while laboratory testing helps us evaluate how individual soil layers will behave under the expected changes,” Ficenec said.

That information becomes the basis for the recommendations in the geotechnical report.

For foundations, engineers evaluate how much load the soil can support and how much settlement will occur. For pavements, the findings can influence subgrade preparation, pavement thickness and drainage. Grading recommendations account for the changes created when soil is added or removed from different locations on the project site.

The investigation can also influence construction materials, equipment use and how the contractor prepares for conditions that may be encountered during the work.

What Borings Cannot Show

A boring records the conditions at one location. Engineers use that information, along with their knowledge of the site’s geology, to estimate what may exist between investigation points. There will always be some uncertainty.

“We can never determine what all is under the ground,” Ficenec said.

Old foundations, undocumented dumping areas and unexpected soil layers may not appear in the available records or borings. That does not mean the investigation failed. It reflects the reality that underground conditions can vary over short distances.

More investigation can fill in some of those gaps, but every additional boring and laboratory test adds cost.

“More borings and laboratory testing help give us a better picture of the subgrade, but that also means a more expensive investigation, and the results of additional testing after the fact may not change the outcome of the recommendations,” Ficenec said.

The goal is to collect enough information to support sound recommendations without performing tests that are unlikely to change the outcome. If crews later encounter conditions that differ from the report, the geotechnical engineer can evaluate what was found and determine whether the recommendations need to change.

A well-planned investigation gives the owner, design team and contractor a better understanding of the site before construction begins. That knowledge can shape the design, prepare the team for field conditions and help address problems while there is still time to respond.

Planning your next project? Reach out to learn how our geotechnical engineers can help!

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