Trial pits are defined by ISO-22475-1 Geotechnical investigation and testing – Sampling methods and groundwater measurements as ‘open excavations constructed to determine the ground conditions in situ, recover samples or carry out field testing’ and should form part of any initial phase or preliminary investigations.
They can be used for geotechnical design, SUDS design, contamination analysis via waste acceptance criteria testing and chemical analysis via pH and sulphate testing.
They are used to quickly investigate the ground at a larger scale and where access allows, field tests can be completed such as field density of coarse soils using a spade or geological pick or undrained shear strength of soft fine soils using a vane. However, typically class 4 samples are taken for particle size distribution tests.
Depths are normally limited to three to five metres depending on soil / rock type, groundwater levels, excavator size, etc. Information derived from boreholes include occurrence of cobbles and boulders, stratigraphy and recognised geological units, stability of walls and groundwater observations.
Trial pits naturally cause more ground disturbance than boreholes or rotary cores and locations must be carefully planned in order to avoid undermining of future formation levels.
They are normally left exposed for a period of 20 minutes to monitor groundwater levels before backfilling with the arisings and compacted in layers.