Utilising mechanical excavators, usually JCB 3CXs but DCE engineers are also experienced with supervising 12 and 20 tonne excavators, can be used to dig three-metre-deep trenches/ pits. These excavations are used in ground investigations and allows engineers to recover bulk samples of soil and potentially rock. Trail Pitting is one of the easiest methods to use in order to determine the composition of the top-most strata, whether it is Made Ground or natural.
Equipment, arisings and spoil shall be placed so as not to present a hazard to the operatives or the general public. All spoil shall be heaped at least 1m from the edge of the pit. The surface material, be it topsoil, turf, ballast, tarmac or concrete shall be heaped in separate piles to avoid mixing where possible. If groundwater is encountered then trial pitting will be limited due to possible pit instability.
Trial pits shall be excavated and backfilled in one shift wherever possible. In the event that a pit has not been completed by the end of a shift or is left unattended, it shall be securely fenced. If there is any doubt over the stability of the walls of the excavation, such that instability shall cause surface settlement which might jeopardise the safety of personnel, equipment or property, the excavation shall be terminated and the pit backfilled. Infiltration testing can also be carried out during the excavation of the investigation area.
Details of the strata encountered, pit stability and other observations will be recorded. A water inflow should be recorded either as a seepage or as an entry. On completion, the pit will either be backfilled with arisings or reinstated with imported material as appropriate. Where possible materials shall be replaced in the pit in the same order that they were excavated. Backfilling shall be carried out in layers, each layer being tamped by the bucket of the excavator.
