Bridge & Bedrock

The purpose of standardised logging is not merely academic. A soil description produced by one engineer on one project should be directly comparable to one produced by another engineer on a different project in the same geological setting. This comparability is essential for building up knowledge of ground conditions over time and across a region, and for producing reports that are meaningful to a wide audience of engineers, geologists, planners, and clients. BS 5930:2015, the Code of Practice for Ground Investigations, provides the standard framework for logging soils and rocks in the UK, and compliance with it is a requirement of most UK ground investigation specifications and a cornerstone of professional geotechnical practice.

Yet despite its importance, soil logging is a skill that is frequently undervalued and poorly taught. It is often delegated to the least experienced member of a team, carried out under time pressure in difficult field conditions, and checked superficially if at all. The result can be borehole logs that are internally inconsistent, that use non-standard terminology, that fail to capture key features of the materials encountered, or that are so vague as to be almost useless for design purposes. Good soil logging takes practice, discipline, and a genuine understanding of what information is needed and why. This post provides an introduction to the BS 5930 logging system and the principles that underpin it.

Why Standardised Logging Matters

The borehole log is the primary factual record of what was found at a particular location and depth during the ground investigation. It is the raw data from which the ground model is built, the geotechnical parameters are derived, and the design is developed. If the log is inaccurate, incomplete, or inconsistent, the errors propagate through every subsequent stage of the project. A missed stratum boundary, a misidentified soil type, or an overlooked feature such as a zone of organic material or a layer of running sand can have serious consequences for design and construction.

Standardisation serves multiple purposes. It ensures that logs produced by different practitioners are comparable, enabling the information from multiple boreholes to be combined into a coherent ground model. It provides a common language for the description of soil and rock that is understood across the profession, reducing the risk of misinterpretation. It requires the logger to describe materials systematically, working through a defined set of parameters for each stratum, which makes it harder to overlook important features. And it produces logs that can be digitised and stored in the AGS data format for electronic data transfer, enabling the information to be used in databases, modelling software, and future investigations.

The BS 5930 Logging System for Soils

BS 5930:2015 sets out a structured approach to soil description based on a defined set of parameters that must be assessed and recorded for each stratum. The principal parameters are: material type, structure, colour, particle size distribution (grading), plasticity (for fine-grained soils), consistency (for cohesive soils) or relative density (for granular soils), strength, and any other relevant characteristics such as organic content, carbonate content, or the presence of particular minerals.

The standard requires that soil descriptions are presented in a specific order, following the mnemonic widely known in UK geotechnics: MSCCORS — Material type, Structure, Colour, Consistency/Compactness, Origin, Relative density, and Secondary characteristics. In practice, descriptions typically begin with the consistency or compactness descriptor (e.g. “Firm”, “Dense”), followed by colour, then material type, and then supplementary information about grading, plasticity, and other characteristics. A complete description might read: “Firm, grey, slightly sandy CLAY with occasional pockets of silt.”

Material Type

The material type is the fundamental classification of the soil, and it is always written in CAPITAL LETTERS in a BS 5930 description to distinguish it from the qualifying terms around it. The principal material types are GRAVEL, SAND, SILT, CLAY, PEAT, and MADE GROUND, reflecting the dominant particle size or origin of the material. Composite soils — those containing significant proportions of more than one particle size — are described using a hierarchical naming convention: if a soil is predominantly sand with a significant gravel fraction, it is described as “gravelly SAND”; if the proportions are approximately equal, it might be described as “SAND and GRAVEL”.

The distinction between SILT and CLAY is particularly important because these two materials have very different engineering behaviours despite similar particle sizes. Clays have significant plasticity and cohesion; silts are non-plastic and prone to liquefaction under vibration. In the field, the distinction is made using simple manual tests: the dilatancy test (a silt will dilate when squeezed and water will appear at the surface), the thread-rolling test (a clay can be rolled into a thin thread, a silt cannot), and the dry strength test (a clay dry pat is hard and cannot be broken by finger pressure, a silt pat can be crumbled easily).

Consistency and Compactness

For cohesive soils (clays and silts), the consistency — which is related to undrained shear strength — is described using standard terms: Very soft, Soft, Firm, Stiff, Very stiff, and Hard. These terms correspond to defined ranges of undrained shear strength, assessed in the field by the resistance of the soil to penetration by the thumb or a thin rod, and quantified by laboratory tests such as the triaxial compression test or the vane shear test. The BS 5930 consistency terms are well established and widely understood, but they are based on subjective field assessment and should be used with an awareness of their limitations.

For granular soils (sands and gravels), the equivalent parameter is relative density (or compactness), described using the terms: Very loose, Loose, Medium dense, Dense, and Very dense. In the field, relative density is estimated from the resistance of the soil to probing with a thin rod, from the ease with which a spade or pick can penetrate the material, or — in boreholes — from the SPT N-value. The correlation between SPT N-value and relative density is well established in the literature, though it is sensitive to the vertical effective stress and the grading of the soil.

Colour and Structure

The colour of a soil is an important but often underappreciated aspect of the description. Colour can indicate mineralogy (reddish colours are associated with iron oxide minerals, black colours with organic matter or manganese), weathering state (paler, more uniform colours typically indicate more advanced weathering), and contamination (unusual colours, particularly bright staining, may indicate the presence of chemical contaminants). Colours should be described using standard Munsell colour notation where precision is important, but in routine logging, descriptive terms such as “grey-brown”, “dark grey”, and “reddish-brown” are generally sufficient provided they are used consistently.

Structure refers to the fabric or internal organisation of the soil mass. Many soils are not homogeneous but contain layers, lenses, pockets, or inclusions of different materials. These features are geologically significant — they often reflect the depositional or post-depositional history of the material — and they may be engineeringly significant: a thin layer of soft clay within a deposit of stiffer material can control the location of a shear surface in a slope stability problem, and a lens of permeable sand within an otherwise impermeable clay can act as a groundwater pathway. Structure should be described using standard terms: laminated, interbedded, fissured, fissile, intact, and so on.

Recording and Presenting Logs

Borehole logs are typically presented in a tabular format, with depth on the vertical axis and columns for the description, the soil profile (shown as a graphical symbol), samples, in-situ test results, and groundwater observations. The graphical symbols used to represent different soil and rock types are standardised in BS 5930, and a legend should be included with every set of logs. The depth of strata boundaries, the positions of samples and tests, and the depths at which groundwater was encountered should all be clearly recorded.

In the UK, ground investigation data are increasingly stored and transferred in the AGS data format — a comma-delimited text format developed by the Association of Geotechnical and Geoenvironmental Specialists that allows borehole log data, laboratory test results, and in-situ test data to be exchanged between different software systems without loss of information. The use of AGS format is increasingly specified in ground investigation contracts and is a requirement for investigations carried out for many public sector clients. AGS version 4 is the current standard.

Logging in the Field: Practical Considerations

The quality of borehole logging is ultimately determined by what happens in the field. Logging must be carried out by a competent person who has been trained in the BS 5930 system and who has sufficient practical experience to make accurate assessments of material type, consistency, colour, and structure under field conditions. It should not be delegated to an untrained operative or carried out by someone who is primarily occupied with supervising the drilling operation.

The logger should examine all material recovered from each borehole run before it is discarded or disturbed by subsequent operations. Samples should be retained in appropriate containers (sealed polythene bags for disturbed samples, waxed tubes for U100 samples) and labelled with the borehole reference, depth, and sample number. Any features that may not be apparent from the sample alone — groundwater strikes, changes in drilling behaviour, the presence of odours or discolouration — should be noted at the time they are observed.

Good soil logging is a combination of observation, knowledge, and discipline. The observation comes with practice in the field and a genuine curiosity about what the ground is telling you. The knowledge comes from training, study, and experience with a wide range of geological materials and settings. The discipline comes from following the BS 5930 system consistently, even under time pressure, and from the professional commitment to producing a record that is accurate, complete, and useful. These three qualities, combined, produce borehole logs that are the foundation of good geotechnical practice — and that is, ultimately, what this series is about.