Digging a Trial Pit for Site Investigations

by | Nov 23, 2023

Prior to carrying out any construction project, you need to determine whether the proposed plan will be feasible by understanding the underlying ground conditions and soil composition. One way to do this is by digging a trial pit.

Here at Borehole Solutions, we’ve helped with countless geotechnical site investigations over the years. If you want to find out more about this investigation technique, then our drilling experts have the answers!

What Are Trial Pits?

Trial pits are between 1 and 4 metres deep. If the hole extends beyond 1.2 metres below ground level, then additional structural supports should be installed. Any pit deeper than this should never be entered for safety reasons. However, if the ground appears to be unstable, then the pit should always be reinforced regardless of depth.

Hand Dug Pits

If it’s a small-scale site investigation, then the trial pit can often be dug by hand. The location is usually scanned to determine whether any underground features are present, such as gas pipelines or electric cables. These shallower pits can be used to obtain soil samples and locate the exact positioning of underground utilities. However, they’re also useful for WAC and contamination testing.

Machine Excavated Pits

If the pit is deeper than 1.5 metres, then window samplers and mini rigs are commonly used. The most common depth of a trial pit is approximately 3 metres as this tends to provide a good visual assessment of the land. Although pits excavated by machinery are cost-effective and quick, they do cause more ground disturbance in comparison to those dug by hand.

What Are Trial Pits Used For?

This geotechnical drilling technique is used to assess the ground, soil profiles and groundwater conditions prior to construction work. They allow a large area of land to be inspected quickly and various sampling methods can be used, including vane shear tests, CBR testing and soakaway tests.

The Advantages

Digging out a trial pit is common practice within the geotechnical industry. After all, nobody wants to start working on a project to then find a structural failure further down the line! There are several benefits to pit digging, including:

  • Cost-effective – trial pits are one of the most practical site investigation techniques available. With minimal, if any, machinery required, you can save on both fuel and transport costs. Moreover, it can often be completed in a single working day, so labour costs should be minimal too.
  • Flexible – pit digging allows you to collect both disturbed and undisturbed samples, as well as vertical and lateral subsurface samples; it’s an incredibly versatile site investigation method!
  • Reduce risk – taking the time to carry out pit digging can help minimise the risk of running into problems in the future. The last thing you need is the costly and time-consuming consequences of damaging any hidden utilities.

Contact Borehole Solutions

If you’re interested in finding out more about trial pit digging services or any other geotechnical drilling technique for that matter, get in touch with us today! Our experts are always happy to help. Give us a call on 01733 200501 or click here to speak with the team.