A well-executed trial pit can reveal far more than a list of soil layers. It provides a direct view of ground conditions, structure, weathering, groundwater, fill, obstructions and the practical behaviour of the excavation. The value of that exposure, however, depends on how clearly and consistently it is recorded.
This guide explains what to capture when logging a trial pit, how to organise the record and how photographs, samples and simple field tests can support the written description.
What is a trial pit log?
A trial pit log is the permanent field record of the ground exposed by an excavation. It normally combines written soil or rock descriptions with depths, strata boundaries, groundwater observations, samples, field-test results, excavation behaviour and photographs.
The finished log should allow another geotechnical professional to understand what was encountered without having been present on site. Consistency matters: soil identification and description should follow the project specification and applicable standards, commonly BS 5930:2015+A1:2020 together with BS EN ISO 14688-1:2018 and BS EN ISO 14688-2:2018. Where rock is exposed, BS EN ISO 14689:2018 provides the corresponding framework for rock identification, description and classification.
Before excavation begins
Confirm the trial pit reference, coordinates, ground level, proposed depth and orientation before work starts. The location should agree with the approved exploratory-hole plan, and the site team should review utility information, permits, access constraints and the project risk assessment and method statement.
Never enter an unsupported trial pit. Logging and photography should be completed from a safe position in accordance with the site-specific RAMS and under competent supervision. Unstable sides, water ingress, plant movements, overhead services and contaminated ground all require appropriate controls.
Essential information to record
Every log should begin with the basic project and excavation details:
- Project name, site and client reference
- Trial pit number and coordinates
- Date, weather and ground conditions
- Ground level and the coordinate system or datum used
- Excavation method, machine and bucket size where relevant
- Pit dimensions, orientation and final depth
- Name or initials of the logger
- Reason for termination, such as target depth, refusal, groundwater or instability
A dedicated A4 Trial Pit Log Book helps keep these details, descriptions, sample references and sketches together. For larger site teams, the pack of 10 trial pit log books provides a consistent format across several engineers and work areas.
Log the sequence, not just the soil
Record each boundary from ground level downwards. Note the top and base depth of every unit and whether the contact is sharp, gradual, irregular or inclined. Thin bands and local lenses can be important even when they do not extend across the full face.
For each soil unit, consider:
- Principal and secondary constituents
- Colour and any mottling
- Moisture condition
- Consistency of fine soils or density of coarse soils
- Particle size, grading and shape where appropriate
- Structure, fissuring, bedding, laminations or fabric
- Roots, organics, cobbles, boulders, inclusions or man-made materials
- Evidence of weathering, disturbance or reworking
- Odour, staining or other potential contamination indicators
Made ground should be described rather than treated as a single generic layer. Its composition, variability, inclusions and apparent method of placement may be central to the engineering interpretation.
Odour, staining or unusual material should be recorded as observations rather than used to make an unsupported contamination diagnosis. Contamination and ground-gas assessment require the project-specific sampling strategy and appropriate specialist guidance; BS 5930 does not itself provide the investigation framework for contaminated land or ground gas.
Record groundwater carefully
Groundwater observations need both a depth and a time. Record the first seepage or strike, the rate and location of inflow, and any rise or stabilised level observed while the pit remains open. A standing water level measured later should not be reported as though it were the original strike.
Also distinguish groundwater from surface runoff, leaking services or water introduced during excavation. Note wet seams, perched water and inflow concentrated along fractures or granular bands.
Photograph the exposed ground consistently
Trial pit photographs should be useful evidence, not simply proof that the excavation took place. Take an overview showing the pit in its surroundings, followed by clear images of the exposed faces and any significant feature.
For reliable photographs:
- Clean the selected face where this can be done safely from outside the excavation
- Keep the camera square to the face where possible
- Include a visible scale and the trial pit reference
- Record depth, project and date consistently
- Avoid deep shadow, glare and plant obscuring the face
- Capture important boundaries, structures, seepage and obstructions separately
- Use a consistent file-naming system linked to the log
An A3 Geotechnical Photo Board places the trial pit reference, depth information, colour scale and measurement reference in the image. This reduces ambiguity when hundreds of photographs are reviewed later and makes the photographic record much easier to audit.
Samples and field tests
Give every sample a unique identifier and record its type, depth or depth range and the stratum from which it was taken. The label on the container, the log and the laboratory schedule must agree. Photograph unusual samples or features before the material is disturbed.
Simple field tests can support the visual description, but their limitations should be understood. A Pocket Penetrometer provides a rapid comparative indication of fine-soil consistency and approximate compressive resistance. It is useful for screening and comparing material across a site, but it does not replace properly selected laboratory testing or engineering interpretation.
Where undrained shear strength of suitable cohesive soil is required, vane testing may be appropriate. See our separate Field Vane Shear Test Guide for equipment selection, procedure and interpretation.
Record excavation behaviour
The way a trial pit behaves can be as informative as the exposed material. Record sidewall collapse, ravelling, running sand, blocky failure, base heave, rapid water ingress and difficulty excavating particular horizons. Note refusal depth and the likely cause, but do not assume that excavator refusal proves bedrock.
Buried concrete, foundations, services, voids, waste, boulders or obstructions should be described, measured and photographed. If the pit is repositioned, record the revised location and explain why.
Common trial pit logging mistakes
- Using vague descriptions such as “brown soil” without engineering detail
- Failing to distinguish topsoil, made ground and natural strata
- Recording groundwater without the observation time
- Giving sample numbers that do not match the containers or schedule
- Taking photographs without a readable pit reference or scale
- Missing thin bands, lenses, local obstructions or irregular boundaries
- Reporting machine refusal as confirmed rock
- Completing the log later from memory rather than during the work
A practical field checklist
- Confirm the pit number, position, level and orientation.
- Record the excavation method, dimensions and start time.
- Log each stratum from the surface down as it becomes visible.
- Mark boundaries and sample depths immediately.
- Record groundwater strikes and subsequent observations with times.
- Photograph the overview, faces and significant features with a scale and reference board.
- Complete any field tests and record the method and result.
- Note stability, obstructions, refusal and termination depth.
- Check sample labels against the log before leaving the location.
- Record backfilling and reinstatement in accordance with the project requirements.
Building a consistent trial pit logging system
Good logging is not about filling every space on a form. It is about creating a clear, traceable record that connects location, description, photographs, samples and test results. Standardised equipment and a repeatable workflow make that much easier across different projects and field teams.
The Ground Investigation Field Kit – Professional Edition brings together practical site logging and measurement equipment, while dedicated log books and photo boards provide consistency from the first excavation through to the final factual report.
Trial pit logging questions
What should a trial pit log include?
A complete trial pit log should identify the location and excavation, describe each stratum and boundary, record groundwater observations, samples and field tests, note excavation behaviour and link the written record to clear photographs.
Which standards are used for trial pit logging in the UK?
Project requirements commonly refer to BS 5930 for ground investigation practice, BS EN ISO 14688-1 and BS EN ISO 14688-2 for soil identification and classification, and BS EN ISO 14689 where rock is exposed. Always use the editions and procedures stated in the project specification.
How deep should a trial pit be?
There is no single correct depth. It depends on the purpose of the investigation, expected geology, groundwater, excavation stability, plant capability and the site-specific safe system of work. The target and termination criteria should be agreed before excavation.
Can a trial pit be entered for closer inspection?
An unsupported excavation should never be entered. Any access must be governed by a suitable excavation design, protective system, risk assessment and competent site supervision.
Related field guides
- Field Vane Shear Test: A Complete Guide
- Dynamic Cone Penetrometer Testing: How to Estimate CBR on Site
Field procedures, soil descriptions and testing should always follow the project specification, current applicable standards and competent geotechnical judgement.