When is it acceptable for workers to excavate below the bottom of a shield?

Excavating below the bottom of a shield is acceptable when the shield is engineered specifically for full depth resistance. Shields, or protective structures, are used in excavation sites to prevent cave-ins and protect workers. If a shield is designed to withstand the pressures and forces exerted by the surrounding soil at its full depth, it provides a safe environment for workers to excavate underneath it.

Other factors like the speed of excavation, moisture levels, or weather conditions do not directly ensure worker safety in this context. Rapid excavation may lead to insufficient support from the shield, and moisture or dry conditions can affect soil stability, but they do not relate to the structural integrity of the shield itself. Thus, the presence of a shield specifically made for full depth resistance is crucial for allowing excavation below it safely.