Trial pits are a crucial component of ground investigations, providing essential data for construction, environmental assessments and geotechnical engineering. They provide quick, cost-effective, visual inspection of the ground making them a preferred method for many site investigations.
Sub Surface perform trial pits, also known as trial holes and trial pitting, throughout the UK for Geotechnical and Environmental Site Investigations.
Trial pits are supervised by our experienced Geotechnical and Geo-Environmental Engineers to provide detailed Engineers logs in accordance with British Standards (BS EN ISO 14688-1 and BS EN ISO 14689-1). Disturbed samples can be taken at regular intervals for contamination and geotechnical testing, and hand shear vane tests can be performed.
Contact your local Sub Surface office to request a quote for Trial Pitting as part of your Ground Investigation.
What is a Trial Pit?
Trial pits are also referred to as trial holes, test pits and trial trenches.
A trial pit is an open excavation used to obtain information about sub-surface ground conditions in geotechnical and geo-environmental site investigations. Trial pits are dug at regular intervals and are a relatively quick, low-cost method used to assess the soil, rock and groundwater conditions of a site.
Trial Pits give a direct visual inspection of the subsurface materials, allowing for easy sampling, testing and analysis. They are typically used in the early stages of construction to evaluate the suitability of a site and inform planning decisions.
There are two methods of excavating a Trial Pit: machine excavation and hand digging.
Machine Excavated Trial Pits
Machine Excavated Trial Pits are the most common method of Trial Pitting and are excavated using a tracked 360° excavator or a hydraulic wheeled backhoe loader (e.g. JCB 3CX). This method is typically used for larger or deeper pits, where hand digging would be impractical and time consuming.
Machine Excavated Trial Pits can reach significant depths up to 4.5m, depending on excavator and site conditions. The ground conditions can quickly be assessed across a larger area and both the vertical and horizontal faces of the Trial Pit can be examined. Soil samples can be easily obtained from various depths during the excavation and taken for environmental or geotechnical Testing.
This method is the preferred option for fast and efficient excavation, in large-scale sites, or where multiple pits are required. However, machinery can cause greater disruption to the soil in comparison to hand digging.
Hand Dug Trial Pits
Hand Dug Trial Pits are dug using hand digging tools to a maximum depth of 1.20m. They are useful for small scale site investigations to assess the near surface soil conditions, obtain soil samples, expose foundations, and locate underground services or structures.
This method is ideal for shallower investigations, in areas where access for machinery is limited, or where precise and controlled excavation is required. Hand Dug Trial Pits are often the preferred method for environmentally sensitive locations, where minimal disruption is required.
Hand digging is more time-consuming but offers a greater level of control and precision compared to machine excavation.
How are Trial Pits used in Site Investigations?
Trial Pits are undertaken for a variety of purposes within Site Investigations, including:
- Soil Profiling: Identifying soil composition, structure, and stratigraphy, both vertical and horizontal.
- Contamination Assessment: Trial pits provide a relatively quick and efficient method to obtain soil and groundwater samples for contamination testing. This is crucial for assessing the potential environmental risks on a site.
- Foundation Design: Data gathered from trial pits can aid engineers in determining appropriate shallow foundation solutions. The strata are logged, in-situ field tests are undertaken, and samples are collected for geotechnical testing. This often includes moisture content, plastic limit and liquid limit tests.
- Existing Foundations: A Hand dug trial pit can be used to expose existing foundations.
- Ground Hazards: Trial trenching can be used to locate mineshafts and infilled solutions features.
- Groundwater Monitoring: Monitoring groundwater levels within Trial Pits provides information on the water table levels and flow characteristics.
- Soakaway and Percolation tests: Undertaking BRE 365 soakaways tests and percolation tests to determine the suitability of infiltration into the ground.
- Waste Classification: Trial Pits offer an efficient method of collecting representative samples for waste classification and WAC testing, allowing us to determine whether waste materials are suitable for disposal at Inert, Non-Hazardous or Hazardous landfill sites.
- Validation: Trial Pits are a key method in the validation phase of a project and are used to confirm site conditions and soil characteristics.
A trial pit investigation enables us to make informed decisions, mitigate risks and ensure compliance with regulations.
Data Collection and Analysis During Trial Pitting
Trail pitting allows for comprehensive data to be collected through:
- Visual Inspection – Large volumes of soil can be inspected, identifying soil texture, colour, consistency, and structure.
- Soil Sampling – Gathering environmental, disturbed and undisturbed samples for laboratory testing.
- Field Testing – Conducting in-situ testing such as hand shear vane tests, in-situ CBR tests, modulus of subgrade reaction tests, PID monitoring, etc.
- Photographs and Logging – recording the site conditions, sample locations and depth measurements.
Request a Quote
If you would like a quote for Trial Pits (Trial Holes) as part of your Site Investigation or have any questions, please do not hesitate to contact your Regional Sub Surface Office.
Other Ground Investigation Methods: Cable Percussive Boreholes, Window Sampling, Rotary Drilling, Dynamic Probing
Trial Pit FAQs
How are Trial Pits excavated?
A Trial Pit is excavated using hand digging tools or using machinery such as a tracked 360° excavator or a hydraulic wheeled backhoe loader.
What is the difference between a borehole and a trial pit?
Unlike boreholes, a Trial Pit gives a direct visual inspection of the subsurface materials and can be excavated relatively quickly. However, a Trial Pit cannot reach the same depths as boreholes. Therefore, boreholes are more suited for the assessment of deeper, more complex subsurface conditions.
The density of granular strata cannot be determined from trial pitting. Therefore, boreholes will be required for foundation recommendations where sands and gravels are present.
How deep is a trial pit?
Hand Dug Trial Pits are usually dug to a maximum depth of 1.20m. Machine Excavated Trial Pits can be excavated to a depth of 4.5m. However, they are generally excavated to around 3.00m, depending on site conditions and project requirements.
How long does Trial Pitting take?
The timeline of trial pitting is dependent on the size and complexity of the ground investigation. Get in touch with our team and we will be happy to discuss the best options for your site!
Can I supply my own excavator?
Yes, as long as you have a fully trained and qualified operator to drive it.