Borehole logging is the systematic recording of subsurface conditions encountered during drilling — capturing soil and rock layer descriptions, sample recovery, in-situ test results, and groundwater observations in a structured, standardised format. Accurate borehole logs form the foundation of every geotechnical site investigation and directly inform foundation design, slope stability analysis, earthworks specification, and ground improvement decisions.

This guide covers everything a practising geotechnical engineer needs to know about borehole logging: what a log contains, how drilling methods affect data quality, how to describe and classify soils in the field, what in-situ tests are performed during drilling, what format standards apply, and how specialist software can automate log generation and reporting.

What is a borehole log? #

A borehole log — also called a boring log or soil boring log — is a structured vertical record of the geological and geotechnical conditions encountered at a specific location during a drilling operation. Each borehole represents a vertical window into the subsurface, and its log is the primary document that communicates what was found there to engineers, clients, and regulators.

A complete borehole log captures every significant change in subsurface conditions from the ground surface down to the termination depth of the drilling. This includes not only what soil or rock types were encountered, but how they behaved during drilling, what samples were retrieved, and what test results were recorded in the field.

Key data fields in a borehole log #

A professional borehole log must contain the following information:

Data field	Description
Boring ID	Unique identifier for the borehole (e.g. BH-01, BH-02)
Project information	Project name, client, location, date of drilling, driller name
Coordinates and elevation	Easting/northing or GPS coordinates; ground surface elevation (datum)
Drilling method	Hollow stem auger, mud rotary, air rotary, etc.
Borehole diameter	Typically 100–200 mm for geotechnical investigation boreholes
Soil/rock layer boundaries	Depth at which material type changes
Soil/rock description	Colour, moisture, consistency/density, structure, plasticity, USCS group symbol
Sample type and depth	Split-spoon (SPT), Shelby tube, bulk, rock core — with depth interval and recovery
SPT N-values	Blow counts for each test interval
Groundwater observations	Depth at which water was struck; stabilised water level after drilling
Refusal depth	Depth at which drilling was stopped due to very hard material or SPT refusal

The significance of borehole log data goes well beyond its immediate use on a single project. Accurate logs create a reusable site database: layers described correctly the first time can be referenced for future work, extensions, or adjacent structures without the cost of repeat investigation.

Borehole drilling methods #

The drilling method used to advance a borehole directly affects the quality of soil samples retrieved, the reliability of in-situ test results, and the cost of the investigation. Selecting the appropriate method for the site conditions is one of the first decisions in planning a borehole programme.

Hollow stem auger #

The hollow stem auger (HSA) is the most widely used method for geotechnical investigation boreholes in cohesive and moderately cohesionless soils. A continuous helical auger rotates into the ground, advancing the borehole while bringing cuttings to the surface along the flights. The hollow centre allows SPT sampling and other tests to be carried out through the auger without removing it from the hole, making it fast and efficient for shallow to medium-depth investigations (typically up to 30 m).

Best for: Clays, silts, and loose to medium-dense sands above the water table. Not suitable for gravels, cobbles, or very dense materials.

Mud rotary drilling #

Mud rotary drilling uses a rotating drill bit with drilling fluid (bentonite slurry or polymer mud) pumped down through the drill rods and back up the annulus. The fluid stabilises the borehole walls and carries cuttings to the surface. It is the preferred method for cohesionless soils below the water table and for deeper boreholes where hollow stem auger is impractical.

Best for: Sands and silts below the water table, deep investigations (30+ m), and sites where borehole stability is a concern. Note that drilling fluid can contaminate samples, so SPT split-spoon samples in mud rotary boreholes require care in description.

Air rotary drilling #

Air rotary uses compressed air instead of drilling fluid to cool the bit and carry cuttings to the surface. It is the standard method for drilling through rock and very hard formations, and is widely used in mineral exploration. In geotechnical investigation it is typically employed where rock coring is required or where the near-surface soils have been successfully penetrated by another method.

Best for: Rock formations, very dense soils, and where water or drilling fluid is undesirable (e.g. contaminated sites where fluid disposal is regulated).

Wash boring #

Wash boring is an older method that uses water jetting to advance the borehole. It is rarely used in modern practice due to severe disturbance of soil samples and limited applicability, but may still be encountered in some regions or on lower-budget programmes in developing markets.

Method comparison #

Method	Best soil types	Typical depth	Sample quality	Relative cost
Hollow stem auger	Clay, silt, loose sand (above GWT)	Up to 30 m	Good (SPT split-spoon)	Low–Medium
Mud rotary	Sand, silt below GWT; deep borings	30–100+ m	Moderate (fluid contamination)	Medium–High
Air rotary	Rock, very hard soils	30–200+ m	Cuttings only (unless coring)	High
Wash boring	Soft soils	Up to 20 m	Poor (significant disturbance)	Low

Soil description and USCS classification #

Consistent, standardised soil description is the single most important skill in borehole logging. A description that varies between field engineers, or that uses non-standard terminology, makes layer boundaries ambiguous and can lead to misinterpretation by the design engineer reading the final log report weeks or months later.

The Unified Soil Classification System (USCS) #

The Unified Soil Classification System (USCS), standardised under ASTM D2487, is the internationally recognised framework for classifying and describing soils in geotechnical engineering. It divides soils into two major groups based on grain size:

Coarse-grained soils — more than 50% retained on the No. 200 sieve (0.075 mm). Subdivided into gravels (G) and sands (S).
Fine-grained soils — more than 50% passing the No. 200 sieve. Subdivided into silts (M) and clays (C) using the Casagrande plasticity chart.

USCS group symbols reference #

Symbol	Soil type	Key characteristics
GW	Well-graded gravel	Wide range of grain sizes, few fines
GP	Poorly-graded gravel	Uniform grain size, few fines
GM	Silty gravel	Gravel with appreciable silt fines
GC	Clayey gravel	Gravel with appreciable clay fines
SW	Well-graded sand	Wide range of grain sizes, few fines
SP	Poorly-graded sand	Uniform grain size, few fines
SM	Silty sand	Sand with appreciable silt fines
SC	Clayey sand	Sand with appreciable clay fines
ML	Silt of low plasticity	LL < 50, plots below A-line
CL	Clay of low plasticity	LL < 50, plots above A-line
MH	Silt of high plasticity	LL ≥ 50, plots below A-line
CH	Clay of high plasticity	LL ≥ 50, plots above A-line
OL	Organic silt/clay, low plasticity	Organic odour, dark colour, LL < 50
OH	Organic clay/silt, high plasticity	Organic odour, dark colour, LL ≥ 50
Pt	Peat / highly organic soil	Fibrous, dark, very compressible

Descriptors used in field soil description #

Beyond the USCS symbol, each soil layer should be described using the following standard descriptors:

Colour: Dominant and secondary colour (e.g. dark brown, grey-green, reddish-brown)
Moisture: Dry / Slightly moist / Moist / Wet / Saturated
Consistency (fine-grained soils): Very soft / Soft / Firm / Stiff / Very stiff / Hard — assessed by thumb penetration or hand shear vane
Density (coarse-grained soils): Very loose / Loose / Medium dense / Dense / Very dense — inferred from SPT N-values
Structure: Intact / Fissured / Laminated / Varved / Interbedded
Plasticity (fine-grained soils): Non-plastic / Low / Medium / High — assessed from thread rolling test in field
Inclusions: Presence of gravel, shells, roots, organics, concretions, etc.

Standard description order: Consistency/Density → Colour → Moisture → Soil name (USCS) → Structure → Inclusions → USCS symbol in brackets

Example: “STIFF, dark grey, moist CLAY, fissured, with occasional shell fragments (CL)”

In-situ testing during drilling #

Borehole logging is not just a visual and descriptive exercise — the borehole provides access to run in-situ tests that measure soil properties directly in the ground, without the sample disturbance that laboratory testing inevitably introduces.

Standard Penetration Test (SPT) #

The SPT is the most common in-situ test performed in boreholes worldwide. A split-spoon sampler (50 mm OD, 35 mm ID, 600 mm long) is driven into the soil at the base of the borehole using a 63.5 kg hammer dropping 760 mm. The number of blows required to drive the sampler through three consecutive 150 mm intervals is recorded. The N-value (sum of blows for the second and third intervals) is the primary output.

The SPT serves a dual purpose: the N-value provides a penetration resistance measure for soil property estimation, while the split-spoon sampler simultaneously retrieves a disturbed soil sample for visual description and laboratory classification testing.

SPT tests are typically performed at 1.0 or 1.5 m depth intervals throughout the borehole, or at every change in material. The raw N-value must be corrected for hammer energy, rod length, borehole diameter, and overburden pressure before being used in correlations — a process covered in full in the SPT guide.

Shelby tube sampling (undisturbed sampling) #

Where laboratory testing of soft to firm cohesive soils is required (consolidation testing, undrained shear strength measurement), thin-walled Shelby tubes (76 mm OD, 1.5 mm wall thickness) are pushed slowly into the soil at the base of the borehole. The goal is to preserve the in-situ structure and moisture condition of the soil as closely as possible. Shelby tube samples are sealed in the field, stored vertically, and transported to the laboratory with minimal vibration.

Vane shear test #

The field vane shear test measures the undrained shear strength (s_u) of soft to medium cohesive soils by rotating a four-bladed vane pushed into undisturbed soil at the base of the borehole. It is particularly valuable where Shelby tube sampling is difficult or where rapid assessment of shear strength with depth is required. Results must be corrected using the Bjerrum (1972) correction factor (μ) before use in design.

Dynamic Cone Penetrometer (DCP) #

The DCP test drives a cone into the soil using a standard drop hammer, measuring penetration per blow. It is faster and simpler than the SPT, and is widely used in road pavement design, compaction quality control, and shallow site investigations in developing regions. DartiGeo includes a DCP module for data entry and interpretation.

Groundwater observations #

Recording groundwater conditions is a mandatory part of every borehole log. Groundwater directly affects bearing capacity calculations (by reducing effective stress), consolidation behaviour, earthworks design, and construction method selection. A borehole log should record:

Seepage depth: The depth at which water was first observed flowing into the borehole during drilling
Cave-in depth: For auger borings in cohesionless soils, the depth at which the borehole walls began to collapse (indicates the water table)
Stabilised water level: The depth to water measured at least 30–60 minutes after completion of drilling (or at the start of the next day), after the borehole has been left open to equilibrate. This is the most reliable groundwater depth measurement.
Artesian conditions: Whether water rose above the ground surface under its own pressure

A single water level observation taken during drilling is often unreliable — particularly in low-permeability soils where equilibration can take hours or days. For critical projects, standpipe piezometers or vibrating wire piezometers are installed in the borehole after drilling to monitor groundwater over time.

Borehole log format and standards #

A professional borehole log follows a standardised format that allows any competent geotechnical engineer — regardless of whether they were present at the site — to read and interpret the subsurface conditions clearly. The relevant standard for field logging of subsurface explorations is ASTM D5434.

Standard borehole log layout #

A typical borehole log is presented in portrait A4 or US Letter format and is divided into vertical columns:

Column	Content
Header	Project name, boring ID, location, coordinates, elevation, drilling date, driller, engineer
Depth scale	Vertical scale in metres or feet from surface
Graphical soil log	Standardised pattern symbols representing each soil/rock type (USCS-based)
Soil description	Full written description of each layer with USCS symbol
Sample column	Sample type (SS, ST, etc.), depth, and recovery percentage
SPT column	Blow counts for each of the three 150 mm drives; N-value; graphical N-value bar chart
Groundwater marker	Horizontal line or symbol at observed and stabilised groundwater depth
Remarks	Drilling progress observations, casing depth, any anomalies

1-page vs 2-page log format #

For shallow boreholes (up to approximately 10–15 m), all information typically fits on a single A4 page. Deeper boreholes continue onto additional pages with the depth scale carrying over from the bottom of the previous page. Some organisations use a 2-page format — one page for the graphical log and a second page for detailed descriptions and test results — for complex or data-heavy boreholes.

Export formats #

Professional borehole log reports are delivered to clients in PDF format for distribution and archiving. Editable Word format may be provided for further customisation. Excel export allows the raw tabular data to be reused in calculations or imported into other platforms.

How DartiGeo handles borehole logging #

DartiGeo’s field tests module provides a complete borehole logging workflow — from field data entry to professional report generation — within the same platform used for SPT processing, CPT interpretation, laboratory test management, and foundation design. There is no re-entry of data between modules.

Within DartiGeo, engineers enter borehole details (ID, coordinates, driller, dates), define soil layers with depths and USCS descriptions, record SPT blow counts at each test interval, log sample types and recovery, and mark groundwater observations. The software automatically generates the graphical soil log column using USCS symbol patterns and plots SPT N-values as a bar chart alongside the layer descriptions.

Reports are generated at the click of a button in 1-page or 2-page borehole log format, exportable as PDF, Word, or Excel. Because SPT data entered in the borehole log feeds directly into the SPT correlations module, there is no duplicate data entry when moving from logging to analysis.

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Frequently asked questions #

What is the difference between a borehole log and a boring log? #

They are the same document. “Boring log” and “soil boring log” are common US terminology; “borehole log” is the internationally used equivalent. Both refer to the structured record of subsurface conditions encountered during a drilled investigation. In the UK, “trial pit log” or “window sampler log” may be used for shallower investigation methods that do not involve rotary or auger drilling.

What software do geotechnical engineers use to create borehole logs? #

Common options include DartiGeo, gINT (Bentley), Keybase, LogPlot, and RSLog. The key differentiator among these tools is integration: DartiGeo is notable for combining borehole logging with SPT correlations, CPT interpretation, laboratory test management, and bearing capacity calculations in a single licence, eliminating the need for separate programs for each stage of the workflow.

What ASTM standard covers borehole logging? #

ASTM D5434 — Standard Guide for Field Logging of Subsurface Explorations of Soil and Rock — is the primary standard covering borehole log content and format. ASTM D1586 governs the SPT procedure performed within boreholes, and ASTM D2487 defines the USCS soil classification system used for soil description.

How deep should a borehole be for building foundation design? #

As a general rule, boreholes for shallow foundation design should extend to at least 1.5 times the width of the largest loaded area below the foundation level, or to the depth at which the stress increase from the applied load has reduced to 10% of the effective overburden stress — whichever is greater. For a typical multi-storey building with a 20 m wide raft foundation, this might require investigation to 20–30 m depth. For preliminary design or smaller structures on known-good ground, 6–10 m boreholes may be adequate.

What does SPT refusal mean in a borehole log? #

SPT refusal occurs when 50 blows are required for any single 150 mm drive increment, or when 100 blows are applied in total over the full 450 mm drive, without the sampler advancing 450 mm. It is recorded on the borehole log as “REF” at that depth and indicates the presence of very dense granular material, hard clay, weathered rock, or intact rock. Drilling can continue past a refusal zone using an appropriate rotary method.

Borehole Logging — A Complete Guide for Geotechnical Engineers · DartisTech