What Borings Don’t Tell You (But Everyone Assumes They Do)

Point data, interpreted conditions, and where projects start filling in the gaps

Borings are treated like a representation of the site.

They’re not.

They’re a set of discrete data points, collected at specific locations, under specific conditions, and interpreted into something that can support design.

That distinction matters more than most projects acknowledge.

Borings Are Not Continuous Data

A typical investigation might include:

• 3–10 borings across a building pad

• Spacing anywhere from 50 to 200+ feet

• Depths driven by anticipated foundation influence

Between those borings, everything is:

interpolated using engineering judgment

Not measured.

What Gets Measured vs What Gets Assumed

Measured:

• Soil classification (USCS)

• Blow counts (SPT N-values)

• Moisture content

• Laboratory index properties (LL, PI, etc.)

Assumed:

• Continuity of strata between borings

• Lateral extent of weak zones

• Uniformity of bearing conditions

• Groundwater behavior outside observed conditions

Those assumptions are necessary—but they’re also where variability lives.

Stratigraphy Is Rarely Clean

Logs tend to suggest layers:

• Fill

• Clay

• Sand

• Rock

In reality, what’s often present is:

• interbedded materials

• lenses

• transitional zones

• abrupt changes over short distances

A boring captures one vertical profile.

It does not define how that profile behaves laterally.

SPT Data: Useful, Not Absolute

SPT N-values are commonly used to:

• estimate density/consistency

• inform bearing capacity

• guide settlement assumptions

But:

• they’re influenced by equipment, operator, and conditions

• they represent a localized resistance, not a uniform layer property

Treating N-values as continuous across a site is convenient—but not strictly accurate.

Groundwater Is a Snapshot

Groundwater observations in borings reflect:

• conditions at the time of drilling

• short-term stabilization (if measured)

They do not necessarily represent:

• seasonal highs

• perched water conditions

• site-wide behavior

Design assumptions often rely on these observations anyway.

Spacing Drives Risk More Than Depth

Projects often focus on:

• “Did we go deep enough?”

Less attention is paid to:

• “Did we space them appropriately?”

Wide spacing increases the likelihood of:

• missing localized weak zones

• underestimating variability

• encountering conditions that were technically “possible,” but not observed

Why This Matters During Construction

When field conditions don’t match expectations, it’s usually not because:

• the borings were “wrong”

It’s because:

• the assumptions between them were optimistic

This shows up as:

• expanded undercut

• variable subgrade response during proofrolling

• inconsistent bearing conditions

• unanticipated stabilization

The Practical Reality

Borings are a sampling tool—not a full site characterization.

They provide:

• a framework for understanding conditions

• a basis for design

They do not eliminate:

• variability

• uncertainty

• the need for field judgment

What Experienced Teams Do Differently

They don’t just look at the logs.

They ask:

• Where could this interpretation break down?

• How sensitive is the design to that variability?

• What’s the cost if one boring doesn’t represent the surrounding area?

That’s where borings stop being data—and start becoming decisions.

Final Thought

Borings don’t tell you exactly what’s in the ground.

They tell you enough to make an informed assumption.

Everything beyond that is how the project chooses to handle uncertainty.