A borehole log — also called a boring log, soil boring log, or drill log — is the primary document produced by a geotechnical site investigation. It is a structured, standardised record of everything encountered during the drilling of a borehole: the soil and rock layers met at successive depths, the results of in-situ tests performed in the hole, the samples recovered and sent to the laboratory, and observations about groundwater. Every foundation design, slope stability assessment, and earthworks specification on a project depends on the accuracy and completeness of its borehole logs.
This article explains what a borehole log contains, how each data field is used, how the standard log layout is structured, which standards govern its format, and what distinguishes a professional borehole log from an incomplete or non-standard one.
Definition and purpose #
A borehole log is a vertical, depth-referenced record of the subsurface conditions encountered at one specific drilling location during a geotechnical site investigation. The log documents conditions from the ground surface down to the termination depth of the borehole, capturing every significant change in material, every test result, every sample, and every field observation.
The borehole log serves several distinct purposes simultaneously:
- Primary data record: It is the authoritative record of what the driller and geotechnical engineer observed in the field. Once drilling is complete, the physical borehole is backfilled and no longer accessible. The log is the only permanent record of what was there.
- Design input: Foundation engineers, slope stability analysts, and earthworks designers read the logs to understand the subsurface profile — soil types, layer thicknesses, consistency, groundwater — and select appropriate design parameters.
- Laboratory test assignment: The log identifies which samples were retrieved from which depths and is used to direct laboratory testing to the most critical or representative intervals.
- Communication document: The log communicates subsurface conditions to everyone involved in a project — client, structural engineer, contractor, and regulator — in a standardised format they can all read and interpret independently.
- Legal document: In many jurisdictions, borehole logs form part of the geotechnical report submitted to planning authorities or building control. They may also be relied upon in disputes about unforeseen ground conditions during construction.
Data fields — what a borehole log must contain #
A professional borehole log must document all of the following information. Each field has a specific purpose; omitting any of them limits the usefulness of the log for downstream design and interpretation.
| Data field | Purpose | Where it appears on the log |
|---|---|---|
| Boring/borehole ID | Unique identifier for this specific borehole (e.g. BH-01, BH-02). Used to cross-reference the log with the site plan, laboratory test results, and design calculations. | Header |
| Project name and number | Identifies the project the borehole belongs to. Critical when files are archived or shared between parties. | Header |
| Client name | The organisation commissioning the investigation. | Header |
| Location / coordinates | The precise position of the borehole — typically easting/northing in the local grid or GPS latitude/longitude. Allows the log to be plotted on the site plan and cross-sections to be drawn between boreholes. | Header |
| Ground surface elevation | The elevation of the ground surface at the borehole collar relative to a datum (mean sea level or local benchmark). Allows depths recorded in the borehole to be converted to absolute elevations for cross-section drawing. | Header |
| Drilling method | The method used to advance the borehole (hollow stem auger, mud rotary, air rotary, etc.). Affects interpretation of sample quality and N-value reliability. | Header |
| Borehole diameter | The nominal diameter of the borehole (typically 100–150 mm for geotechnical investigation). Required for SPT correction (borehole diameter factor Cb). | Header |
| Dates of drilling | Start and completion dates. Relevant when groundwater observations are timed relative to drilling, and for regulatory compliance. | Header |
| Driller name / company | Accountability and quality assurance. The driller’s experience and hammer type affect N-value reliability. | Header |
| Logged by | The geotechnical engineer or geologist who described the samples and oversaw the test programme. | Header |
| Hammer type and energy | The SPT hammer type (safety, donut, automatic) and measured or assumed energy ratio. Essential for applying the energy correction factor Ce to all N-values in this borehole. | Header |
| Depth scale | Vertical axis of the log, in metres or feet from ground surface. All other depth-referenced fields (soil layers, samples, SPT tests, groundwater) are plotted against this scale. | Depth column |
| Soil/rock layer boundaries | The depth at which each change in material occurs. Horizontal lines on the graphical log column and description column mark these boundaries. | Graphical log + description |
| Graphical soil symbols | Standardised pattern symbols representing each USCS soil type (dots for sand, hatching for clay, triangles for gravel, etc.). Allows rapid visual reading of the stratigraphy. | Graphical log column |
| Soil description | Written description of each layer: colour, moisture, consistency/density, structure, inclusions, and USCS group symbol. The full USCS description in the standard sequence. | Description column |
| Sample type, depth, and recovery | For each sample: type (SS = split-spoon, ST = Shelby tube, B = bulk, C = core), depth interval, and recovery (length of sample retained in the sampler, expressed in mm or as a percentage). Recovery below 100% affects the reliability of the soil description for that interval. | Sample column |
| SPT blow counts | The number of blows for each of the three 150 mm drives: seating drive / second drive / third drive. The raw N-value (sum of second + third drives) is plotted as a bar on the SPT chart column. Individual drive counts allow detection of anomalous tests. | SPT column |
| Groundwater observations | The depth at which water was first encountered during drilling (seepage depth or cave-in depth) and the stabilised water level measured after drilling ceases or at the start of the next day. | Groundwater marker |
| Termination depth and reason | The final depth of the borehole and the reason drilling stopped: target depth reached, SPT refusal, budget limit, or equipment limitation. | Final depth notation |
| Remarks | Any observations that do not fit the structured columns: casing depth, loss of drilling fluid, unusual drilling conditions, colour of drilling water, obstructions encountered. | Remarks column |
Header section #
The header occupies the top portion of the first log page and contains all project identification, location, and procedural information. A complete header allows the log to be understood in isolation — without reference to any other project document. Missing header information is one of the most common quality deficiencies in borehole logs submitted for regulatory review, and it cannot be reconstructed after the drilling programme is complete.
At minimum, the header must include: boring ID, project name, client, location coordinates, ground elevation, drilling method, borehole diameter, drilling dates, driller, and logged-by engineer. The hammer type and energy ratio should also appear in the header because they apply to all SPT tests in the borehole. Some organisations also include the geotechnical firm’s logo, the driller’s licence number, and the applicable investigation standard in the header.
Graphical soil log column #
The graphical column is a visual representation of the soil and rock stratigraphy drawn to the depth scale of the log. Each soil layer is filled with a standardised symbol pattern corresponding to its USCS classification:
| Symbol pattern | Soil type | USCS symbols |
|---|---|---|
| Blank / white | Fill or made ground | — |
| Triangles / angular fragments | Gravel | GW, GP, GM, GC |
| Dots / stipple | Sand | SW, SP, SM, SC |
| Horizontal dashes | Silt (low plasticity) | ML |
| Angled hatching | Clay | CL, CH |
| Hatching + dashes | Sandy clay / clayey sand | SC, CL |
| Organic texture / irregular | Organic soil / peat | OL, OH, Pt |
| Brick pattern | Rock / bedrock | — |
Symbol conventions vary slightly between organisations and national standards. The important requirement is that the legend is provided — either on the log itself or in the geotechnical report — so any reader can interpret the patterns correctly. Layer boundaries on the graphical column are drawn as solid horizontal lines for definite contacts and as dashed lines for gradational or uncertain boundaries.
The graphical column is typically 15–25 mm wide on an A4 log. It gives an immediate visual impression of the stratigraphy — alternating sand and clay layers, a deep gravel stratum, a thin soft zone — before reading a single word of description. This is the column that design engineers scan first.
Soil description column #
The description column, which occupies the widest part of the log, contains the written geotechnical description of each soil or rock layer identified in the borehole. The description is written by the logging geotechnical engineer based on visual and manual assessment of the split-spoon sample recovered at each test depth, supplemented by observation of the drill cuttings and drilling behaviour between samples.
Each layer description follows a standardised sequence of descriptors to ensure consistency and completeness:
- Consistency (fine-grained) or Relative Density (coarse-grained) — the first and most important descriptor: VERY SOFT / SOFT / FIRM / STIFF / VERY STIFF / HARD for clays and silts; VERY LOOSE / LOOSE / MEDIUM DENSE / DENSE / VERY DENSE for sands and gravels
- Colour — dominant colour first, then secondary: e.g. dark grey, reddish-brown, olive green
- Moisture — DRY / SLIGHTLY MOIST / MOIST / WET / SATURATED
- Soil name — the USCS group name: e.g. CLAY, SAND, SANDY SILT, SILTY GRAVEL
- Structure — INTACT / FISSURED / LAMINATED / VARVED / INTERBEDDED — only if notable
- Inclusions and secondary material — e.g. “with occasional fine gravel”, “containing shell fragments”, “with rootlets to 1.0 m”
- USCS group symbol in brackets — e.g. (CL), (SP-SM), (CH)
A well-written layer description example: “STIFF, dark grey, moist CLAY, fissured with occasional silt partings, becoming very stiff below 4.5 m (CL)”
For a complete guide to field soil description techniques and the full USCS symbol table, see How to describe soil layers in a borehole log and USCS soil classification — field guide for borehole logging.
Sample column #
The sample column records each sample taken from the borehole — its type, depth interval, and recovery. Sample notation typically uses single-letter or two-letter abbreviations:
| Abbreviation | Sample type | Quality and use |
|---|---|---|
| SS | Split-spoon sample (from SPT) | Disturbed. Suitable for visual description, grain size, Atterberg limits. Not suitable for consolidation or undrained shear strength testing. |
| ST | Shelby tube (thin-walled tube sample) | Nominally undisturbed. Suitable for consolidation, UCS, triaxial, and moisture content testing. Requires slow careful push, not driving. |
| B | Bulk sample (from cuttings or auger) | Disturbed and mixed. Suitable for classification and compaction testing only. |
| CS | Chunk sample (hand-cut from trial pit) | Relatively undisturbed if carefully cut. Suitable for similar tests as Shelby tube. |
| C | Rock core | Recovered by rotary core drilling. Recovery and Rock Quality Designation (RQD) are recorded. |
Sample recovery is recorded as the length of soil actually retained in the sampler expressed as a fraction of the driven length — for example, “380/450 mm” or “84%”. Recovery below 100% for split-spoon samples means some soil was lost during retrieval, which may affect the reliability of the description for that interval. Recovery below about 60% for Shelby tubes indicates significant sample disturbance and should be flagged in the geotechnical report.
SPT column #
The SPT column records the penetration resistance measured at each Standard Penetration Test interval. It typically contains two related elements:
- Blow count notation: The three individual drive counts recorded separately — for example “4 / 7 / 9” meaning 4 blows for the seating drive, 7 for the second drive, and 9 for the third drive. The N-value is 7 + 9 = 16. Recording all three counts rather than only the N-value allows the logger to identify anomalous tests (e.g. a very high seating drive suggesting a gravel layer just above the test depth, or a first drive that is higher than the third suggesting loose, collapsing soil).
- N-value bar chart: A horizontal bar plotted proportionally to the N-value at each test depth, providing an immediate visual profile of relative resistance with depth. Abrupt increases in the bar length indicate a transition to denser or harder material; abrupt decreases indicate softer zones.
SPT refusal is recorded as “50/Xmm” where X is the actual penetration achieved with 50 blows (e.g. “50/120mm” means the sampler advanced only 120 mm before 50 blows were reached), or simply “REF” if the sampler did not advance at all. Refusal depth and reason are also noted in the Remarks column.
For the full procedure governing SPT testing and the correction factors that must be applied to N-values before use in design, see the SPT complete guide and SPT N-value correction factors.
Groundwater column #
Groundwater observations are recorded as horizontal markers on the depth scale, with a note of the time and date of each observation. Two types of observation are distinguished:
- Seepage depth: The depth at which water was first observed running into the borehole during drilling. This is noted on the log but is not a reliable indicator of the true water table — drilling fluid, artesian conditions, or perched water zones can all give misleading seepage observations.
- Stabilised water level: The depth to water measured after drilling has stopped and the borehole has been allowed to equilibrate — ideally measured at the start of the next drilling day, or at least 30–60 minutes after completion in permeable soils. This is the most reliable field measurement of the groundwater table depth and is the value used in design calculations.
In low-permeability soils (silts and clays), hours or days may be required for the borehole to equilibrate. Where accurate groundwater levels are critical — for example for dewatering design, pile design, or below-ground structure design — standpipe piezometers or vibrating wire piezometers are installed in the borehole after drilling to monitor levels continuously.
For more on groundwater recording and its influence on bearing capacity calculations, see Groundwater observation in boreholes — recording and reporting and the foundation design guide.
Remarks column #
The remarks column captures observations that are important but do not fit the structured fields described above. Typical remarks entries include:
- Casing installed and depth (e.g. “Steel casing installed to 3.0 m to prevent collapse in loose sand”)
- Loss of drilling fluid circulation (may indicate a highly permeable or open-jointed zone)
- Colour changes in return water (may indicate groundwater quality issues or change in formation)
- Unusual drilling resistance — rapid advance, spinning, rod drop (indicating cavities, soft pockets, or open joints)
- Odour from samples (organic soil, contamination)
- Obstructions encountered (cobbles, masonry, old foundations)
- Deviation from standard test procedure and reason
- Core recovery and RQD for rock sections
Remarks are often the most valuable part of a borehole log for identifying ground conditions that don’t fit the standard fields — and they are the most frequently omitted section in poorly maintained field logs. A driller who notes “rapid rod drop of 200 mm at 8.5 m depth” is alerting the design team to a potential cavity or very soft lens that the N-value alone would not reveal.
Standards and references — ASTM D5434 #
The primary standard governing the content and format of borehole logs in geotechnical engineering is ASTM D5434 — Standard Guide for Field Logging of Subsurface Explorations of Soil and Rock. ASTM D5434 defines the minimum information that must be recorded on a borehole log to meet professional practice standards in North American and internationally referenced practice.
Additional standards that govern specific elements recorded on the log include:
| Standard | Covers |
|---|---|
| ASTM D2487 | Unified Soil Classification System (USCS) — the classification system used for all soil descriptions and USCS group symbols on the log |
| ASTM D1586 | Standard Penetration Test (SPT) — the procedure generating the N-values recorded in the SPT column |
| ASTM D1587 | Thin-walled tube sampling (Shelby tube) — governs the ST samples recorded in the sample column |
| ASTM D6032 | Rock core drilling and logging — governs RQD and rock description entries |
| BS EN ISO 14688 | Geotechnical investigation — identification and classification of soil (European equivalent of USCS for projects following Eurocode 7) |
In practice, many organisations have their own internal borehole log templates that comply with the applicable standard while incorporating their own header layout, graphical symbols, and report formatting. The key requirement is that the log contains all mandatory data fields, uses a recognised soil classification system consistently, and is comprehensible to any competent geotechnical engineer reading it without additional context.
Log formats — one-page vs two-page #
Borehole logs are produced in two primary formats depending on the depth and complexity of the borehole:
One-page (single-sheet) borehole log #
For shallow to medium-depth boreholes — typically up to 10–15 m — all data fits comfortably on a single A4 or US Letter page. The depth scale, graphical column, description, sample, SPT, and groundwater columns are all presented side by side. This is the most common format for routine site investigations for building foundations, small infrastructure projects, and preliminary investigations. DartiGeo’s one-page borehole log format generates this layout automatically from entered data.
Two-page (continuation-sheet) borehole log #
For deeper boreholes, the log continues onto additional pages. The depth scale carries forward from the bottom of the previous page. Two-page formats are also used in some organisations’ standards regardless of depth — keeping all graphical columns on one page and all detailed descriptions, sample details, and test results on a facing page. DartiGeo supports both one-page and two-page log formats and generates continuation sheets automatically as required.
Digital and paper log formats #
Traditionally, borehole logs were completed on paper forms in the field and later transferred to a CAD or word processor template. Modern practice increasingly uses field tablet software for direct digital logging, reducing transcription errors and allowing the log to be reviewed in the site office while drilling is still in progress. The final deliverable to clients is almost universally a PDF borehole log. Word format (.docx) may be provided for projects where the client needs to edit or incorporate the log into their own documents. Excel export provides the underlying tabular data for import into analysis software and databases.
Boring log vs borehole log — is there a difference? #
No — “boring log” and “borehole log” refer to the same document. The terminology difference is regional:
- “Boring log” or “soil boring log” is the standard term in the United States and is used in ASTM standards and North American geotechnical practice.
- “Borehole log” is the equivalent term used internationally — in the UK, Europe, Australia, the Middle East, and most countries following ISO and British Standards.
- “Drill log” is sometimes used informally, particularly in mining-influenced geotechnical practice.
All three terms describe the same standardised document. Search engines index all three — which is why this article uses both “borehole log” and “boring log” to ensure it is found by engineers from all regions searching for the same concept.
A related but distinct document is the trial pit log (or test pit log), which records observations from an excavated open pit rather than a drilled borehole. Trial pits allow direct visual inspection of the soil profile and collection of bulk samples, but are limited to shallow depths (typically 2–4 m) and cannot be used in unstable soils or below the water table.
How DartiGeo generates borehole logs automatically #
Creating a professional borehole log manually — formatting the graphical column, positioning soil symbols at the correct depths, plotting SPT bar charts, placing groundwater markers, and paginating long boreholes across continuation sheets — is time-consuming and introduces formatting errors. DartiGeo’s field tests module automates the entire log generation process from entered data.
The workflow in DartiGeo is straightforward:
- Enter borehole header information: Project name, boring ID, coordinates, ground elevation, drilling method, borehole diameter, drilling dates, driller, and hammer type.
- Define soil layers: For each layer, enter the depth to the top and bottom of the layer, select the USCS soil type from a dropdown (which assigns the correct graphical symbol automatically), and type the full soil description in the description field.
- Enter SPT data: For each SPT interval, enter the three drive blow counts. The software calculates the N-value and plots the bar automatically.
- Record samples: Enter sample type, depth interval, and recovery percentage for each sample taken.
- Record groundwater: Enter the seepage depth and stabilised water level observations.
- Generate the log: Click the report generation button. DartiGeo produces a formatted borehole log — one-page or two-page format as selected — with the graphical soil log column, description column, SPT bar chart, sample column, and groundwater marker all laid out correctly to scale.
- Export: The finished log is exported as PDF, Word, or Excel.
Because the same project file also holds the SPT correction factors and correlations, bearing capacity calculations, CPT interpretation, and laboratory test results, there is no re-entry of data between modules. Layer descriptions and SPT N-values entered in the borehole log feed directly into the SPT correlations module and the foundation design module.
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Frequently asked questions #
What is the difference between a borehole log and a site investigation report? #
A borehole log is a data record — a structured, standardised document showing exactly what was found in a single borehole. A site investigation (geotechnical) report is an interpretive document that synthesises data from all boreholes, laboratory tests, and in-situ tests across the entire site, draws conclusions about the subsurface conditions, and makes recommendations for foundation design, earthworks, or other engineering actions. The borehole logs are always appended to the geotechnical report as primary supporting data. The report cannot substitute for the logs, and the logs without the report lack the engineering interpretation needed for design.
Who fills in a borehole log? #
The borehole log is the joint responsibility of the field driller and the supervising geotechnical engineer or engineering geologist. The driller records blow counts, drilling observations, and sample depths in real time during drilling. The geotechnical engineer — who should be present at the borehole during drilling — describes and classifies each sample, assigns USCS group symbols, records groundwater observations, and notes any unusual drilling behaviour in the remarks column. The completed field log is then used to generate the final formatted log, which is reviewed and signed off by the geotechnical professional responsible for the investigation.
How long does it take to drill one borehole? #
Drilling time depends on borehole depth, soil conditions, and drilling method. A routine 10 m hollow stem auger borehole with SPT tests at 1.5 m intervals in cohesive to medium soils typically takes 3–6 hours including set-up, drilling, testing, and backfilling. A 20 m borehole in varied conditions with more frequent testing may take a full working day. Harder conditions, water-table complications, or gravel layers requiring casing can extend this significantly. As a rule of thumb, budget one borehole per day per rig for routine 10–15 m investigations in typical soil conditions.
What depth should a borehole be for a building foundation? #
The required borehole depth for building foundation design depends on the foundation type, building size, and soil conditions. A general rule is to investigate to at least 1.5 times the width of the largest loaded foundation element below the foundation level, or to the depth at which the applied stress increment falls below 10% of the effective overburden stress — whichever gives the greater depth. For a single-storey residential building on a 0.6 m wide strip footing, a 5–6 m borehole is typically sufficient. For a multi-storey building on a wide raft foundation, investigation to 20–30 m may be required. The foundation design guide discusses investigation depth in more detail.
Can a borehole log be completed entirely in the field? #
The field log — a rough handwritten or tablet-entered record of observations made during drilling — is always completed in the field. The formatted final borehole log for submission to the client and inclusion in the geotechnical report is typically completed in the office, using the field log as the source document and software such as DartiGeo to generate the standardised formatted output with correct scaling, graphical symbols, and SPT bar chart. Field logging apps that produce a formatted PDF directly from tablet entry are increasingly common and reduce the office formatting step, but the geotechnical engineer must still review and approve the log before it is submitted.
Related articles #
- Borehole Logging — complete guide
- USCS soil classification — field guide for borehole logging
- Borehole log format and required data fields (ASTM D5434)
- How to describe soil layers in a borehole log
- Hollow stem auger drilling — method, advantages and limitations
- Groundwater observation in boreholes — recording and reporting
- Borehole log report — how to generate a professional PDF
- Borehole log software — features to look for
- Standard Penetration Test (SPT) — complete guide
- SPT N-value correction factors explained
- Foundation design — bearing capacity and settlement guide
