Final answer:
The depth requiring a registered professional engineer for an excavation might traditionally be considered at 5 feet, but it can vary based on various factors and local regulations. It's critical to consult with an engineer for specific evaluations, and to follow current industry standards.
Step-by-step explanation:
The depth at which an excavation needs attention from a registered professional engineer is not strictly defined by a rule of thumb and can vary based on several factors including soil conditions, excavation size, and the presence of water or other risk factors. However, as a general guideline, traditionally, attention from a professional engineer might be considered necessary for depths greater than 5 feet. It is important to also consider that local regulations and industry standards could dictate specific requirements. Moreover, the depth might also be determined as specified by the engineer who is responsible for assessing the specific conditions and risks associated with the excavation work.
As the information provided mentions an older 'rule of thumb' and a newer suggestion regarding diameters deep, it can be inferred that advancements in engineering practices can lead to changes in safety standards and recommendations. Therefore, while early estimates may have considered 5 feet or 10 feet as significant depths, it is critical to follow the most current engineering standards and to consult with a registered professional engineer for site-specific evaluations.