EXPERTS IN GEOSCIENCE
Trial pits are used for recovering near surface large bulk samples of granular soil, or if close visual examination of the strata is required. The advantage of this method over drilling is the relative speed at which the work can be carried out, and because of this they can often provide the most practitcal and economical method of site investigation and assesment.
Trial pits can be formed where the ground will be able to stand temporarily unsupported. Where there is water present in the excavation, problems may be presented due to instability of the sides and the difficulty of obtaining representative samples of the ground (finer material tends to wash out with the water as the sample is collected).
Entry of the excavation by personnel must be avoided, since the unsupported sides of a trial pit can collapse. If it is essential that an excavation be entered, e.g. for the collection of undisturbed samples or to enable in-situ testing to be carried out, then shoring must be used or the sides of the trial pit battered back to a safe angle.
Trial pits can be safely manually excavated down to about 1.2m, after this depth the sides must be properly supported if personell are to enter the pit. Hand excavation is necessary if services (water, gas, electricity, etc.) are known to exist in the vicinity and particularly if their location is uncertain. Once the base of the excavation is below the depth at which any services may exist, then the excavation can be continued by machine.
Machine excavated trial pits can be as deep as the capability of the machine, this is usually anything up to 6m. For aquiring samples from locations deeper than this it is normally required to drill a borehole.