Trial Pits & Holes
ESP’s experienced team delivers trial pit investigations that provide reliable, expert insight into ground conditions and subsurface composition
Trial pits are holes dug into the ground to examine the subsurface’s structure and composition. Also called test pits, trial pitting and trial trenches – they help to examine soil, rocks and groundwater. Trial hole investigations take place before beginning construction projects so the conditions and composition of the soil and ground can be properly assessed.
They can often provide the most practical and economical method of site investigation and assessment. ESP often utilises trial pits to obtain samples and engineering logs of shallow strata and we consider the large in-situ exposure of soils to be of importance when dealing with variable ground conditions.
Trial pits are usually completed before construction works to assess the geology and water table of the site. Pits are excavated in intervals, typically 1 to 4 meters deep, by hand or machine. In a situation where there is any risk of coming into contact with utilities like water pipes or electric cables then digging by hand is the preferred approach.
If the hole required is deeper than 1.2 metres then reinforcement precautions are used to solidify the pit to ensure safety.
What Is a Trial Pit?
Recent Trial Pit Projects
Case Study 1: Geotechnical Support Including Trial Pit
As part of this ongoing commission, Invertek Drives asked us to undertake an investigation and assessment to assist in the design of geotechnical and drainage aspects for a proposed large extension to the existing warehousing/fabrication facilities.
Our site investigation has included trial pits, large-scale infiltration testing, dynamic cone penetrometer testing to derive CBR values, plate load testing, and laboratory testing.
Over several phases, we were appointed to undertake a SAB-compliant infiltration investigation to allow the relocation and appropriate sizing of sustainable drainage features (SuDS). This was followed by a detailed geotechnical Desk Study and Site Investigation to enable the design of the proposed extension buildings and associated slabs, which were to support high-level racking, and specification for backfilling of former drainage features.
Case Study 2: Targeted Trial Pit Near Plymouth
ESP undertook targeted trial pit investigations for a proposed commercial and business park development near Langage, Plymouth, providing essential near-surface ground information to support early-stage design.
We excavated and logged trial pits across the site to assess Made Ground, topsoil and shallow geological conditions, while also identifying potential contamination risks and constraints affecting development.
Our findings informed a refined ground model and provided clear recommendations for further detailed investigation, supporting the progression of planning and design across the wider site.
Explore our project gallery to see our team in action. From initial investigations to final delivery, these images represent our commitment to high quality data collection and the professional standards we bring to every project.
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Frequently Asked Questions
What Types of Trial Pits are there?
1. Hand Dug Trial Pit
Hand-dug trial pits are ideal for small-scale site investigations, especially where subsurface risks like pipes and cables are present. Typically, manual excavation pits are excavated to a depth of 1.2m to 1.5m. Before excavation, the surface can be scanned using a standard or advanced CAT scanner to detect underground obstructions such as utilities and structures.
Applications:
- Soil Sampling: Hand-excavated trial pits are often used to collect near-surface soil samples.
- Utility Location: They help in locating underground utilities and structures.
- Foundation Exposure: This method is also used to expose shallow, existing foundations for further assessment.
2. Machine Dug Trial Pits
Machine-dug trial pits are suitable for larger sites where extensive excavation is required. These pits typically range from 1.5m to 6m in depth, with a standard depth of around 3m being sufficient for most assessments.
Key Considerations:
- Ground Disturbance: The use of large machinery can cause significant ground disturbance, which must be managed carefully along with trial pit safety.
- Supplementary Testing: For deeper investigations, smaller test holes or boreholes may be recommended. These can be used for window sampling and mini rigs, offering a less disruptive alternative.
Why Use Trial Pits?
Here are some of the benefits of trial pits:
- The SI enables information to be collected to further characterise the Conceptual Model (S-P-R)
- The presumed Conceptual Ground Model can be updated with actual site data in real time
- The siting can be justified and compared with assumptions reasonably developed during desk-study
- It is not limited to the collection and analysis of samples to simply ascertain contaminant concentrations – the input to engineering geology and geotechnical assessments is valuable
- The sampling strategy can be modified whilst the investigation is in progress for little cost
It is also important to consider the Health and Safety implications of trial pitting, both during and after the investigation.
When Are Trial Pits Required?
Trial pits conducted as part of site investigations are often needed for the below situations:
- Site pre-purchase due diligence studies
- Contaminated land risk assessments
- Foundation design
- Subsidence surveys
- Part IIa investigations
- Environmental monitoring
- Insurance claims
- Mining surveys
- Relict foundation and obstruction assessment
- Ground improvement design
What is a Trial Hole Used For?
Often more cost-efficient than using a borehole, trial pits are used before work on a development site begins. They are conducted on stable ground, to assess the ground conditions as part of a site investigation. Trial pits are used to investigate large sample areas to examine the strata and conclude if any additional investigations are needed on the construction site.
Trial pitting is a helpful tool to examine:
- Sub-surface structures
- Variable ground conditions
- Soil conditions and profile
We can employ the use of a trial pit to take distributed and undistributed samples along with the use of other sampling methods such as:
- Vane Shear tests
- Soakaway tests
- CBR testing (per BRE 365 guidance)
- In-situ strength testing
Client Testimonials
Speak To Our Experts Today
Earth Science Partnership is a leading expert in trial pits across Wales, England, Scotland, Northern Ireland and Ireland. We have decades of combined experience in our team of industry-recognised experts.
Our work, for homeowners, land owners, developers, and investors is completed in compliance with the latest, relevant industry best practices and guidance. We understand that every project is unique, so we deliver our services tailored to your needs.
If you’re looking to complete a trial pit, the ESP team can support you with a straightforward, cost-effective solution for your planning permission application.
Author
I am a UK Registered Ground Engineering Specialist and Chartered Engineering & Environmental Geologist with significant experience in Site Hazard Assessment, Remediation, Design, and Implementation across the Geoenvironmental and Geotechnical sectors in the UK and abroad.
I often work in multi-disciplinary teams advising other engineering and design professionals on the implications of ground hazards.
My work involves undertaking design, planning, management, and reporting of ground investigations. For example investigations for new developments and many bespoke studies including:
- Assessment of hydrogeological settings, materials for earthworks.
- Impact of abandoned mine workings.
- Remediation and stability assessment of soil and rock slopes/landslides, most notably the Pantteg Landslide in the Swansea Valley for which I have guided and presented assessments to Local Authority officers and the general public.