What conditions necessitate the use of a trench shield or shoring?

The use of a trench shield or shoring is primarily required when a trench reaches a depth of 1.5 metres or more and is situated in unstable soil. This is critical for ensuring the safety of workers in and around the trench. Unstable soil can collapse easily, posing a significant risk to those working within the trench. A trench shield or shoring provides essential support to the walls of the trench, reducing the chance of cave-ins and protecting workers from potential hazards.

When the trench reaches the significant depth of 1.5 metres, the risk factors increase as the weight of the soil above can generate substantial lateral pressure against the walls. In contrast, trenches that are less than 1.5 metres deep may have different regulations, and the general guidance is that the risk of collapse is lower, although caution is still necessary. Additionally, simply having rocky soil does not necessarily indicate a need for trench shields unless the depth and overall soil condition require reinforced support.

The final option mentions the use of shoring solely during backfilling, which is not an accurate representation of when this safety equipment should be utilized. It's essential to apply shoring or shielding while workers are in the trench, rather than just during backfilling processes.