A trial hole, or trial pit, is an excavation dug with a machine to expose the soil profile so it can be inspected, logged and tested. Trial holes are the standard way of checking ground conditions and groundwater levels on house sites, and they are also used in percolation testing for wastewater systems.
A trial hole is usually excavated with a tracked excavator to a depth below the likely founding level. The engineer records the sequence of soils, the depth at which each layer changes, the depth to rock if reached, and whether water enters the pit and at what level. Photographs and a scale rod are normally included in the report.
In site characterisation for wastewater treatment, a trial hole is used to check the depth of subsoil above bedrock and the water table, and a separate percolation test hole is used to measure how quickly water drains. Both are required by the EPA Code of Practice.