Site Investigation

Trial pit excavations

Trial pits are carried out in order to recover large bulk samples of soil or if detailed visual examination of the strata is required. The main advantage of this method compared with light-cable percussion drilling or window sampling is the relative speed with which the work can be carried out. The disadvantage is the level of surface disturbance and the difficulty in carrying out effective reinstatement of the excavations.

Trial pits are usually carried out when the ground is able to stand temporarily unsupported. Where there is water present in the excavation, problems may be encountered due to instability of the side walls.  There can also be difficulty obtaining representative samples of the ground due to fine material washing out with the water as the sample is recovered.

Manned entry must be avoided because the unsupported sides of a trial pit can collapse. If manned entry is unavoidable, e.g. for the recovery of undisturbed samples or to enable in-situ testing to be carried out, then temporary shoring must be used or the sides of the trial pit stepped or battered back to a safe angle.

Trial pits can usually be safely excavated to a depth of about 1.2m using hand tools, assuming the sides of the trial pit are stable. Hand excavation is necessary if underground services (water, gas, electricity, etc.) are known to exist and particularly if their location is uncertain. When the base of the excavation is below the depth at which any services may exist, then the trial pit can be continued by machine.  Hand pits are frequently used to identify the construction of existing foundations prior to development or demolition.

A wheel-driven back-hoe excavator is appropriate for excavating trial pits to a depth of about 3m. Some machines of this type are capable of excavating down to about 4.5m. A track-driven excavator is necessary to reach depths beyond 4.5m. However, if groundwater is encountered, deeper excavation and sampling may be difficult and of limited value.