How to Read a Soil Boring Log: What Geotechnical & CMT Professionals Look For

A boring log is the single most information-dense document to come out of a subsurface investigation. One page tells you what's under a site, how strong it is, where the water sits, and how confident you should be in all of it. But if you're new to geotechnical or CMT work, that page can look like a wall of abbreviations, hatch patterns, and numbers.

Here's how to read a soil boring log, section by section, and what the data actually means for the project.

View the Full Example Boring Log (PDF)

Start With the Header

Before you look at a single soil layer, check the header. It tells you whether the rest of the log is even relevant to your question. Every boring log should identify:

• Project name, number, and client
• Boring ID and its location (coordinates or station/offset)
• Ground surface elevation
• Drilling contractor, rig type, and drilling method
• Date drilled and who logged it

That elevation number matters more than people realize. Depths on the log are measured from the ground surface, so if you're comparing borings across a sloped site, you need elevation to line up the strata. Two borings might both show clay at 10 feet, but if one was drilled 8 feet higher, you're looking at two different layers.

The drilling method matters too. Hollow stem auger, mud rotary, and air rotary each disturb the soil differently and affect how groundwater readings should be interpreted.

The Depth Column & Stratigraphy

The left side of a soil boring log is a vertical depth scale, usually in feet, with horizontal lines marking where one soil layer ends and the next begins. Make sure to look at the reference point where the measurements start. Some borings are measured from the ground surface during drilling but are converted later to Mean Sea Level (MSL) or to a specific City Datum or reference point. Next to it you'll find a graphic column of hatch patterns representing each material, plus a written description.

Those descriptions follow a consistent order per ASTM D2488. A typical line reads something like: "Very Stiff Gray Silty Clay (ML), moist." That's strength, color, group name, USCS symbol, then moisture. The two-letter symbols come from the Unified Soil Classification System, defined in ASTM D2487, which classifies soils by particle size characteristics, liquid limit, and plasticity index. Field descriptions are based on visual observation and get confirmed or revised once laboratory testing is complete . Once you know a handful of symbols (CL for lean clay, SP for poorly graded sand, ML for silt), the descriptions start reading like plain language.

Pay attention to transition lines. A solid line between layers means the driller observed a distinct change. A dashed or gradational line means the change was interpreted between samples, so the actual boundary could sit anywhere in that interval.

Blow Counts & N-Values

This is the column that gets the most attention, and for good reason. During a standard penetration test, performed per ASTM D1586, a split-barrel sampler is driven into the soil at the bottom of the borehole with a drop hammer, and the crew records the number of hammer blows needed to advance it. Blow counts are recorded for each 6-inch interval, and per the TxDOT Geotechnical Manual, the sum of the blows for the second and third 6-inch drives is the standard penetration resistance, or the uncorrected N-value.

The N-value is your quick read on strength. Low N-values generally indicate loose sands and soft clays, while high N-values point to dense sands, stiff clays, or hard soils. A column of single digits in the upper 15 feet tells a very different foundation story than a column of 30s and 40s.

A few things to watch:

• Refusal. If you see "50/3in" or similar, the sampler stopped advancing. Could be bedrock, could be a boulder. The log notes should say which the driller suspected.
• WOH or WOR. Weight of hammer or weight of rods. The sampler sank under static weight alone, which flags very soft or loose material.
• Corrections. Raw N-values are typically corrected for hammer efficiency and overburden pressure before they're used in design. Hammer efficiency calibrations must be performed on a regular basis by the drilling company at the intervals established by the local DOT, and the results of each calibration should be provided to the geotechnical engineer. MnDOT's boring log terminology, for example, reports N60 values normalized to 60% of the system's potential energy. Don't plug field numbers straight into a correlation without checking whether they've been adjusted.

Recovery matters too. If the log shows 4 inches of recovery from an 18-inch drive, treat that sample's description with some skepticism. The soil that stayed in the spoon may not represent the full interval.

Groundwater Observations

Groundwater entries usually appear as symbols in the margin with a date and time. Read them carefully, because a single reading is a snapshot, not the water table.

Most logs record water levels at multiple points: during drilling, at completion, and sometimes 24 hours later after the hole has stabilized. In clays and silts, water seeps in slowly, so the level at completion can be well below the true static level. Seasonal variation adds another layer of uncertainty. When groundwater drives the design, one reading is a starting point, not an answer.

Before settling for a water level, consider the weather conditions and the season. In fall and spring, snowmelt can artificially raise water levels. Similarly, perched water in granular material can cause changes in the measured levels. Also verify the drilling method. If mud rotary was used, water levels will be inaccurate or very difficult to obtain. If accurate groundwater levels are needed, consider talking with your drilling subcontractor about installing monitoring wells.

Lab Results & Remarks

Many logs include columns for moisture content, Atterberg limits, unconfined compressive strength, or other lab data tied to specific samples. These values confirm or refine the field descriptions, and they're where the field log connects to your lab testing and reporting workflow.

Don't skip the remarks section at the bottom. Notes about drilling fluid loss, caving, artesian conditions, or equipment changes often explain anomalies in the data above. A sudden drop in blow counts makes a lot more sense when the remarks mention a void.

Reading Between the Borings

A single log describes one point on a site, and conditions between borings are always interpolated. That's why engineers read logs as a set, building a subsurface profile and flagging where the story doesn't hold together. If boring B-3 shows fill twice as deep as B-2 fifty feet away, somebody needs to explain why before the foundation design moves forward. Boring data also feeds directly into fieldwork downstream, from soil compaction testing to field inspections during construction.

Borings can be interpolated and plotted on a single image, which is known as a profile. The profile gives you a complete picture of the strata across the site and makes it easier to identify discrepancies or special conditions between borings. One important note: when plotting a profile, make sure the elevation references are correlated across all borings so the strata line up accurately.

Where the Data Lives Matters

A soil boring log is only useful if the people who need it can find it, trust it, and connect it to the rest of the project record. That's harder than it sounds when field notes live in a truck, lab results live in a spreadsheet, and the final log lives in a PDF attached to an email. And every time boring data gets retyped from one system into another, transcription errors get a chance to creep into the record.

Omnant keeps boring data, lab results, and project documentation in one system built for geotechnical and CMT firms. And with our BoreDM integration, boring log data moves from BoreDM into Omnant in AGS format with no manual re-entry, so the log you read is tied to the samples, tests, and reports behind it. Schedule a demo to see how it works.

*Image courtesy of Flood Testing Labs