A borehole log has a dense column of depths, codes, hatched symbols and abbreviations. If you know what each column is telling you, it's just a structured record of what a geologist found in the ground, in a strict order.
This is a walk-through of what's actually on a borehole log, how the soil and rock descriptions are structured, what the abbreviations mean, and how to read the numbers.
What a Borehole Log Actually Is
A borehole log is the factual record of a single drilled hole: what materials were encountered at what depth, what samples and tests were taken, and what groundwater was observed. It's deliberately kept separate from the engineering interpretation, recommendations and design advice that come later in a geotechnical report. The log is the evidence; the report is the argument built on it.
That distinction matters practically. Borehole logs are usually attached to geotechnical reports as factual appendices, and they're often relied on directly in contracts, disputes and later investigations on the same site, sometimes years after the original drilling. A log that's ambiguous, inconsistent, or missing key information can cause real problems long after the project that commissioned it is finished.
In Australia, the content and structure of borehole logs is based on AS 1726 Geotechnical Site Investigations, and most drilling contractors and road authorities (Queensland's Transport and Main Roads publishes one of the more detailed public guidelines) follow broadly the same conventions, even though the exact log sheet layout varies between firms.
What's in the Header
Before any subsurface information, every log carries project and location details:
- Project name, project number and job number
- Borehole number and page/sheet number
- Coordinates (easting, northing) and surface reduced level (RL), tied to a stated datum
- Date started and date completed
- Drilling contractor, driller, and drill rig
- Logged by and reviewed by (initials of the geologist and the person who checked the log)
If any of this is missing or looks inconsistent between logs on the same project, that's worth querying before you rely on the data.
Borehole Numbering and Location
Each borehole gets a unique identifier that ties it back to the site plan, so anyone reading the log can see exactly where that data point sits on the ground. Common prefixes:
| Abbreviation | Meaning |
|---|---|
| BH | Borehole |
| TP | Test pit |
| MW | Monitoring well |
| CPT | Cone penetration test |
| RC | Rock core hole |
| DH | Drill hole |
Location matters more than it might seem: ground conditions can change significantly over short distances, and it's entirely normal for one borehole to show rock at 1.5 m while another 20 m away shows it at 6 m. That's exactly why a single borehole should never be read as if it represents the whole site.
The ground surface level at each borehole is usually given as a reduced level (RL) against a datum, commonly the Australian Height Datum (AHD) or a project-specific benchmark. Depths on the log are measured downward from that surface, so converting a depth to an actual elevation is simple subtraction: if the surface is RL 52.30 m AHD and a clay layer starts at 2.00 m depth, that layer starts at RL 50.30 m AHD. That conversion matters whenever you're comparing layers between boreholes with different ground levels, or checking a layer against a known structure level.
The Main Columns
Layouts vary between firms, but most Australian borehole logs are built around the same core set of columns:
| Column | What it shows |
|---|---|
| Depth / RL | Depth below the ground surface and the corresponding reduced level, keyed to the log's vertical scale |
| Graphic log / lithology | A symbol or hatch pattern representing the material at each depth, keyed to a legend |
| Material description | The structured soil or rock description (see below) |
| USCS / weathering | The Unified Soil Classification System group symbol for soil, or the weathering grade for rock |
| Samples / tests | Sample type and depth (for example U50, SPT, Bulk) |
| Additional data and test results | In-situ and laboratory results: SPT N-values, point load index, UCS, moisture content, and so on |
| RQD / core recovery | For rock core: percentage of intact pieces over 100 mm long (RQD) and total recovered length as a percentage of the drilled run |
| Groundwater | Depth of water strikes and, separately, the stabilised water level with the date it was measured |
The graphic log column is the part people read fastest and understand least: it's only meaningful with its legend, since hatch patterns aren't universally standardised between companies. Always check the legend, usually on the same sheet or in the report, before assuming what a pattern means.
How Soil Descriptions Are Built
Soil descriptions on an AS 1726-based log follow a fixed order, arranged in rows so the same type of information always appears in the same place:
- Soil name: minor fraction as a lower-case prefix, major fraction in upper case, followed by the depositional origin in brackets, for example Gravelly SAND (Alluvium) or Silty CLAY (Residual).
- Primary descriptors: colour, moisture condition, consistency or density, in that order, for example pale grey brown, wet, medium dense.
- Secondary descriptors: grain size and grading, particle shape, plasticity, organic content, cementation, for example fine grained sand, rounded grains, poorly graded.
Composite soils use modifier terms based on the proportion of each fraction: broadly, up to about 5% fines in a coarse soil is described as a "trace," 5-12% as "with," and above roughly 12% the soil is named as clayey or silty SAND or GRAVEL instead. The equivalent thresholds for fine-grained soils with a coarse fraction are roughly 15% ("trace"), 15-30% ("with"), and above 30% ("sandy" or "gravelly") CLAY or SILT. The exact percentages vary slightly by reference, so treat these as the general pattern rather than an exact cutoff.
What Soil Type, Colour and Origin Actually Tell You
The soil name is only the headline. Three other details on the same line carry real engineering meaning.
Primary soil type sets the expected behaviour:
| Soil type | Typical engineering behaviour |
|---|---|
| CLAY | Can be compressible, shrink-swell prone, low permeability |
| SILT | Fine-grained, less plastic than clay, can be erosion- and moisture-sensitive |
| SAND | Drains well; strength depends heavily on density |
| GRAVEL | Usually strong when dense, high permeability |
| COBBLES / BOULDERS | Obstructions for drilling, excavation and piling rather than a "layer" in the usual sense |
| FILL | Variable by definition; treat with caution until proven otherwise |
| ORGANIC SOIL / PEAT | Usually weak and highly compressible |
Colour isn't cosmetic, it's often a direct clue to drainage and history:
| Colour | Possible meaning |
|---|---|
| Brown / red-brown | Oxidised, often above the water table |
| Grey / blue-grey | Reduced conditions, often saturated or poorly drained |
| Black | Organic matter, peat, or possible contamination |
| Orange mottling | Iron staining from fluctuating groundwater |
| White / pale | Weathered rock, carbonate, ash or leached material |
Grey clay with orange mottling, for example, is a fairly reliable sign that a layer has been through repeated wetting and drying or sits near a fluctuating water table.
Geological origin, shown in brackets after the soil or rock name (Alluvium, Residual, Fill, and so on), tells you how the material got there, which affects how consistent it's likely to be:
| Origin | Typical behaviour |
|---|---|
| Fill | Variable, may settle |
| Alluvium | Layered, may contain soft clay or loose sand lenses |
| Colluvium | Slope-derived, thickness varies |
| Residual soil | Weathered in place from the underlying rock |
| Marine / estuarine clay | Often soft, compressible and sensitive |
| Aeolian sand | Often relatively uniform, may be loose |
Fill Gets Special Attention for a Reason
Fill is man-placed material, and it's flagged differently from natural ground because its behaviour depends entirely on how it was placed, not just what it's made of.
| Fill type | What it means |
|---|---|
| Controlled fill | Placed and compacted under specification; generally more predictable |
| Structural fill | Engineered specifically for load support; usually acceptable once verified |
| Uncontrolled fill | Placement and compaction unknown or undocumented; real settlement risk |
| General fill | Placed to raise levels, quality variable |
| Contaminated fill | Contains waste, debris or chemical contamination; an environmental issue as well as a geotechnical one |
A log entry like "FILL: gravelly sand with brick fragments, concrete, glass" is telling you two things at once: the material is likely to be inconsistent, and it may need to be removed, improved, or avoided as a founding layer rather than built on directly. This is one of the more common reasons a straightforward pad footing gets ruled out in favour of piles, a raft, or ground improvement.
How Rock Core Is Described
Rock descriptions follow a similar row-based structure, but with an extra layer for the rock mass itself:
- Rock name, in upper case, with the geological unit in brackets.
- Weathering grade and primary descriptors: colour, grain size, texture, fabric and strength.
- Secondary descriptors: secondary minerals, alteration, staining or other distinctive features.
- Discontinuity sets: type, angle, frequency, roughness, aperture and infilling, listed in order of prevalence.
Weathering is described on a scale from fresh rock through to residual soil:
| Symbol | Grade |
|---|---|
| FR | Fresh rock, no sign of decomposition or staining |
| SW | Slightly weathered, little or no change in strength from fresh rock |
| MW | Moderately weathered, noticeable strength reduction |
| HW | Highly weathered, strong discolouration, strength much weaker than fresh rock |
| XW | Extremely weathered, has soil-like properties but original rock fabric is still recognisable |
| RS | Residual soil, structure and fabric no longer evident |
Where extremely weathered rock is encountered, it's typically logged as "recovered as" followed by a normal soil description, since at that point it behaves like soil even though it started as rock.
Rock Quality Designation (RQD) is calculated from the core itself: the combined length of intact core pieces longer than 100 mm, expressed as a percentage of the total length drilled in that run. It's a defect-frequency indicator, not a direct strength measure, and it should be read alongside the discontinuity descriptions rather than on its own.
Intact rock strength is usually estimated in the field first, then confirmed by point load or UCS testing:
| Term | Point load index Is(50), MPa | Field guide |
|---|---|---|
| Extremely low | ≤0.03 | Easily remoulded by hand |
| Very low | 0.03-0.1 | Crumbles under firm pick blows |
| Low | 0.1-0.3 | Easily scored with a knife |
| Medium | 0.3-1.0 | Readily scored with a knife; a 150 mm core piece can be broken by hand with difficulty |
| High | 1-3 | Cannot be broken by hand; breaks with a single firm pick blow |
| Very high | 3-10 | Breaks with more than one pick blow |
| Extremely high | >10 | Requires many blows to break |
A commonly used, and approximate, rule of thumb converts point load index to unconfined compressive strength as UCS ≈ 20 × Is(50), though the actual factor varies with rock type and specimen size and should never replace a direct UCS test where the design depends on it.
Total core recovery is a related but different figure from RQD, it's simply the length of core actually retrieved as a percentage of the length drilled:
Core recovery (%) = (core recovered / core run length) x 100
A 1.5 m run that returns 1.2 m of core has 80% recovery. Low recovery on its own can point to highly fractured rock, weak seams, cavities, or drilling technique, and it's worth reading alongside RQD rather than instead of it.
RQD itself is graded against a standard scale:
| RQD | Rock quality |
|---|---|
| 0-25% | Very poor |
| 25-50% | Poor |
| 50-75% | Fair |
| 75-90% | Good |
| 90-100% | Excellent |
A 1.5 m run containing 0.9 m of pieces longer than 100 mm gives an RQD of 60%, fair quality, though RQD alone says nothing about defect orientation, infill or groundwater, which is why it's read alongside the discontinuity descriptions rather than on its own.
Making Sense of "Stiff" and "Medium Dense"
Consistency and density terms on a log aren't just descriptive adjectives, they correspond to fairly specific ranges, generally assessed from SPT N-values (with laboratory or field strength testing to confirm where available):
Cohesive soils (consistency):
| Term | SPT N | Undrained shear strength su (kPa) |
|---|---|---|
| Very soft | 0-2 | <12 |
| Soft | 2-4 | 12-25 |
| Firm | 4-8 | 25-50 |
| Stiff | 8-15 | 50-100 |
| Very stiff | 15-30 | 100-200 |
| Hard | >30 | >200 |
Granular soils (density):
| Term | SPT N | Density index |
|---|---|---|
| Very loose | 0-4 | 0-15% |
| Loose | 4-10 | 15-35% |
| Medium dense | 10-30 | 35-65% |
| Dense | 30-50 | 65-85% |
| Very dense | >50 | 85-100% |
These ranges are a general guide rather than a universal conversion, they can vary between references and are affected by soil type, gravel content and testing method, but they're close enough to what most Australian logs use that you can sanity-check a description against the reported N-value.
Reading the Abbreviations
A finished log is dense with shorthand. The most common you'll run into:
| Abbreviation | Meaning |
|---|---|
| SPT | Standard Penetration Test |
| CPTu | Cone Penetration Test with pore pressure measurement (piezocone) |
| DCP | Dynamic Cone Penetrometer |
| U50 / U100 | Undisturbed tube sample, 50 mm or 100 mm nominal diameter |
| PP | Pocket penetrometer test |
| FSV | Field shear vane |
| DST | Direct shear test |
| UCS | Unconfined compressive strength |
| Is(50) | Point load strength index, corrected to a 50 mm diameter |
| PI / LL / LI | Plasticity index / liquid limit / liquidity index |
| su / qu | Undrained shear strength / unconfined compressive strength |
| c' / φ' | Effective cohesion / effective friction angle |
| RQD / R | Rock quality designation / total core recovery |
SPT results are usually reported as three blow counts, such as "4, 7, 11," representing the seating drive and the two 150 mm test intervals. The reported N-value is the sum of the second and third counts (in this example, N = 18), not all three. If the sampler meets refusal, the log will show it as a fraction, such as "30/80 mm," meaning 30 blows achieved only 80 mm penetration, and no N-value is reported for that interval.
Other Quick Field Tests You'll See Referenced
Two simpler tests often show up alongside SPT results. A pocket penetrometer gives a quick, approximate undrained shear strength reading on a clay sample, useful for comparing consistency between layers but not a substitute for laboratory testing. A vane shear test (FSV), run in the borehole or on a recovered sample, gives a more reliable undrained shear strength specifically for soft to firm clays, where SPT is less useful and sample disturbance from other methods can be a problem.
SPT itself has real limitations worth remembering when you're relying on N-values: results are sensitive to drilling method and hammer energy efficiency, gravel can produce misleadingly high counts, very soft soils can be disturbed by the sampler itself, and N-values are generally less reliable for continuous profiling than CPT. Treat a single N-value as one data point, not a verdict.
What "Refusal" Actually Means
Refusal shows up on both drilling records and SPT results, and it's one of the easiest things to misread. On an SPT, refusal is reported as a fraction, such as "50 blows for 75 mm," meaning the sampler couldn't achieve the standard penetration under the standard effort, and no N-value is reported for that interval.
Drilling refusal is different, and it doesn't automatically mean the borehole has reached bedrock. It can equally mean a boulder, dense gravel, an old concrete footing, buried steel, or construction debris. Assuming refusal equals competent rock is one of the more common misreadings of a borehole log, especially on sites with a history of filling or previous construction. The log should record why drilling stopped, target depth reached, refusal, or another reason, and that reason is worth checking rather than assuming.
Drilling Behaviour as a Risk Signal
What happens during drilling, not just what's recovered, tells you something too:
| Observation | Possible meaning |
|---|---|
| Borehole collapsing (sand) | Excavations at this location may need support |
| Loss of drilling fluid | Permeable gravel, fractured rock, or a void |
| Difficult drilling / slow progress | Dense gravel, boulders, or hard rock |
| Rapid groundwater inflow | Dewatering may be needed during construction |
| Squeezing ground | Soft, weak, saturated soil |
These notes are often tucked into remarks rather than given their own column, but they're some of the most direct warnings a log gives about construction risk.
Groundwater on the Log
Groundwater is recorded twice, and it's worth knowing the difference. The depth water is first encountered during drilling reflects what the drilling process disturbed, which can be affected by drilling fluid, casing, or a perched layer. The stabilised water level, measured after the hole has been left to equilibrate (commonly checked again 24 or more hours later), is a better indicator of the actual groundwater table, and it's shown on the log with the date it was measured. Where a project needs reliable long-term data, a standpipe or piezometer is installed and monitored rather than relying on a single reading.
From Log Data to Foundation Type
Reading a log correctly matters because it's the evidence an engineer uses to decide what kind of foundation the site actually needs. A profile showing loose sand, soft compressible clay, or uncontrolled fill near the surface points toward settlement risk, and often toward piled foundations, a raft, or ground improvement rather than a simple pad footing. A shallow, competent rock layer is usually good news for bearing capacity, though it can mean more difficult and costly excavation or drilling to found into it properly. High or seasonally variable groundwater shows up in the log too, and it influences both the construction method (dewatering, casing, sequencing) and the long-term durability of whatever foundation is chosen.
This is also why some boreholes don't get backfilled once drilling finishes: where groundwater behaviour matters to the project, the hole is completed as a standpipe or piezometer instead, so the water table can be tracked over months or seasons rather than judged from a single reading. That longer record feeds directly into settlement and durability predictions for the finished structure. For how these findings translate into an actual foundation choice, see Bearing Capacity of Soil and Foundations, Foundation Design for Slabs and Footings and Pile Capacity.
Draft vs Final Logs
Logs go through a review cycle before they're something you should rely on for design. In the field, the geologist enters preliminary descriptions, provisional USCS symbols in brackets, and field strength estimates as drilling proceeds. Back in the office, the draft is checked against adjoining boreholes so the geological picture makes sense, laboratory results come in and confirm or correct the field estimates, and a senior reviewer signs it off. Only then does the log become "final," with brackets removed and any provisional calls updated.
If you're working from a "preliminary" or "draft" log, treat any bracketed classification or unconfirmed strength estimate as exactly that, an estimate awaiting confirmation, not a design input.
Borehole Log vs Geotechnical Report
It's worth being precise about what a log is and isn't, since the two documents get treated as interchangeable more often than they should:
| Borehole log | Geotechnical report |
|---|---|
| Describes ground conditions | Interprets ground behaviour |
| Records depths, samples, tests | Gives design recommendations |
| Factual observation | Includes engineering judgement |
| Shows soil/rock layers | Provides bearing pressures, pile design, settlement estimates |
A log is evidence. A report is the argument built from that evidence, informed by the site geology, groundwater behaviour and the specific project. Don't use borehole logs alone as a design input unless you're qualified to do that interpretation yourself.
Digital Logs and Data Exchange
Increasingly, logs exist as structured digital data rather than (or alongside) a scanned PDF sheet, so they can be loaded directly into modelling and reporting software instead of retyped. In Australia, the common exchange format for this is AGS (Association of Geotechnical and Geoenvironmental Specialists format, with an Australian variant maintained by the Australian Geomechanics Society). See Geotechnical Data Management and the AGS Format for how that data structure works.
Geophysical and Downhole Testing Beyond the Standard Log
Everything described above comes from physical samples and mechanical in-situ tests, drilled, retrieved, described and tested by hand. On deeper or more complex investigations, that can be supplemented with downhole geophysical logging: gamma ray, resistivity or sonic (acoustic) tools lowered into the hole to record a continuous physical-property profile rather than discrete sample points. This is particularly useful for correlating conditions between boreholes and filling in the gaps between physical samples, and it's covered in more depth in Geophysical Methods for Geotechnical Investigations.
Two more specialised downhole techniques are worth knowing, mainly so they aren't confused with routine investigation logging. Downhole magnetic resonance (NMR) gives an in-situ estimate of porosity, permeability and moisture content directly in the hole; it's more established in groundwater and petroleum work and appears only occasionally on larger geotechnical projects. Sonic (or cross-hole sonic) integrity testing is a different animal again: it's run after a cast-in-place concrete pile has been constructed, not during site investigation, to check the finished pile for defects or discontinuities. Both are supplementary tools used selectively on larger, deeper or higher-risk projects, not part of a routine borehole log.
Laboratory Tests Linked to the Log
The log tells you where a sample came from; the laboratory tells you what it's actually made of. Common tests run on samples identified in the log include:
| Test | Purpose |
|---|---|
| Moisture content | In-situ water content at the time of sampling |
| Atterberg limits | Plasticity (liquid limit, plastic limit, plasticity index) |
| Particle size distribution | Proportions of gravel, sand, silt and clay |
| Linear shrinkage | Shrink-swell indicator |
| UCS / point load | Rock (or stiff soil) strength |
| Oedometer consolidation | Settlement behaviour |
| Triaxial / direct shear | Shear strength parameters |
| CBR | Pavement subgrade strength |
| Permeability | Rate of water flow through the material |
| Sulfate / chloride / pH | Aggressivity to buried concrete or steel |
Plasticity is worth a specific mention, since it drives some of the biggest risks in Australian residential and light commercial construction. The plasticity index (PI = LL minus PL, the difference between liquid limit and plastic limit) indicates how much a clay is likely to shrink and swell with moisture change. High-plasticity clays logged on a site are a direct flag for potential slab movement, pavement distress and site classification implications, well before any lab certificate confirms the exact number. See Atterberg Limits Indices: PI, LI and CI for how those numbers are derived and used.
The log and the lab results should always be read together. A description alone is an engineer's field estimate; the lab result is what confirms or corrects it.
Example Interpretation
Put a few of these ideas together, and a log starts to read like a story rather than a table. Take this simplified profile:
| Depth | Description | Test |
|---|---|---|
| 0.0-0.5 m | FILL: silty sand with gravel, loose, moist | - |
| 0.5-2.5 m | CLAY, high plasticity, soft to firm, moist | SPT N = 4 |
| 2.5-6.0 m | CLAY, medium plasticity, very stiff, moist | SPT N = 18 |
| 6.0-8.0 m | SAND, medium dense to dense, wet | SPT N = 25 |
| 8.0-10.0 m | SANDSTONE, highly weathered, low strength | Core recovery 70%, RQD 35% |
A reasonable first-pass reading: the shallow fill is unlikely to be suitable for direct footing support without further checking. The soft to firm, high-plasticity clay immediately below it is a settlement and shrink-swell risk, worth flagging even before lab results confirm the plasticity number. The very stiff clay from 2.5 m offers noticeably better support. The wet sand at 6 m is a sign that dewatering or groundwater control may be needed if excavation reaches that depth. The sandstone at 8 m is not automatically "good rock": it's highly weathered with only fair-to-poor RQD (35%), so if the design depends on rock as a founding stratum, piles or piers may need to extend deeper into less weathered material rather than stopping at the first rock encountered.
None of this replaces an actual geotechnical assessment, but it's the kind of read-through that tells you which parts of a log deserve a closer look, and which questions to bring back to the engineer who wrote the report.
Resources
- Borehole Drilling and Soil Logging
- Geotechnical Site Investigation
- Bearing Capacity of Soil and Foundations
- Foundation Design for Slabs and Footings
- Pile Capacity
- Geophysical Methods for Geotechnical Investigations
- Standard Penetration Test (SPT) Procedure
- Cone Penetration Test (CPT)
- Dynamic Cone Penetrometer (DCP) Testing
- Rock Mechanics Testing
- Soil Classification in Geotechnical Engineering
- Atterberg Limits Indices: PI, LI and CI
- Geotechnical Data Management and the AGS Format
- Soil Sampling for Geotechnical Investigations