Trial Pit Excavation for ground investigation
A trial pit is a shallow excavation, typically up to about 3 to 4 metres deep, dug with an excavator to expose the ground directly. It lets engineers see and log the soil layers, fill and groundwater, take disturbed and block samples, and carry out in-situ tests such as density tests and plate load tests at the right level. Trial pits also locate buried services and foundations, and check the condition of fill before and after treatment.
When the test is used
Typically used for…
- Shallow ground investigation and fill characterisation.
- Checking soil layers between boreholes and CPTs.
- Locating services, old foundations and obstructions.
- Sampling for laboratory testing and borrow material assessment.
- Inspecting treated ground or fill at depth.
Limitations
- Depth is limited by the excavator reach and the stability of the pit sides.
- Below the water table, or in loose sand, the sides may collapse.
- Pits disturb the ground and must be backfilled and compacted.
- Workers do not enter unsupported pits deeper than allowed by the safety rules.
The test type, number and locations are agreed with the consultant for each project.
How the test works
The pit location is checked for services, then excavated in layers. The exposed faces and the spoil are logged by an engineer from outside the pit, with photographs and measurements.
Samples are taken from the bucket or, where entry is allowed with support, from the faces. In-situ tests are carried out as required, and the pit is backfilled in compacted layers.
Test planning inputs
- Purpose and the information required.
- Locations and target depths.
- Service records and permits to dig.
- Sampling and testing schedule.
- Backfill requirements.
Calibration and equipment checks
- Service scan and permit before excavation.
- Excavation method and edge protection agreed for the expected ground.
- Sampling and logging procedure confirmed.
Test execution controls
- Pit position and ground level surveyed.
- Logging to the agreed standard, with photographs and scale.
- Groundwater inflow and side stability observed.
- Barriers and edge protection while open.
- Backfill compacted in layers.
Interpretation and reporting
The test plan, the raw record and the interpretation are kept separate, so it is always clear what was specified, what was measured and what it shows.
What is specified
Investigation scope and locations agreed with the consultant.
What was measured
Trial pit logs, photographs and sample records.
What it shows
Laboratory and in-situ test results, in the factual report.
Deliverables
- Location plan
- Trial pit logs
- Photographs
- Sample and test records
- Factual report
Discuss the testing your project needs.
Share the project stage, the ground conditions and the specification or acceptance criteria.