Hand dug trial pits/test pits/trial holes typically extend to 0.75m or 1.50m. They are useful to locate safely services such as electricity and gas ahead of Borehole Drilling. It is recommended that the location of any trial pit or borehole is first swept with a CAT scanner or cable detection tool to avoid such dangers.
Machine dug trial pits can extend to between 1.5m or 6m depth but typically 3m and provide very good visual assessment. They can be dug using micro, mini, midi and maxi diggers with increasing size and depth capability. They are cheap, quick and flexible but larger excavations and machines can cause considerable disturbance and damage to the ground and the site which is becoming less acceptable these days. For this reason large trial pit excavation is being replaced by smaller test holes made by micro and mini digger and Boreholes using window sampler and tracked mini rigs.
However large trial pits can still often provide the most practical and economical method of Site investigation and assessment and Geoinvestigate still uses trial pits to obtain samples and engineering logs of shallow strata and we consider the large in-situ exposure of soils to be of importance when dealing with variable ground conditions and particularly ground contamination.
The safety issues of trial pitting, both during and after the investigation require careful consideration. Temporary “Heras” fencing and safety signage may be needed to safeguard the public from falling hazards created by deep excavations and moving plant and there remains the problems of tripping or stumbling hazards created by hazards caused by raised mounds left over when the pit is infilled and subsequent depressions caused by later subsidence of the infill.
There may be instances where it may be necessary to return to a site which has been intensely trial pitted to level or infill. Geoinvestigate has seen sites which have been so intensely pitted and the ground so badly damaged that houses have required raft foundations rather than normal strips.
Careful consideration also need to be given to personal safety and whether it is safe to enter a trial pit (even momentarily) to recover samples and log the sidewalls. Several instances of serious injury and even death have occurred where excavations have partially or totally collapsed. Even collapse of a 1.25m deep pit can kill.